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  • Atomic, Molecular and Optical Physics  (2,025)
  • Physical Chemistry  (1,410)
  • Ultrastructure  (820)
  • 1990-1994  (3,105)
  • 1970-1974  (1,150)
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  • 1
    ISSN: 1432-072X
    Keywords: Key words     Extremely thermophilic eubacterium ; Calderobacterium hydrogenophilium ; Ultrastructure ; Electron microscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract       Calderobacterium hydrogenophilum is an extreme thermophilic, obligately chemoautotrophic, hydrogen-oxidizing bacterium. The cells were shown to be non-motile straight rods of average size 0.4 × 2.5 μm. After negative-staining of the whole cells, no flagella were observed. The multilayered cell wall was of type 1 and possessed a crystalline proteinaceous surface layer exhibiting p4 symmetry. The square unit cells had a lattice constant of approximately 11 nm. Cell division occurred by a constriction mechanism. C. hydrogenophilum differred from a similar hydrogen-oxidizing eubacterium, Hydrogenobacter thermophilus, by the absence of intracytoplasmic membrane structures in chemically fixed cells. However, an electron-dense intracytoplasmic hemispherical structure adhering to the inner membrane was frequently observed.
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  • 2
    ISSN: 1432-1106
    Keywords: Dopaminergic grafts ; Neuropeptide Y ; Ultrastructure ; Striatum ; Rat
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract In a previous study we demonstrated that grafted dopamine (DA) neurons are able to induce an early and widespread normalization of DA-neuropeptide Y (NPY) interactions in the host striatum previously deprived of its DA input. Since similar recoveries were found to occur in striatal areas densely or poorly reinnervated by the graft, the question was raised as to what mechanisms (synaptic or volumic release) were involved in these functional effects. Ultrastructural analysis of graft-to-host relationships was performed using single — and double — immunolabelling techniques to detect neurons containing tyrosine hydroxylase (TH) and NPY, with a view to analysing the early establishment of synaptic connectivity in various areas of the host striatum. Within 1 month of the grafting, TH-immunoreactive (TH-IR) neurons showed most of the normal intrinsic morphological features characteristic of adult rat neurons and were found to have established direct relationships with various striatal neuronal populations. TH-NPY relationships were observed only in the area most densely reinnervated by the graft, and their relative frequency was found to be roughly the same as that determined in the intact striatum. Three months after the grafting, this percentage decreased, probably owing to the further elongation in TH-IR axons resulting in a wider distribution of the TH-NPY associations over the host striatum. In the zones distal from the graft, the reinnervation was far from complete and the few TH-IR fibres projected only to some unlabelled elements, mainly of the spiny type, which have been shown to interact normally with both DA afferents and NPY cells and therefore may relay the DA action over the whole striatum on the NPY population. It can be concluded from these data that the rapid and extensive functional normalization of the TH-NPY interactions previously found to occur in the entire striatum may depend on the restoration of direct and indirect synaptic relationships. A diffuse action of DA through non-synaptic mechanisms may also account for the fact that the amine has access to broader striatal populations than to those presumably reached by DA fibres arising from the graft.
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  • 3
    ISSN: 1433-2981
    Keywords: ADP ; Snake ; Thrombocyte ; Ultrastructure ; Waglerophis merremii
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The ultrastructure of the resting thrombocytes of the snake Waglerophis merremii and the process of ADP aggregation are studied. These thrombocytes are nucleate, ellipsoidal, and contain a marginal band. Endoplasmic reticulum and ribosomes are few. An open canalicular system, Golgi complex, granules resembling the alpha-granules in human platelets, and structures similar to secondary lysosomes are also present. The activation response of W. merremii thrombocytes to ADP is examined and compared to the morphological alterations in human platelets. The thrombocytes, normally ellipitical in shape, become spheroid with cytoplasmic protrusions, and adhere to one another. The open canalicular system undergoes dilation and the granules cluster at the centre of the thrombocytes. The release reaction of these granules occurs after stimulation by ADP, similar to what happens in human platelets. This similarity may suggest a process of evolutional specialization since thrombocytes in the majority of non-mammal vertebrates are not aggregated by ADP.
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  • 4
    ISSN: 1432-2307
    Keywords: Mast cell ; Compound 48/80 ; Ultrastructure ; Quick-freezing
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The ultrastructure of mast cells stimulated with compound 48/80 was examined by quick-freezing and deep-etching (QF-DE) or freeze-substitution (QF-FS) methods. Peritoneal cells including mast cells of adult male rats were stimulated in vitro with compound 48/80 at 17° C for 0, 10, 30, 60 or 180 s. The QF-DE replicas revealed that the mast cells stimulated with compound 48/80 for 30 s decreased filamentous actin around secretory granules. In the QF-FS specimens, perigranular membranes in mast cells stimulated for 60 s formed pentalaminar structures between adjacent granules in their cytoplasm prior to degranulation. These findings suggest that preparatory states for degranulation occur in the whole cytoplasm of stimulated mast cells at early stages. Moreover, both QF-FS specimens and QF-DE replicas revealed a compact morphological appearance of discharged granules in the extra-cellular space, indicating the existence of considerable content within the granules. Skeletal structures in the granules were also demonstrated on QF-DE replicas prepared after extracting soluble elements from the cytoplasm. It is suggested that the granular contents associated with the skeletal structures are gradually detached from the discharged granules to ensure local concentration in the tissues.
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  • 5
    ISSN: 1432-2307
    Keywords: Rat ; Pancreatic beta cells ; Immunocytochemistry ; Ultrastructure ; Insulin
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract When studied morphologically in semi-thin sections in the rat in vivo, pancreatic beta cells displayed heterogeneous immunoreactivities for insulin and amylin, depending on the islet size and the intra-islet position of the beta cells. In larger islets, cortical beta cells (beta cells with contacts with all islet cell types and with the exocrine parenchyma) which are located in the periphery were more densely immunostained for insulin and amylin than medullary beta cells (beta cells with contacts only with other beta cells) which are located in the centre of the islet. Ultrastructurally, these findings were accompanied by differences in the number of secretory granules and mitochondria. Beta cells in small islets and at extra-islet sites exhibited a dense immunoreactivity. After administration of glibenclamide, immunoreactivities for insulin and amylin were diminished in a time-dependent manner, decreasing first in medullary and thereafter in cortical beta cells of larger islets. Ultrastructurally, the beta cells exhibited the typical signs of stimulation. A minority of beta cells in small islets and all beta cells in extra-islet locations remained unchanged. Thus pancreatic beta cells under basal and stimulatory conditions in vivo exhibit heterogeneity in hormone content and in ultrastructural features. These differences may represent the basis for a functional heterogeneity of the insulin secretory response of the individual beta cell both in vivo and in vitro in states of normal and impaired insulin secretion. As heterogeneity was observed only among beta cells in islets, while single beta cells surrounded by acinar cells exhibited no changes in insulin immunoreactivity, interactions between beta cells as well as between beta cells and other endocrine cells may be critical for expression of heterogeneity within the beta cell population.
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  • 6
    ISSN: 1432-2307
    Keywords: Pituitary adenoma ; Sellar gangliocytoma ; Immunohistology ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Three cases of a composite sellar tumour composed of a gangliocytoma and an adenoma are presented. Two patients who showed acromegaly and hyperprolactinaemia had a gangliocytoma and a growth hormone (GH)-prolactin cell adenoma in close proximity. The gangliocytoma contained growth hormone-releasing hormone (GHRH) by immunohistochemistry. At the electron microscopical level, the gangliocytoma was characterized by numerous synaptic vesicles. The third patient, a child with Cushing's disease, presented a corticotropin-releasing hormone (CRH)-positive gangliocytoma in close contact with an adrenocorticotropic hormone (ACTH) secreting adenoma, the latter a typical densely granulated ACTH cell adenoma. Ultrastructurally, the gangliocytoma revealed synaptic vesicles and sparse secretory granules. The results suggest that gangliocytomas may promote the development of pituitary adenomas by hypersecretion of releasing hormones. Whereas 20 cases of sellar GHRH producing gangliocytomas in acromegaly are reported in the literature, the combination of a CRH-positive gangliocytoma and an ACTH cell adenoma in Cushing's disease is apparently the first case.
