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  • Polymer and Materials Science  (22,814)
  • Biochemistry and Biotechnology  (3,598)
  • 1985-1989  (17,469)
  • 1970-1974  (8,943)
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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
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  • 2
    ISSN: 0887-3585
    Keywords: template-assembled synthetic protein (TASP) ; 4-helix bundle ; β-barrel structure ; protein de novo design ; peptide synthesis ; peptide conformation ; orthogonal protection ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The construction of a template-assembled synthetic protein (TASP) designed to contain both a 4-helix bundle and a β-barrel as two folding “domains” is described. For the de novo design of proteins, amphiphilic helices (α) and β-sheets (β) are covalently attached to a template peptide (T) carrying functional side chains suitably oriented to promote intarmolecular folding of the secondary structure blocks into a characteristic packing arrangement, i.e., T8-(4α)(4β). The design of this new macromolecule was assisted by computer modeling, which suggested a low-energy conformation with tight hydrophobic packing of the secondary structure subunits. Solid-phase synthesis of the “two-domain” TASP molecule was achieved using orthogonal protection techniques. The solution properties as well as circular dichroism (CD) and infrared spectroscopy (IR) data under various experimental conditions are consistent with the folded conformation suggested by modeling.
    Additional Material: 6 Ill.
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  • 3
    ISSN: 0887-3585
    Keywords: reconstitution ; membrane protein folding/unfolding ; pH effects ; differential scanning calorimetry ; circular dichroism ; electron microscopy ; HPLC ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Thermal unfolding experiments on bacteriorhodopsin in mixed Phospholipid/detergent micelles were performed. Bacteriorhodopsin was extracted from the purple membrane in a denatured state and then renatured in the micellar system. The purpose of this study was to compare the changes, if any, in the structure and stability of a membrane protein that has folded in a nonnative environment with results obtained on the native system, i.e., the purple membrane. The purple membrane crystalline lattice is an added factor that may influence the structural stability of bacteriorhodopsin. Micelles containing bacteriorhodopsin are uniformly sized disks 105 ± 13 Å in diameter (by electron microscopy) and have an estimated molecular mass of 210 kDa (by gel filtration HPLC). The near-UV CD spectra (which is indicative of tertiary structure) for micellar bacteriorhodopsin and the purple membrane are very similar. In the visible CD region of retinal absorption, the double band seen in the spectrum of the purple membrane is replaced with a broad positive band for micellar bacteriorhodopsin, indicating that in micelles, bacteriorhodopsin is monomeric. The plot of denaturational temperature vs. pH for micellar bacteriorhodopsin is displaced downward on the temperature axis, illustrating the lower thermal stability of micellar bacteriorhodopsin when compared to the purple membrane at the same pH. Even though micellar bacteriorhodopsin is less stable, similar changes in response to pH and temperature are seen in the visible absorption spectra of micellar bacteriorhodopsin and the purple membrane. This demonstrates that changes in the protonation state or temperature have a similar affect on the local environment of the chromophore and the protein conformation. We conclude that the tertiary structure of the bacteriorhodopsin monomer is essentially the same in micelles and the purple membrane. On the other hand, in the synthetic mixed micelle system, the packing between the nonnative amphiphiles and bacteriorhodopsin is probably not optimal, protein-protein interactions have been lost, and thehelical packing may be looser because the crystalline lattice is absent. Itis likely that a combination of these effects leads to the decreased stability of micellar bacteriorhodopsin.
    Additional Material: 7 Ill.
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  • 4
    ISSN: 0887-3585
    Keywords: folding intermediate ; molten globule state ; protein folding ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Previous results from equilibrium and kinetic studies of the folding of bovine growth hormone (bGH) have demonstrated that bGH does not follow a simple two-step folding mechanism. These results are summarized and interpreted according to the “molten globule” model. The molten globule state of bGH is characterized as a folding intermediate which largely a-helical, retains a compact hydrodynamic radius, has packing of the aromatic side chains that is similar to the unfolded state, and possesses a solvent-exposed hydrophobic surface along helix 106127 that readily leads association.
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  • 5
    ISSN: 0887-3585
    Keywords: protein conformation ; dynamics ; Monte Carlo simulation ; conformational energy ; minimization ; plastic deformation ; conformational heterogeneity ; hierarchy in dynamics ; trypsin inhibitor ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Differences in atom packing are studied in the minimum energy conformations derived from the record of the Monte Carlo simulation of conformational fluctuation in the native state of a globular protein, bovine pancreatic trypsin inhibitor. It is found that local deformations observed among the minima which are found in the previous paper are accompanied by rearrangement of atom packing. Spatial locations of the local deformations in the three-dimensional folded structure are also studied. It is foundthat the local deformations are distributed in space in several clusters inthe folded structure. The size and location of the clusters characterize the respective fluctuations of the first and the second levels observed in the simulation. In the fluctuations of the first level local deformations, each of which usually involves a few side chains and one main chain local segment, are thermally exited independently of each other near thesurface of the molecule. The observed fluctuation of the second level involves a cooperative deformation involving many side chains and local main chain segments all in one cluster, which goes though the core of the molecule. The collective local deformations observed both in the first and second levels are plastic in the sense that they are accompanied with rearrangement of atom packing.