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  • 7
    ISSN: 1432-0533
    Keywords: Corticobasal degeneration ; Ultrastructure ; Tau ; Glial inclusions ; Progressive supranuclear palsy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract We have studied brain tissues from three patients with corticobasal degeneration (CBD) histologically, ultrastructurally and immunohistochemically. Ballooned neurons in the cerebral cortex and severe degeneration of the substantia nigra were observed in them all and weakly basophilic neurofibrillary tangles (NFTs) were distributed widely in the basal ganglia and brain stem. Ultrastructural examination demonstrated that the NFTs comprised characteristic 15-nm-wide straight tubules, which showed positive immunohistochemical staining with an antibody against tau, but not ubiquitin. Tau-immunoreactive neuronal cell bodies without NFTs also were found in the cerebral cortex and subcortical nuclei, predominantly in the brain stem, and the greatest number of tau-positive glial inclusions occurred in the cerebral gray and white matter of the pre- and post-central gyri. These inclusions comprised tubular structures with diameters of about 15 nm and were localized in the oligodendroglial cellular cytoplasm and processes. These findings indicate that there is a close cytoskeletal pathological relationship between CBD and progressive supranuclear palsy.
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  • 8
    ISSN: 1432-0533
    Keywords: Vacuolization ; Neurotoxicity ; Neuropathology ; Electron microscopy ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Cytoplasmic vacuoles appear in neurons of the posterior cingulate/retrosplenial cortex (PC/RS) of rats after treatment with N-methyl-d-aspartate (NMDA) receptor antagonists. Prominent dilatation of mitochondria and endoplasmic reticulum has been described within 2 h; however, the ultrastructural features of vacuole formation are unknown. To investigate this, the present study examined the PC/RS cortex of male rats (age 60–70 days) at 15, 30, 45, 60, 90, and 120 min after subcutaneous treatment with 1 mg/kg of the noncompetitive NMDA antagonist MK-801 (dizocilpine maleate, 5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine). Subtle mitochondrial dilatation was identified in a few neurons as early as 15 min postdose (MPD). By 30 MPD, dilatation was more pronounced in mitochondria and also involved the endoplasmic reticulum and perinuclear space. Ribosomal disaggregation and degranulation were also evident by 30 MPD. At all subsequent time points, dilatation of mitochondria and endoplasmic reticulum progressed in severity. Although the relative involvement of mitochondria and endoplasmic reticulum varied, glia were not involved. These ultrastructural data suggest that after treatment with MK-801, mitochondrial dilatation precedes involvement of endoplasmic reticulum in vacuolization of susceptible PC/RS cortical neurons. The early mitochondrial effects identified in this study suggest an initial metabolic insult that rapidly progresses to affect endoplasmic reticulum and ribosomes. This strengthens the relationship between the ability of certain NMDA antagonists to induce energy perturbations and neuronal vacuoles in the same region of the rat cerebral cortex.
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  • 9
    ISSN: 1432-0533
    Keywords: Key words Vacuolization ; Neurotoxicity ; Neuropathology ; Electron microscopy ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Cytoplasmic vacuoles appear in neurons of the posterior cingulate/retrosplenial cortex (PC/RS) of rats after treatment with N-methyl-D-aspartate (NMDA) receptor antagonists. Prominent dilatation of mitochondria and endoplasmic reticulum has been described within 2 h; however, the ultrastructural features of vacuole formation are unknown. To investigate this, the present study examined the PC/RS cortex of male rats (age 60 – 70 days) at 15, 30, 45, 60, 90, and 120 min after subcutaneous treatment with 1 mg/kg of the noncompetitive NMDA antagonist MK-801 (dizocilpine maleate, 5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine). Subtle mitochondrial dilatation was identified in a few neurons as early as 15 min postdose (MPD). By 30 MPD, dilatation was more pronounced in mitochondria and also involved the endoplasmic reticulum and perinuclear space. Ribosomal disaggregation and degranulation were also evident by 30 MPD. At all subsequent time points, dilatation of mitochondria and endoplasmic reticulum progressed in severity. Although the relative involvement of mitochondria and endoplasmic reticulum varied, glia were not involved. These ultrastructural data suggest that after treatment with MK-801, mitochondrial dilatation precedes involvement of endoplasmic reticulum in vacuolization of susceptible PC/RS cortical neurons. The early mitochondrial effects identified in this study suggest an initial metabolic insult that rapidly progresses to affect endoplasmic reticulum and ribosomes. This strengthens the relationship between the ability of certain NMDA antagonists to induce energy perturbations and neuronal vacuoles in the same region of the rat cerebral cortex.
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  • 10
    ISSN: 1432-0533
    Keywords: Amyotrophic lateral sclerosis ; Anterior horn neuron ; Synapse ; Active zone ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract This report deals with an ultrastructural investigation of the synapses of anterior horn neurons in the lumbar spinal cords of five patients with amyotrophic lateral sclerosis (ALS) who had mild neuronal depletion. Specimens from five age-matched, neurologically normal individuals served as controls. In each instance, the autopsy was performed within 3 h after death. A statistically significant decrease in cell body area, number of synapses and total synaptic length was found in the normal-appearing neurons of the ALS patients. The alterations were more pronounced in neurons with central chromatolysis. However, despite an approximately 20% reduction in the number of synapses, the length of the active synaptic zone of the normal-appearing neurons in the ALS patients was not diminished. This observation may be accounted for by a plasticity to the loss of synapses which maintained the active zone of the remaining synapses to increase synaptic efficiency. It is suggested that when the plasticity of the active zone reaches its limit, the continuing loss of synapses may lead to functional impairment. The capacity of the active synaptic zone to respond to progressive denervation of the anterior horn neurons may preserve motor function or slow the development of motor deficits in the early stage of degeneration of the lower motor neurons.
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  • 11
    ISSN: 1432-0533
    Keywords: Human pineal gland ; Pineal parenchymal tumors ; Ultrastructure ; Chromogranin A
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract We have studied 20 pineal parenchymal tumors (PPT) and 4 normal or cystic pineal glands both by light and electron microscopy and immunohistochemistry with antibodies against glial markers [glial fibrillary acidic protein (GFAP) and protein S-100] or neural/neuroendocrine markers [neurofilaments (NF), synaptophysin and chromogranin A]. Light microscopy revealed the cellular organization of pinealocytes in the normal gland and in different morphological types of pineal tumors (typical pineocytomas, PPT with intermediate differentiation, mixed PPT exhibiting elements of both pineocytoma and pineoblastoma and pineoblastomas). Immunohistochemistry showed the presence of GFAP and protein S-100 in interstitial cells in nonneoplastic pineal gland. Cell processes were labeled with anti-synaptophysin and anti-NF antibodies. No immunoreactivity was found for chromogranin A in non-neoplastic pineal gland. In pineocytomas, GFAP and protein S-100 were observed in interstitial cells. Synaptophysin and NF were present in the large rosettes of pineocytomas. Synaptophysin, NF and chromogranin A were present in pineocytomas with a lobular arrangement of cells. Anti-chromogranin A immuno-reactivity was also seen in lobular areas of some PPT with intermediate differentiation. Analysis of normal human pineal gland by electron microscopy showed the presence of vesicle-crowned rodlets (VCR or synaptic ribbons), fibrous filaments (F), paired twisted filaments but few dense-core vesicles (DCV) in normal pinealocytes. Tumoral pineal cells appeared to differentiate either towards a neurosensory pathway characterized by the presence of sensory cells elements (VCR and F), or towards a neuroendocrine pathway, with the occurrence of many DCV. Immunogold labeling demonstrated the presence of chromogranin A in neurosecretory granules.
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  • 12
    ISSN: 1432-0568
    Keywords: Oocyte ; Ultrastructure ; Zebu ; Nucleolus
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The objective of the study was to investigate the light and ultrastructural morphology of dominant and subordinate oocytes from zebu (Bos indicus) cattle. Healthy cycling animals, which had a well-developed corpus luteum as judged by rectal palpation, were administered cloprostenol to induce luteolysis and therefore ovulation. The animals were slaughtered at days 3–11 post-ovulation, but those slaughtered at days 8–11 received a second injection of cloprostenol at day 7. Cumulus-oocyte complexes were aspirated from the largest (dominant) and the second largest (subordinate) follicles, and processed for transmission electron microscopy. Up to day 7, the dominant oocyte presented a peripherally located spherical oocyte nucleus with a compact dense fibrillar nucleolus. After day 7, the nuclear envelope became undulated and the nucleolus vacuolated. The nuclei contained an average of four nucleoli. In addition to vesicles, mitochondria, smooth endoplasmic reticulum, Golgi complexes and cortical granules, the ooplasm contained annulate lamellae and microtubules. Moreover, mitochondrial granules and pleomorphic forms of mitochondria were commonly observed. Some subordinate oocytes exhibited advanced stages of meiotic maturation. It is concluded that (1) the dominant oocyte undergoes certain prematurational changes, including nucleolus vacuolation, in the period from luteolysis up to the presumptive occurrence of the LH peak and (2) subordinate oocytes may undergo meiotic maturation.
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  • 13
    ISSN: 1432-0533
    Keywords: Key words Amyotrophic lateral sclerosis ; Anterior horn neuron ; Synapse ; Active zone ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract This report deals with an ultrastructural investigation of the synapses of anterior horn neurons in the lumbar spinal cords of five patients with amyotrophic lateral sclerosis (ALS) who had mild neuronal depletion. Specimens from five age-matched, neurologically normal individuals served as controls. In each instance, the autopsy was performed within 3 h after death. A statistically significant decrease in cell body area, number of synapses and total synaptic length was found in the normal-appearing neurons of the ALS patients. The alterations were more pronounced in neurons with central chromatolysis. However, despite an approximately 20  % reduction in the number of synapses, the length of the active synaptic zone of the normal-appearing neurons in the ALS patients was not diminished. This observation may be accounted for by a plasticity to the loss of synapses which maintained the active zone of the remaining synapses to increase synaptic efficiency. It is suggested that when the plasticity of the active zone reaches its limit, the continuing loss of synapses may lead to functional impairment. The capacity of the active synaptic zone to respond to progressive denervation of the anterior horn neurons may preserve motor function or slow the development of motor deficits in the early stage of degeneration of the lower motor neurons.
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  • 14
    ISSN: 1432-0533
    Keywords: Key words Human pineal gland ; Pineal parenchymal tumors ; Ultrastructure ; Chromogranin A
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract We have studied 20 pineal parenchymal tumors (PPT) and 4 normal or cystic pineal glands both by light and electron microscopy and immunohistochemistry with antibodies against glial markers [glial fibrillary acidic protein (GFAP) and protein S-100] or neural/neuroendocrine markers [neurofilaments (NF), synaptophysin and chromogranin A]. Light microscopy revealed the cellular organization of pinealocytes in the normal gland and in different morphological types of pineal tumors (typical pineocytomas, PPT with intermediate differentiation, mixed PPT exhibiting elements of both pineocytoma and pineoblastoma and pineoblastomas). Immunohistochemistry showed the presence of GFAP and protein S-100 in interstitial cells in non-neoplastic pineal gland. Cell processes were labeled with anti-synaptophysin and anti-NF antibodies. No immunoreactivity was found for chromogranin A in non-neoplastic pineal gland. In pineocytomas, GFAP and protein S-100 were observed in interstitial cells. Synaptophysin and NF were present in the large rosettes of pineocytomas. Synaptophysin, NF and chromogranin A were present in pineocytomas with a lobular arrangement of cells. Anti-chromogranin A immunoreactivity was also seen in lobular areas of some PPT with intermediate differentiation. Analysis of normal human pineal gland by electron microscopy showed the presence of vesicle-crowned rodlets (VCR or synaptic ribbons), fibrous filaments (F), paired twisted filaments but few dense-core vesicles (DCV) in normal pinealocytes. Tumoral pineal cells appeared to differentiate either towards a neurosensory pathway characterized by the presence of sensory cells elements (VCR and F), or towards a neuroendocrine pathway, with the occurrence of many DCV. Immunogold labeling demonstrated the presence of chromogranin A in neurosecretory granules.
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  • 15
    ISSN: 1432-0533
    Keywords: Key words Extraskeletal osteosarcoma ; Brain neoplasms ; Ultrastructure ; Multinucleated giant cells
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract A 57-year-old woman with primary intracerebral osteosarcoma is reported. The tumor was identified by computed tomography as a mass with hemorrhage in the right parietal lobe. The surgical and pathological examinations confirmed an osteosarcoma of intracerebral origin. She suffered from repeated local recurrence of the tumor and died about 1 year after the onset. The pathological findings showed features of osteoblastic osteosarcoma with numerous osteoclast-like multinucleated giant cells. Immunohistochemically, tumor cells were positive for vimentin, and partially for actin. Multinucleated giant cells were reactive with vimentin and CD68 antibodies. Ultrastructurally, tumor cells were rich with rough endoplasmic reticulum. These findings are consistent with the histological features of skeletal or extraskeletal osteosarcoma. This is the third case of primary intracerebral osteosarcoma reported in the literature and the first one analyzed ultrastructurally.
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  • 16
    ISSN: 1432-0533
    Keywords: Extraskeletal osteosarcoma ; Brain neoplasms ; Ultrastructure ; Multinucleated giant cells
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract A 57-year-old woman with primary intracerebral osteosarcoma is reported. The tumor was identified by computed tomography as a mass with hemorrhage in the right parietal lobe. The surgical and pathological examinations confirmed an osteosarcoma of intracerebral origin. She suffered from repeated local recurrence of the tumor and died about 1 year after the onset. The pathological findings showed features of osteoblastic osteosarcoma with numerous osteoclastlike multinucleated giant cells. Immunohistochemically, tumor cells were positive for vimentin, and partially for actin. Multinucleated giant cells were reactive with vimentin and CD68 antibodies. Ultrastructurally, tumor cells were rich with rough endoplasmic reticulum. These findings are consistent with the histological features of skeletal or extraskeletal osteosarcoma. This is the third case of primary intracerebral osteosarcoma reported in the literature and the first one analyzed ultrastructurally.
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  • 17
    ISSN: 1432-0568
    Keywords: Chick embryo ; Bursectomy ; Female gonads ; Steroidogenic cells ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract We examined the ultrastructural characteristics of the medullary steroidogenic cells in left and right female gonads of surgically bursectomized chick embryos killed on the 17th day of incubation. The steroidogenic cells of the bursectomized embryos have a more developed system of cisternae in the smooth endoplasmic reticulum than controls, and their mitochondria show some alterations in the density of the matrix and in the shape of the cristae. On the basis of these results, an enhancement of the steroidogenic activity in both gonads is suggested.
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  • 18
    ISSN: 1432-0568
    Keywords: Cattle ; Epithelium ; Oviduct ; Secretion ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The non-ciliated (NC) cells of the bovine oviduct epithelium, have been shown to release embryotrophic substances to the oviduct lumen. The aim of the present study was to investigate the ultrastructure, focusing on aspects of the secretory machinery, of NC cells in different segments of the oviduct during and after transoviduct migration of zygotes and embryos. Dairy heifers (n=8) were superovulated with an ECG/cloprostenol regimen, and the time of ovulation was estimated by ultrasound scanning. Samples from the infundibulum, ampulla, isthmus and uterotubal-junction of the oviduct were surgically collected from animals at 19–96 h and 7 1/2; –8 1/2 days after ovulation and processed for transmission electron microscopy, following standard procedures. The NC cells contained characteristic membrane-bound secretory granules composed of a lamellar cortex encaging an amorphous medulla. The two components could still be recognized during extrusion of the granule content into the oviduct lumen by exocytosis. During granulogenesis, small maturing granules without the lamellar structure were observed, but distinct condensing vacuoles were absent. An abundance of granules was found in the early versus the late group. In both groups the uterotubual junction was almost free of granules. This segment, on the contrary, was characterized by the presence of primary and secondary lysosome-like bodies. In the early group the intracellular location of the granules varied between oviduct segments. In the infundibulum they were placed in the supranuclear cytoplasm, in the isthmus they were found in the most apical part of the cells, while in the ampulla an intermediate granule position was noticed. In both groups the uterotubal junction was almost free of granules. This segment, on the contrary, was characterized by the presence of primary and secondary lysosome-like bodies. Cytoplasmic protrusions, often containing nuclei, were more frequent in the late than in the early group. This phenomenon may represent epithelial renewal. In conclusion, the NC cell of the bovine oviduct epithelium possesses an extensive capacity for protein synthesis and secretion. The numbers and location of secretory granules show cyclic and segmental variations. Most granules are present in the infundibulum and ampulla during the period of transoviduct migration of zygote and embryo.