    Additional Material: 3 Ill.
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  • 6
    ISSN: 0887-3585
    Keywords: solution scattering ; low-angle scattering ; spherical averaging ; spherical harmonics ; spherical Fourier transform ; bound water ; solvent structure ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: If one expands the structure factor equation in spherical coordinates, rotational averaging of the molecular Fourier transform, which leads directly to the solution scattering profile, is greatly simplified. It becomes a projection in the polar and azimuthal angular variables. The profile is given by The index j runs over all atoms; r, θ, φ are atomic coordinates and ε and N are constants; the Ym,n are complex spherical harmonics, and Jn are spherical Bessel functions; R = 2 sin θ/λ. The effects of solvent have been modeled by subtracting from each protein atom a properly weighted water. Hydrogens have been included by using scattering curves fj derived from the spherical averaging ofprotein atoms with their attached hydrogens. This approach may also be satisfactory for neutron scattering. Published scattering profiles2 for lysozyme and BPTI have been accurately matched in less than one-tenth the time required by other methods. Separate, adjustable temperature factors for the protein, solvent waters, and bound watersare used, and appear to be needed. In the case of BPTI, as suggested by NMR observations, the observed diffraction pattern was much better accounted for by including only 4 tightly bound waters rather than the roughly 60 seen by crystallography.
    Additional Material: 5 Ill.
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  • 7
    ISSN: 0887-3585
    Keywords: enhanced stability ; λCro ; genetic suppression ; intracellular proteolysis ; antibody screen ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: A mutant Cro protein, which bears the Ile-30→ Leu substitution, is thermally unstable and degraded more rapidly than wildtype Cro in vivo. Using an antibody screen, we have isolated five different second site suppressor substitutions that reduce the proteolytic hypersensitivity of this mutant Cro protein. Two of the suppressor substitutions increase the thermal stability of Cro by 12°C to 14°C. These amino acid substitutions affect residues 16 and 26, which are substitutions affect residues 16 and 26, which are substantially exposed to solvent in the crystal structure of wild-type Cro.
    Additional Material: 4 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: aconitase ; iron-sulfur enzyme ; crystal structure ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The crystal structure of the 80,000 Da Fe—S enzyme aconitase has been solved and refined at 2.1 Å resolution. The protein contains four domains; the first three from the N-terminus are closely associated around the [3Fe-4S] cluster with all three cysteine ligands to the cluster being provided by the third domain. Associationof the larger C-terminal domain with the first three domains createsan extensive cleft leading to the Fe—S cluster. Residues from all four domains contribute to the active site region, which is defined by the Fe—S cluster and a bound SO42-ion. This region of the structure contains 4 Arg, 3 His, 3 Ser, 2 Asp, 1 Glu, 3 Asn, and 1 Gln residues, as well asseveral bound water molecules. Three of these side chains reside on a threeturn 310 helix in the first domain. The SO42-ion is bound 9.3 Å from the center of the [3Fe-4S] cluster by the side chains of 2 Arg and 1 Gln rsidues. Each of 3 His side chains in the putative active site is paired with Asp or Glu side chains.
    Additional Material: 13 Ill.
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  • 10
    ISSN: 0887-3585
    Keywords: sequence conservation ; exon ; gene duplication ; protein folding ; structure-function ; X-ray structure ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Detailed analysis of the CuZn superoxide dismutase (SOD) structure provides new results concerning the significance and molecular basis for sequence conservation, intron-exon boundary locations, gene duplication, and Greek key β-barrel evolution. Using 15 aligned sequences, including a new mouse sequence, specific roles have been assigned to all 23 invariant residues and additional residues exhibiting functional equivalence. Sequence invariance is dominated by 15 residues that form the active site stereochemistry, supporting a primary biological function of uperoxide dismutation. The β-strands have no sequence insertions and deletions, whereas insertions occur within the loops connecting the β-strands and at both termini. Thus, the β-barrel with only four invariant residues is apparently over determined, but dependent on multiple cooperative side chain interactions. The regions encoded by exon I, a proposed nucleation site for protein folding, and exon III, the Zn loop involved in stability and catalysis, are the major structural subdomains not included in the internal twofold axis of symmetry passing near the catalytic Cu ion. This provides strong confirmatory evidence for gene evolution by duplication and fusion followed by the addition of these two exons. The proposed evolutionary pathway explains the structural versatility ofthe Greek key β-barrel through functional specialization and subdomain insertions in new loop connections, and provides a rationale for the size of the present day enzyme.
    Additional Material: 6 Ill.