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  • 19
    ISSN: 1432-0827
    Keywords: Collagen ; Crystal habit ; Ultrastructure ; Turkey leg tendon
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Physics
    Notes: Abstract Transmission electron micrographs of fully mineralized turkey leg tendon in cross-section show the ultrastructure to be more complex than has been previously described. The mineral is divided into two regions. Needlelike-appearing crystallites fill the extrafibrillar volume whereas only platelike crystallites are found within the fibrils. When the speciment is tilted through a large angle, some of the needlelike-appearing crystallites are replaced by platelets, suggesting that the needlelike crystallites are platelets viewed on edge. If so, these platelets have their broad face roughly parallel to the fibril surface and thereby the fibril axis, where the intrafibrillar platelets are steeply inclined to the fibril axis. The projection of the intrafibrillar platelets is perpendicular to the fibril axis. The extrafibrillar volume is at least 60% of the total, the fibrils occupying 40%. More of the mineral appears to be extrafibrillar than within the fibrils. Micrographs of the mineralized tendon in thickness show both needlelike-appearing and platelet crystallites. Stereoscopic views show that the needlelike-appearing crystallites do not have a preferred orientation. From the two-dimensional Fourier transform of a selected area of the cross-sectional image, the platelike crystallites have an average dimension of 58 nm. The needlelike-appearing crystallites have an average thickness of 7 nm. The maximum length is at least 90 nm. Atomic force microscopy (AFM) of unstained, unmineralized turkey leg tendon shows collagen fibrils very much like shadow replicas of collagen in electron micrographs. AFM images of the mineralized tendon show only an occasional fibril. Mineral crystallites are not visible. Because the collagen is within the fibrils, the extrafibrillar mineral must be embedded in noncollagenous organic matter. When the tissue is demineralized, the collagen fibrils are exposed. The structure as revealed by the two modalities is a composite material in which each component is itself a composite. Determination of the properties of the mineralized tendon from the properties of its elements is more difficult than considering the tendon to be just mineral-filled collagen.
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  • 20
    ISSN: 1432-0827
    Keywords: In vitro ; Bioactive glass ceramic ; Mineralization ; Bone bonding mechanisms ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Physics
    Notes: Abstract Rat bone cells were cultured in the presence of bioactive glass-ceramic containing crystalline apatite and wollaston te. Scanning electron microscopy observations of the surface of the seeded ceramic disks revealed that cells attached, spread, and proliferated on the material surface. Soaking in cell-free culture medium showed that no change occurred in the surface structure. However, when cultured with bone cells and observed under a transmission electron microscope, an electron-dense layer was noted initially at the surface of the material, before bone formation occurred. In addition, energy-dispersive X-ray microanalysis demonstrated the presence of calcium and phosphorus in this layer. Progressively, during the following days of culture, active osteoblasts synthetized and laid down an osteoid matrix composed of numerous collagen fibrils arranged either parallel or perpendicularly to the first-formed electron-dense layer. Mineralization initiated on the ceramic surface dispersed then along the collagenous fibrils, leading to a mineralized matrix which surrounded the ceramic particles. These results demonstrate the capacity of apatite-wollastonite glass ceramic to initiate biomineralization in osteoblast cultures and to achieve a direct bond between the surface apatite layer of the bioactive glass-ceramic and the mineralized bone matrix.
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  • 21
    ISSN: 1432-072X
    Keywords: Extremely thermophilic eubacterium ; Calderobacterium hydrogenophilium ; Ultrastructure ; Electron microscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Calderobacterium hydrogenophilum is an extreme thermophilic, obligately chemoautotrophic, hydrogen-oxidizing bacterium. The cells were shown to be nonmotile straight rods of average size 0.4x2.5 μm. After negative-staining of the whole cells, no flagella were observed. The multilayered cell wall was of type 1 and possessed a crystalline proteinaceous surface layer exhibiting p4 symmetry. The square unit cells had a lattice constant of approximately 11 nm. Cell division occurred by a constriction mechanism. C. hydrogenophilum differred from a similar hydrogen-oxidizing eubacterium, Hydrogenobacter thermophilus, by the absence of intracytoplasmic membrane structures in chemically fixed cells. However, an electron-dense intracytoplasmic hemispherical structure adhering to the inner membrane was frequently observed.
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  • 22
    ISSN: 1434-0879
    Keywords: GnRH-DT vaccine ; Testosterone ; Ultrastructure ; Rat ; Prostate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract To evaluate the effects of active immunization against gonadotropin-releasing hormone (GnRH) on the ultrastructure of the rat ventral prostate, male Sprague-Dawley rats received three consecutive intramuscular injections of 10 μg/100g body weight (D-Lys6)-GnRH-diphtheria toxoid conjugate (GnRH-DT vaccine). Following immunization, test animals developed sufficiently high antibody titres to block the pituitary gonadal axis. Consequently testosterone values dropped to the levels in castrates. This therapy leads to atrophy of the prostate. Following immunization a strong immunological response, indicating the presence of considerable amounts of a GnRH-like peptide, was observed in the ventral prostates as early as 14 days after the first injection of GnRH-DT. Immunoneutralisation of GnRH-like activity may contribute to the effects observed.
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  • 23
    ISSN: 1434-4726
    Keywords: Cochlea ; Ultrastructure ; Stria vascularis ; Development
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The mammalian stria vascularis undergoes certain developmental changes in the postnatal rat. The present study was designed to examine the ultrastructure of the stria vascularis in rat pups from immediately after birth to 20 days postpartum. The cochlea were removed with the animals under xylazine (Rompun) anesthesia and were prepared for transmission electron microscopy. Each of the three cell types in the stria were found to contain kinocilia up until 12–17 days of age. The presence of kinocilia in the intermediate and basal cells has not been previously described. Findings suggest that these organelles may serve a motile and/or sensory function to assist in the maturation of cell functions, particularly ion transport, during early stages of development.
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  • 24
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    Protoplasma 180 (1994), S. 14-28 
    ISSN: 1615-6102
    Keywords: Golgi apparatus ; Dictyosome ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Golgi apparatus (GA) of eukaryotic cells consist of one or more stacks of flattened saccules (cisternae) and an array of fenestrae and tubules continuous with the peripheral edges of the saccules. Golgi apparatus also are characterized by zones of exclusion that surround each stack and by an assortment of vesicles (or vesicle buds) associated with both the stacks and the peripheral tubules of the stack cisternae. Each stack (sometimes referred to as Golgi apparatus, Golgi complex, or dictyosome) is structurally and functionally polarized, reflecting its role as an intermediate between the endoplasmic reticulum, the cell surface, and the lysosomal system of the cell. There is probably only one GA per cell, and all stacks of the GA appear to function synchronously. All Golgi apparatus are involved in the generation and movement of product and membrane within the cell or to the cell exterior, and these functions are often reflected as structural changes across the stacks. For example, in plants, both product and membrane appear to maturate from the cis to the trans poles of the stacks in a sequential, or serial, manner. However, there is also strong ultrastructural evidence in plants for a parallel input to the stack saccules, probably through the peripheral tubules. The same modes of functioning probably also occur in animal GA; although here, the parallel mode of functioning almost surely predominates. In some cells at least, GA stacks give rise to tubular-vesicular structures that resemble the trans Golgi network. Rudimentary GA, consisting of tubular-vesicular networks, have been identified in fungi and may represent an early stage of GA evolution.
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  • 25
    ISSN: 1615-6102
    Keywords: α-Amylase ; (1-3, l-4)-β-Glucanase ; Hormones ; Monensin ; Transfection ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary A procedure has been developed to isolate protoplasts from mature aleurone layers of the malting variety Alexis and four other barley genotypes. It combines induction of endogenous cell wall degrading enzymes together with use of Onuzuka cellulase R 10 and driselase and results in better yields for two varieties than can be obtained with the huskless variety Himalaya. The viability of the freshly isolated protoplasts is greater than 90% and in spite of the presence of gibberellic acid during isolation procedures, most of the protoplasts are at an early developmental stage, as judged by ultrastructure. Gibberellic acid-induced changes in protoplast structure resemble those reported for Himalaya protoplasts. The protoplasts secrete both α-amylase (EC 3.2.1.1) and (1-3, 1-4)-β-glucanase (EC 3.2.1.73) into the surrounding medium. Transfection studies using a low pI α-amylase promoter to direct chloramphenicol acetyltransferase expression in aleurone protoplasts from Alexis and Himalaya revealed significant differences in their hormone responsiveness. In the absence of hormones, low levels of expression of the reporter enzyme were obtained in Alexis protoplasts, while high levels were characteristic for Himalaya protoplasts. An 8-fold increase in the expression of the reporter gene was induced by supplying the transfected Alexis protoplasts with gibberellin A3, whereas expression in Himalaya protoplasts remained unchanged. When Himalaya protoplasts were isolated from aleurone layers that had not been incubated with GA3 during the initial stages of protoplasting (the classical procedure), the hormone response of the promoter was 2.5-fold. It is thus possible to optimize the aleurone protoplast isolation procedure for different barley genotypes and mutants of interest in studies of transgenic gene expression and hormone induced secretion of proteins from this unique secretory plant tissue.
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  • 26
    ISSN: 1618-0860
    Keywords: Coccolithophorid ; Cruciplacolithus neohelis ; Flagellar apparatus ; Haptophyceae ; Prymnesiophyceae ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The flagellar apparatus ofCruciplacolithus neohelis (McIntyre and Bé) Reinhardt including its transition region is described. The transition region contains a hat-shaped structure, which is suggested to be one of the common features of the Prymnesiophyceae. Its flagellar root system resembles that of most coccolithophorids examined so far, except that only one vestigial crystalline root is present associated with root 1. Two well-developed crystalline roots associated with roots 1 and 2, respectively, appear in the preprophase of nuclear division, suggesting conversion to a mitotic spindle. The taxonomic and evolutionary significance of the flagellar apparatus is discussed.
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  • 27
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    Springer
    Protoplasma 179 (1994), S. 142-150 
    ISSN: 1615-6102
    Keywords: Dinoflagellate ; Eyespot ; Gymnodinium natalense ; Ontogeny ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Ultrastructure and ontogeny of a new type of eyespot in dinoflagellates is described. A marine tidal poolGymnodinium natalense is found to possess a highly organized eyespot whose structure is unique among dinoflagellates. The eyespot is rectangular in ventral view, C-shaped in apical view, and is located posterior to the sulcus. The eyespot is independent of the chloroplast and consists of several (typically six) layers of hemi-cylindrical walls which are concentrically arranged with narrow spacing between them. Each hemicylindrical wall is enclosed by a single unit membrane and is composed of many regularly arranged rectangular crystalline bricks. These crystalline bricks are produced in small vesicles which are formed in the invaginations of the chloroplast. The vesicles containing newly formed crystalline bricks are then transported to the sulcal area to assemble the eyespot. The crystalline bricks are arranged in a neat row within the vesicle termed “eyespot forming vesicle” (EFV), which is located near the sulcus. The hemi-cylindrical wall is constructed within the EFV. Based on the structure of the eyespot, viz. consisting of concentric multi-layered walls, the eyespot is thought to act as a quarter-wave stack antenna.
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  • 28
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    Protoplasma 180 (1994), S. 169-184 
    ISSN: 1615-6102
    Keywords: Actin ; Cell-cell communication ; Plasmodesmata ; Regulation ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary We have used several methods to localise actin associated with plasmodesmata. In meristematic plant material fixed in 0.1% glutaraldehyde/1% paraformaldehyde and embedded in LR White resin, actin was localised (in TEM using 5 nm gold-labelled secondary antibody to C4 anti-actin primary antibody) in the neck region by the plasma membrane and endoplasmic reticulum, and also down the length of the plasmodesma, deep in the cell wall. When the chemical fixation was replaced by rapid freezing in liquid propane (without cryoprotectants) and substitution in acetone, the plasmodesmata were labelled in similar positions, but with less background label on sections. While only 8–20% of plasmodesmata were labelled, the label was 10 to 100 fold denser over plasmodesmata than over the surrounding wall indicating specific association with plasmodesmata. We presume the apparent extracellular location of some label was due to the size of the antibodies between the site of attachment and the observed position of the gold particle. Gold label was found in similar locations in material fixed in 3% paraformaldehyde, infiltrated with sucrose, frozen, sectioned (10–12 μm thick), then labelled with antibodies before resin embedding. Furthermore, cell walls in epidermal peels stained with rhodamine-phalloidin showed localised patches of fluorescence, presumably at the site of plasmodesmata (or primary pit-fields), which were connected on either side to fluorescent strands of actin in the cytoplasm. Suspension cultured cells ofNicotiana plumbaginifolia similarly stained showed very faint, narrow fluorescent strands crossing the walls of sister cells, which may indicate actin associated with individual plasmodesmata, shown in TEM to be sparsely distributed in these walls. In addition, the neck regions of cytochalasin-treated plasmodesmata were greatly enlarged and lacked the normal extracellular ring of particles. We propose that actin associated with plasmodesmata stabilizes the neck region and possibly also the cytoplasmic sleeve, and may be actively involved in regulating cell-to-cell transport.
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  • 29
    ISSN: 1615-6102
    Keywords: Saprolegnia ; Lectins ; Concanavalin A ; Wheat germ agglutinin ; Monoclonal antibodies ; Ultrastructure ; Pathogenesis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The importance of the surface structure and chemistry in zoospores and cysts of oomycetes is briefly reviewed and the organelle systems associated with encystment described. The surface structure and chemistry of primary and secondary zoospores and cysts ofSaprolegnia diclina (a representative saprophytic species) andS. parasitica (a representative salmonid fish pathogen) were explored using the lectins concanavilin A (Con A) and wheat germ agglutinin (WGA) and monoclonal antibodies (MAbs) raised against a mixed zoospore and cyst suspension ofS. parasitica. The binding of lectins and antibodies to spores was determined using immunofluorescence microscopy with fluorescein isothiocyanate-labelled probes and with electron microscopy with gold-conjugated probes applied to spore suspensions post-fixation. In both species Con A, which is specific for glucose and mannose sugars, bound to both the surface of primary and secondary zoospores (the surface glycocalyx) and their cyst coats and readily induced zoospore encystment. The binding to the cysts appeared to be mainly associated with the matrix material released from the primary and secondary encystment vesicles and which appeared to diminish with time. No binding to germ tube walls was observed with this lectin. The MAb labelling showed a generally similar binding pattern to the primary and secondary cysts to that observed with Con A, although the binding to zoospores was more variable. Primary zoospores bound the antibodies but secondary zoospores appeared less reactive. It is suggested that the MAbs share a common epitope with one or more of the Con A-binding components. In both species WGA, which is specific for amongst other things the sugar N-acetyl glucosamine, bound to localised apical patches on the primary zoospores. This lectin also binds to the ventral groove region of secondary zoospores ofS. diclina, which were induced to encyst by this lectin. In contrast secondary zoospores ofS. parasitica were not induced to encyst by the addition of WGA and showed a patchy dorsal binding with this lectin. WGA also binds to both the inner wall of discharged primary cysts and the young germ tube walls of both species. These observations are discussed both in relation to other oomycete spores and to their possible functional and ecological significance.
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  • 30
    ISSN: 1615-6102
    Keywords: Carbohydrates ; Chytridiomycetes ; Extracellular material ; Membranes ; Ultrastructure ; Zoospores
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary In development of the primitive fungi, chytridiomycetes, unwalled zoospores bearing single, posterior flagella are transformed into walled, round-cells which elaborate the thallus. Production, structural modification, or release of extracellular material are involved with each transition of developmental stage. This article reviews the variety and developmental changes of extracellular materials found at the cell surface of chytridiomycetes. A cell coat, produced from Golgi-derived vesicles during zoosporogenesis, is visible around free swimming zoospores of some chytridiomycetes. How the zoospore surface receives and transduces signals is not widely explored, but it is known that fenestrated cisternae and simple cisternae, which are integrated into the microbody-lipid globule complex, are spatially and structurally associated with the plasma membrane and flagellar apparatus. This spatial association, as well as the cytochemical localization of calcium in fenestrated cisternae, suggest a mechanism for signal transduction and for regulation of zoospore motility. Zoospores become encased in a new layer of extracellular material as the zoospore encysts. Among some chytrids the source of this material is preexisting vesicles which fuse with the plasma membrane. Among other zoospores, a readily identifiable population of encystment vesicles is not apparent, demonstrating that there is no single pattern or mechanism for zoospore encystment in chytridiomycetes. Encysted zoospores developing into thalli, typically produce cell walls with a microfibrillar substructure. Ultrastructural analysis of walls reveals distinctive architecture and remarkable sculpturing which have been used in systematics of some members of chytridiomycetes. Nothing is known as to underlying controls of cytoskeletal elements and plasma membrane enzyme complexes in wall biogenesis. Many changes in cell surface structures accompany thallus maturation. Septa, many traversed with plasmodesmata, are produced in most chytrid thallus types. As sporangia and resting spores prepare for the production and release of zoospores, additional extracellular layers of material are frequently produced. Polarized deposits of extracellular material become discharge plugs, discharge vesicles, or endoopercula. Interstitial material is also released into cleavage furrows. Circumscissile or localized digestion of walls produce operculate or inoperculate exit ports for zoospore release. Cryofixation preserves more extensive extracellular material than does conventional chemical fixation, and broader application of cryofixation may radically alter our current view of cell surface structure. Thus chytridiomycetes exhibit a range in patterns for the occurrence and subsequent modifications of extracellular materials, even for members within the same order. The most universally recognized role for these extracellular materials is protection. Although there is a reasonable view of the types of extracellular material involved in chytridiomycete development, we have only limited understandings of their biogenesis or roles in regulation and communication, areas awaiting more investigations.
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  • 31
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    Protoplasma 178 (1994), S. 34-47 
    ISSN: 1615-6102
    Keywords: Appressorium ; Cochliobolus sativus ; Electron microscopy ; Thigmotropism ; Ultrastructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary GerminatingCochliobolus sativus spores were induced to form appressoria on a variety of artificial surfaces, including replicas of the barley leaf surface. Evidence was obtained for the involvement of chemical and topographic signals during induction of appressorium formation inC. sativus. Germ tube thigmotropism was also observed in vitro. Ultrastructure relevant to appressorium formation was observed, including the germ tube apex, apical swelling of the germ tube apex prior to appressorium formation, the appressorium with associated septation and the penetration peg. Cytochemical probes applied to germlings at the electron microscope level failed to detect α-D-mannan, α-D-glucan, β-D-galactan, D-glcNAc or D-galNAc polymers in the extracellular mucilage associated with the fungal germlings. The ultrastructure of hyphal apices from germlings grown under different nutritional conditions differed with respect to Spitzenkörper morphology, apex shape and in the quantity of associated extracellular mucilage. Experimental findings are discussed relative to current understanding of appressorium induction in more extensively studied systems.
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  • 32
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Semi-empirical (AM1) calculations on the 12 isomeric products and the corresponding transition states of ketene-1-azabutadiene [4 + 2] and [2 + 2] cycloaddition reactions as a model system for the reaction of ketenes with 4-iminobenzylfuran-2,3-diones are presented. A [4 + 2] type of reaction of the ketene C=C double bond leading to a six-membered lactam compound is found to be highly favoured both thermodynamically and kinetically. [2 + 2] Cycloadducts generally have significantly higher activation energies. Reactions involving the formation of a carbon-nitrogen bond proceed in most cases via attack of the nitrogen lone pair leading to a zwitterionic intermediate. Depending on the respective cycloadduct, both two-step and concerted, albeit asynchronous, processes were obtained. Based on the structures of the various transition states, some predictions with respect to substituent effects are made. Similarities to and differences from the analogous reaction of ketenimines with oxa-1,3-dienes are discussed.
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  • 33
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Reaction of aryliminodimagnesium [ArN(MgBr)2, IDMg] with nitrobenzene (Ar1NO2) in tetrahydrofuran (THF) gives unsymmetrical (unsym) azoxybenzene, which is deoxygenated to give unsym-azobenzene. The reaction is utilized for the independent preparation of ONN and NNO isomers of unsym-azoxy compounds. The effects of the relative value of the difference between oxidation and reduction potentials of para-substituted reactants, special effects of ortho-substituents of the substrate and also effects of structure and concentration of mono- and bis-IDMg reagents were studied previously, and ‘cooperation of their aggregate excess needed for product formation’ was proposed. This unfamiliar concept for reactions of magnesium reagents was studied further. Supporting evidence was obtained from the retarding effect of trimethylene chains of nitro [3.3] metacyclophanes and the derived azoxy [3.3] metacyclophanes and also from the effect of the length of bis-IDMg's central α,ω-polymethylenedioxy chain appropriate for azoxy deoxygenation. By comparison with deoxygenation by X3P reagents (X = EtO, Me2N), the general needs of cooperation of excess magnesium reagents were confirmed and its role in their reactions is discussed.
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  • 34
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The magnitude ρx(ρnuc) is shown to decrease with increase in the extent of bond making estimated by the kinetic isotope effect for the reactions of Y-benzoyl chlorides with anilines, XC6H4NH2, in acetonitrile at 25·0°C.
    Additional Material: 2 Tab.
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  • 35
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The role of orbital control in product selectivity during electrophilic aromatic alkylation catalysed by zeolites was studied both theoretically and experimentally. In order to discuss this, the alkylation of toluene and m-xylene by methanol was carried out on a series of large-pore zeolites (HY). The changes in the para to ortho ratio observed on changing the framework Si/Al ratio of the zeolites were related to ab initio molecular orbital calculations of the LUMO energy of structurally alike model clusters but containing different tetrahedral cations around the active site. The observed correlation is discussed in terms of the HSAB principle by taking into account the influence of the catalyst composition on the reactivity of the electrophilic reagent.
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  • 36
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: N-Substituted carbonimidodithioic acid dimethyl esters, when reacted with an active methylene compound such as nitromethane, undergo carbon-carbon bond formation followed by carbon-sulphur bond cleavage in the presence of zeolite catalysts to give 1-substituted amino-1-methylthio-2-nitroethenes. This carbon-sulphur bond cleavage is facilitated by the presence of rare earth cations in the zeolite-Y framework. The shape selectivity of the zeolite also plays an important role in this cleavage reaction. Force field calculations adopted for the molecules involved in the reaction indicate the geometry and conformational flexibility of these molecules. The computer-simulated model for the zeolite-Y is correlated with the conformation and shape of the reactant and product molecules to explain the variation in yields obtained in the conversion of substituted dimethyl esters.
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  • 37
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effects of the concentrations of LiCl, NaCl, KCl, CsCl, Na2CO3 and BaCl2 on the rates of reactions of hydroxide ion with ionized N-hydroxyphthalimide (NHP-) at 30°C and in H2O-MeCN solvents containing 10, 50, 60, 66, 70, 76 and 80% (v/v) MeCN reveal the formation of ion-pair complexes between cations of the salts, which probably exist in solvent-separated loose ion-pair forms (Mn+⃛Xk-) and NHP-. An increase in MeCN content from 2 to 76% (v/v) causes an increase in the association constants (K) by factors of 40, 21 and 9 for LiCl, NaCl and KCl, respectively, while the respective increase in the rate constants (k′s1) for the collapse of the ion-pair complexes to product is ca two-fold. The values of k′s1 remain almost unchanged whereas the values of K increase 7·5-fold with an increase in MeCN content from 2 to 70% (v/v) for CsCl. Ion-pair complex formation was not detected in the presence of Me4NCl at 70% (v/v) MeCN. The observed pseudo-first-order rate constants are highly sensitive to the valence state of cations and almost insensitive to the valence state of the anions of the salts.
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  • 38
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rates and products of solvolyses of 4-methylene-2ax- and -2eq-adamantyl p-toluenesulphonates (tosylates) (4a-OTs and 4e-OTs, respectively) were studied. Compound 4a-OTs solvolysed more slowly than 2-adamantyl tosylate (1) in methanol and 2,2,2-trifluoroethanol (TFE) by factors of 2·3 and 2·5, respectively, at 25°C. However, by taking the inductive decelerating effect of a β-methylene substituent into account, the rates were revealed to be enhanced by α-participation by a factor of 50. The products of solvolyses of 4a-OTs in methanol, 80% acetone and TFE at 100°C were 2ax- and 2eq-alkoxy(or hydroxy)-4-methyleneadamantanes (4a-OR and 4e-OR, respectively), exo-4-alkoxy(or hydroxy)-5-methyleneprotoadamantane (exo-5-OR) and 5-[alkoxy(or hydroxy)methyl]-4-protoadamantene (6-OR) with adamantyl to protoadamantyl product ratio of 39:61 (in methanol), 56:44 (in 80% acetone) and 71:29 (in TFE). Despite the nearly symmetric nature of the intermediate cation, the 4a-OR: 4e-OR product ratio was essentially constant with 83:17 (in methanol), 85:15 (in 80% acetone) and 82:18 (in TFE). The formation of considerable amounts of 4e-OR was interpreted as showing the intermediacy of a pair of rapidly equilibrating classical ions. The rates of 4e-OTs were 2300-4300 times faster than those expected from inductive electron-withdrawing effect of a β-methylene substituent. The major product (84·5% in methanolysis and 98·7% in trifluoroethanolysis) was 4e-OR accompanied by small amounts of 2-alkoxy-2,4-methanoadamantane (9-OR) and 2-(alkoxymethyl)-2,4-didehydroadamantane (10-OR), no formation of 4a-OR having been observed. These results suggested that 4e-OTs solvolyses via a π-bridged intermediate cation.
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  • 39
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    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 40
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Molecular mechanics (MM3) calculations were carried out on the title compounds. Comparison between the MM3 results and those of semi-empirical and ab initio calculations and experiment indicates that the MM3 results are at least as good as results of much more expensive calculations. The MM3 calculations predict that unlike corannulene and cyclopentacorannulene, the transition state of the bowl-to-bowl inversion of the related C30H10 is non-planar, and the activation barrier is too high for this motion to occur.
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  • 41
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Reactions of benzo-, naphtho- and anthraquinone derivatives (unsubstituted and substituted) with aryliminodimagnesium [ArN(MgBr)2] and aryloxymagnesium of a weak electron-donating ability were studied. In addition to the reduction products (quinhydrones and hydroquinones), nuclear substitution and condensation products were formed. The efficiency of single electron transfer (SET) from ArN(MgBr)2, evaluated by the relative values of the difference between the oxidation and reduction potentials of the reactants (ΔE = Eox - Ered), varies with the electron-accepting power of quinones. ΔE governs the modes of semiquinone appearance (ESR signals) and the relative amounts of the heat of reactions, reflecting the types and yields of the products. It is concluded that condensation products of both components are produced in the reactions of quinones with the lower SET efficiency by the presence of a fused benzene ring or MeO substituent. The structure-reactivity relationship proposed previously for reactions of various magnesium reagents is extended in the reactions with a variety of quinones even in cases of higher SET efficiency.
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  • 42
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The low temperature 1H NMR spectra of 2,2-bis(3,5-dibromomesityl)ethenol in CS2-CD2Cl2 (3:7) display new signals which indicate the presence of four enol species whose OH is hydrogen bonded. Oligomerization to intermolecularly hydrogen-bonded enol dimers or tetramers is suggested.
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  • 43
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    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 44
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Measurements of absorption and emission spectra, fluorescence quantum yields (Φf) and lifetimes (τf) of ω-(1-pyrenyl)alkyl para-substituted benzoates PnX (X = H, Cl, CF3, CN, NO2, with n = 1-4) were carried out in solvents of various polarity. Intramolecular interaction in the ground state is not observed in any of these compounds. PnCN (n = 2-4) shows an intramolecular exciplex emission in solvents of low to high polarity. The broad, structureless emission at longer wavelength observed in ethyl acetate solution of P1CN is ascribed to an ‘exciplex-type’ emission which does not require actual overlapping of the two chromophores. P2CF3 also shows an exciplex emission in solvents of medium to high polarity. The solvent dependence of both Φf and τf increases as the electron-withdrawing ability of the para-sustituents increases. The relationship between fluorescence quenching by electron transfer and para-substituent of PnX is discussed by means of the free energy for electron transfer, ΔGET, obtained from the oxidation and reduction potentials of pyrene and methyl para-substituted benzoates in acetonitrile, respectively. PnH, with positive ΔGET, does not show a solvent dependence of Φf, except for P1H, in which ca 40% of the fluorescence is quenched in acetonitrile. PnCl, with slightly negative ΔGET, shows more efficient quenching, but does not show exciplex emission. PnCN, PnCF3 and PnNO2 have ΔGET values between -0·36 and -0·65 eV, and their fluorescence is fairly efficiently quenched. The fluorescence of PnCN is concluded to be strongly quenched by intersystem crossing from the singlet exciplex to the locally excited pyrene, and by electron transfer from the pyrene part to the benzoate part. That the formation of a singlet exciplex is necessary for intersystem crossing in bichromophoric compounds containing pyrene is thus clearly illustrated.
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  • 45
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The evidence for the mechanisms proposed for aromatic nucleophilic substitution reactions by primary and secondary amines in aprotic solvents of low relative permittivity is reviewed.
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  • 46
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A rigorous procedure was developed for measuring the relative rates of addition of bromine atoms to eleven substituted α-methylstyrenes (1-Y, with Y = CF3, NO2, F, CN, Cl, Br, CO2Me, Me, COMe, OMe and SMe). The reaction was run in tetrahydrofuran in the presence of HBr, O2 and dibutyl peroxyoxalate at 30°C. All products were derived from the YC6H4CMeCH2Br adduct radicals, which were immediately intercepted by O2. Correlation analysis of all the data confirmed the proposition that in the absence of measurable steric effects, the relative rates for radical additions can be correlated only by a dual-parameter equation and not by a single-parameter equation. Among various combinations of σ* and σx, the (σJJ* + σmb) combination yields the best correlation.
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  • 47
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Protonation of tetrafluoro-2,11-dithia [3·3]paracyclophane and tetrafluoro-2,11-dithia [3·3]metaparacyclophane in high-acidity super acid media, namely FSO3H.SbF5 (1:1) ‘magic acid’-SO2CIF, gave their corresponding acidic disulfonium ions. Additional ring protonation to give a disulfonium-monoarenium trication was not observed. With the non-fluorinated 2,11-dithia [3·3] cyclophanes, the disulfonium ions can be ring protonated in equilibrium to give a dynamic disulfonium-monoarenium trication. Tetrafluoro [2·2]-metaparacyclophane is monoprotonated at the meta ring and gives a complex mixture of conformational isomers. Multinuclear magnetic resonance data on the cyclophane precursors and their derived cations are compared and analysed. The energies, conformations and charge distributions of the isometric fluorinated and non-fluorinated [2·2] - and dithia [3·3] cyclophanes were calculated by the AM1 and PM3 methods, respectively. In all but one case the cyclophane arenium ions predicted by theory to be energetically most favoured are those observed in solution under stable ion conditions. In agreement with experiment, the instabilities of S,S,C-cyclophane trications are also theoretically predicted.
    Additional Material: 10 Ill.
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  • 48
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Ionization constants for 214 dye molecules were calculated from molecular structures using the chemical reactivity models developed in SPARC (SPARC Performs Automated Reasoning in Chemistry). These models used fundamental chemical structure theory to predict chemical reactivities for a wide range of organic molecules from molecular structure. The energy differences between the protonated state and the unprotonated state for a molecule of interest are factored into mechanistic components including the electrostatic and resonance contributions and any additional contributions to these energy differences. The RMS deviation was found to be less than 0·62 pKa units, which is similar to the experimental error.
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  • 49
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 169-169 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 50
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvent effects on the absorption spectra of N-(p-nitrophenyl)dimethylenediamine (Ia), N-(p-nitrophenyl)trimethylenediamine (Ib), N-(p-nitrophenyl)tetramethylenediamine (Ic). N-methyl-N-(p-nitrophenyl)tetramethylenediamine (II), N-butyl-p-nitroaniline (III) and N-(o-nitrophenyl)trimethylenediamine (IV) were studied at different temperatures. Whereas II, III and IV do not show any variation in their spectra characteristics with changes in temperature, I shows a hypsochromic shift with a hypochromic effect when the temperature is increased. The Kamlet and Taft solvatochromic comparison method was applied. A strong effect of the β parameter on I and III was interpreted as being due to the hydrogen bond donor ability of the H atom in the aromatic amino groups. In I, the β influence increases with increase in temperature. These facts are explained by proposing the formation of intramolecular hydrogen bonds between amine groups in all compounds I, besides the intermolecular interactions between compounds I and the solvent. On the other hand, the values of vo, s and b for Ib are smaller than the corresponding values for Ia and Ic. Since in Ib a six-membered ring may be formed, a more stable bond is expected. Comparative 1H NMR of the aniline hydrogen for I and IV (in non-hydrogen bond acceptor solvents) shows a particular downfield chemical shift for I which suggests hydrogen bond formation. Since this effect is independent of concentration, the hydrogen bond is assumed to be intramolecular, in agreement with solvatochromic and thermochromic studies. These conclusions were corroborated by IR spectroscopy in the solid state and in chloroform solutions.
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  • 51
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 52
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The secondary α-deuterium isotpe effect on the equiliburium constant (KD/KH) for the hydration of p-nitrobenzaldehyde is 1·39 ± 0·05. The Kinetic secondary α-deuterium isotope effect (KD/KH) for the addition of water to p-nitrobenzaldehyde is 1·18 ± 0·07. This result demonstrates that there is extensive carbon-oxygen bond formation in the transition state for addition of water to the carbonyl group of p-nitrobezaldehyde.
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  • 53
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rate of the reaction of 2-chloroquinoxaline with piperidine in dimethyl sulphoxide was measured over a wide range of amine concentrations and at several temperatures. It was found that the order with respect to the nucleophile is close to 1 between 300 and 320 K, but is definitely less at lower and higher temperature. It is suggested that below 300 K an unreactive charge-transfer complex is formed between the reactants which dissociates at higher temperatures, whereas at temperatures higher than 320 K an unproductive σ complex is formed, the concentration of which increases with increase in temperature.
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  • 54
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The enthalpic contribution to the anomeric effect in r-2-carboethoxy-trans-4,trans-6- and r-2-carboethoxy-cis-4,cis-6-dimethyl-1,3-dithianes was determined by reaction-solution calorimetry. The enthalpy of solution of both isomers in pure p-dioxane and the enthalpy of solution and isomerization in the same solvent and in the presence of trifluoroacetic acid were experimentally measured. From these results the corresponding enthalpies of isomerization in solution were calculated, and were found to be ΔHax→eq = -0·16 ± 0·04 kcal mol-1 (-0·67 ± 0·18 kJ mol-1) and ΔSax→eq = -2·68 ± 0·1 cal K-1 mol-1 (-11·2 ± 0·4 J K-1 mol-1). The slightly negative ΔH term nevertheless reflects a substantial anomeric effect owing to the countervailing steric effects in the axial isomer. The significant entropy loss in the equatorial isomer was explained in terms of intramolecular electrostatic effects. The results are in agreement with those obtained from NMR studies of the conformational behaviour of 2-carboethoxy-5-methyl-5-aza-1,3-dithiacyclohexane.
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  • 55
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 591-609 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The MM3 molecular mechanics program calculates a fair representation of hydrogen bonding interactions, but to improve the MM3 hydrogen bond potential, a directional term has been added to the hydrogen bonding function. The resulting total function was reoptimized. Comparisons of the hydrogen bonding potential functions from ab initio, the original MM3, the current MM3(92) force field and the reoptimized MM3 force field MM3(94) for a variety of C,N,O systems are described.
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  • 56
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The title sulphenamides were pyrolysed in a stirred-flow reactor at temperatures of 310-410°C, pressures of 8-15 Torr and residence times of 0·4-2 s using toluene as the carrier gas. N-(tert-Butylthio)allylamine formed 73 ± 4% isobutene, 23 ± 3% propene and N-allylthiohydroxylamine. The first-order rate coefficients for the formation of isobutene and propene, respectively, followed the Arrhenius equations kC4(s-1) = 1012·52 ± 0·36 exp(-163 ± 5 kJ mol-1 RT) and kC3(s-1) = 1010·99 ± 0·29 exp(-151 ± 4 kJ mol-1 RT) N-(tert-Butylthio)diethylamine gave 97 ± 1% isobutene, 1·9 ± 0·4% isobutane and N,N-diethylthiohydroxylamine. The first-order rate coefficients for isobutene elimination followed the Arrhenius equation k(s-1) = 1013·45 ± 0·24 exp(-164 ± 3 kJ mol-1 RT). The formation of the products is interpreted in terms of an elimination reaction with a unimolecular, four-centered, cyclic transition state. The reactivity of these sulphenamides was found to be much higher than that of previously studied alkyl or aryl tert-butyl sulphides and disulphides.
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  • 57
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The aromatic odd-alternant phenalenyl anion and a number of its derivatives were prepared in order to study the perturbation of this conjugated anion by methyl and methoxy groups. The conjugated anions were studied by means of 1H and 13C NMR spectrometry, alkylation experiments and semi-empirical calculations. It was found that a substituent at a charged carbon atom perturbs the entire conjugated system, whereas substituents at inactive (uncharged) carbon atoms have a large effect on the positions ortho to the substituent.
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  • 58
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The spontaneous hydrolysis of p-nitrobenzyl cellulose xanthate (CelXNB) with a degree of substitution (DS) in the range 2-9 was studied in 10% aqueous ethanol at pH 10, and was followed spectrophotometrically by the appearance of p-nitro-α-toluenethiol, in a continuous-flow system where the reactor was shaken. CelXNB was characterized by solid-state 13C NMR spectra. The reaction occurs through two parallel processes due to two xanthate ester groups with different reactivities. The fast hydrolysis was ascribed to the reaction of the C-2 + C-3 isomers, whereas the slow hydrolysis was due to the C-6 isomer. The percentage of the latter is much higher than C-2 + C-3. The solvent isotope effect of the fast hydrolysis (k′H2O/)k′H2D 11 was 2·22 ± 0·16 and the proton inventory indicated that there is only one proton transfer involved in the transition state, where a second water (or a neighbouring OH group) acts as a general base. The entropy of activation of the fast hydrolysis was only 3·3 ± 0·8 e.u., suggesting that the water molecules involved are highly oriented with respect to the coordinates required to reach the transition state. It is proposed that they form part of the three-dimensional hydrogen-bonded ice-like structure that involves the cellulose matrix.
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  • 59
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 316-322 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rates of hydrolysis of the vinyl ether functional groups of (Z)- and (E)-β-methoxyacrylic acid and (Z)-and (E)-β-methoxymethacrylic acid and their methyl esters were measured in aqueous perchloric acid solution. Additional rate measurements were also made for one substrate, (Z)-β-methoxymethacrylic acid, in buffer solutions down to pH 7, and a rate profile was constructed. The results show that the β-carboxy and β-carbomethoxy substituents produce strong rate retardations, ranging from 2000- to 25 000-fold, for both Z- and E-isomers in both the acrylic and methacrylic acid series. The rate profile for (Z)-β-methoxymethacrylic acid indicates that ionization of this substrate to the carboxylate ion form rises the rate of hydrolysis by a factor of 240. It is argued that this difference in reactivity of ionized and non-ionized forms of the substrate is due to conjugative and inductive effects of the substituents, rather than β-lactone formation as suggested in an earlier observation of the same phenomenon in a different system.
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  • 60
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The stereochemistry and kinetics of the methoxide ion substitution reactions (Z)- and (E)-methylbenzohydroximoyl cyanidc [PhC(CN)=NOCH3] were investigated. The reaction of the (Z)-hydroximoyl cyanide with sodium methoxide in DMSO-methanol (9:1) solution at 44·8°C gives a mixture of methyl (Z)-O-methylbenzohydroximate [PhC(OCH3)=NOCH3] and the O-methyloxime of α-ketophenylacetamide PhC(CONH2)=NOCH3. The (E)-hydroximoyl cyanide undergoes methoxide ion-catalyzed isomerization to the E-isomer faster than it undergoes nucleophilic substitution. These observations were interpreted in terms of an addition-elimination mechanism in which the rate-limiting step is elimination of the nucleofuge (AN + DN#).
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  • 61
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 394-394 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 62
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reaction of nitrosobenzene with aniline, to give azobenzene, in basic conditions was studied. It was shown that the reaction exhibits general base catalysis by different buffers giving a Brønsted coefficient β = 0·318. As in previous studies, a two-step process with a first step of attack of aniline on nitrosobenzene to give an addition intermediate and a second step of dehydration of this intermediate is proposed to interpret the mechanism of the reaction. The analysis of the Brønsted relationship and of the intermediate of the reaction led to the suggestion that hydoxide ion catalyses the reaction by a mechanism of general base catalysis in the dehydration step.
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