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  • 11
    ISSN: 0887-3585
    Keywords: ribonuclease ; active site ; conformational change ; protein-nucleic ; acid interactions ; fluorescence depolarization ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Molecular dynamics simulations were performed on free RNase T1 and the 2′GMP-RNase T1 complex in vacuum and with water in the active site along with crystallographically identified waters, allowing analysis of both active site and overall structural and dynamics changes due to the presence of 2′GMP. Difference in the active site include a closing in the presence of 2′GMP, which is accompanied by a decrease in mobility of active site residues. The functional relevance of the active site fluctuations is discussed. 2′GMP alters the motion of Tyr-45, suggesting a role for that residue in providing a hydrophobic environment for the protein-nucleic acid interactions responsible for the specificity of RNase T1. The presence of 2′GMP causes a structural change of the C-terminus of the α-helix, indicating the transmission of structural changes from the active site through the protein matrix. Overall fluctuations of both the free and 2′GMP enzyme forms are in good agreement with X-ray temperature factors. The motion of Trp-59 is influenced by 2′GMP, indicating difference in enzyme dynamics away from the active site, with the calculated changes following those previously seen in time-resolved fluorescence experiments.
    Additional Material: 10 Ill.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
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  • 14
    ISSN: 0887-3585
    Keywords: protein conformation ; dynamics ; Monte Carlo simulation ; conformational energy ; minimization ; spin glass ; conformational substates ; conformational heterogeneity ; hierarchy in dynamics ; trypsin inhibitor ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: A computer experiment of protein dynamics is carried out, which consists of two steps: (1) A Monte Carlo simulation of thermal fluctuations in the native state of a globular protein, bovinepancreatic trypsin inhibitor; and (2) a simulation of the quick freezingof fluctuating conformations into energy minima by minimization of the energy of a number of conformations sampled in the Monte Carlo simulations sampled in the Monte Carlo simulation. From the analysis of results of the computer experiment is obtained the following picture of protein dynamics:multiple energy minima exist in the native state, and they are distributedin clusters in the conformational space. The dynamics has a hierarchical structure which has at least two levels. In the first level, dynamics is restricted within one of the clusters of minima. In the second, transitions occur among the clusters. Local parts of a protein molecule, side chains and local main chain segments, can take multiple locally stable conformations in the native state. Many minima result from combinations of these multiple local conformations. The hierarchical structure in the dynamics comes from interactions among the local parts. Protein moleculeshave two types of flexibility, each associated with elastic and plastic deformations, respectively.
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  • 15
    ISSN: 0887-3585
    Keywords: protein conformation ; dynamics ; Monte Carlo simulation ; conformational energy ; minimization ; hierarchy in dynamics ; conformational heterogeneity ; flexibility ; trypsin inhibitor ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Distances between centers of gravity of individual residues are compared among the minimum energy conformations derived from the recordof the Monte Carlo simulation of conformational fluctuations in the native state of a globular protein, bovine pancreatic trypsin inhibitor. It is found that local deformations originating from the multiplicity of localconformations cause deformations of the whole structure of the molecule in various ways, which can be classified into two types. Type 1:When a local deformation occurs in a region consisting of a few residues near the surfaceof the molecule, the whole shape of the molecule responds by deforming elastically. The magnitude of this deformation is in the range of thermalfluctuations calculated by the harmonic approximation around a singleminimum. Type 2: We have observed one case belonging to the second type in which local deformations occur cooperatively in an extended region. This regiongoes across the whole molecule and divide the remaining parts into two. Atom packing changes in and around the extended region of local deformations. For this reason deformation in this region is plastic. Relative locationand orientation between the divided two parts change very much. Deformationof the whole shape in this case, associated with the plastic deformationin an extended region, demonstrates that protein molecules have a flexibility beyond the harmonic limit.
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  • 16
    ISSN: 0887-3585
    Keywords: protein architecture ; packing ; evolutionary relationships ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: α/β barrel structures very similar to that first observed in triose phosphate isomerase are now known to occur in 14 enzymes. To understand the origin of this fold, we analyzed in three of these proteins the geometry of the eight-stranded β-sheets and the packing of the residues at the center of the barrel. The Packingin thisregion is seen in its simplest form in glycolate oxidase. It consists of 12 residues arranged in three layers. Each layer contains four side chains. The packing of RubisCO and TIM can be understood in terms of distortions of this simple pattern, caused by residues with small side chains at someof the positions inside the barrel. Two classes of packing are found. In one class, to which RubisCO and TIM belong, the central layer is formed by a residue from the first, third, fifth, and seventh strands; the upper and lower layers are formed by residues fromthe second, fourth, sixth, and eighth strands. In the second class, to which GAO belongs, this is reversed: it is side chains from the even-numbered strands that form the central layer, and side chains from the oddnumbered strands that form the outer layers. Our results suggest that not all proteins with this fold are related by evolution, but that they represent a common favorable solution to the structural problems involved in the creation of a closed β barrel.
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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: α-crystallin ; enthalpy ; entropy of solution ; light scattering ; second virial coefficient ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Light scattering studies were performed on bovine α-crystallin measuring the scattering intensities as a function of scattering angle, concentration, and temperature. The data yielded the molecular weight, radius of gyration, and second virial coefficient of α-crystallin at different temperatures. The second virial coefficient increased with increasing temperature. Both the enthalpy and entropy of solution of α-crystallin are positive. The Flory thetatemperature was found to be 271 K.
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  • 18
    ISSN: 0887-3585
    Keywords: crystallographic refinement ; restrained least-squares refinement ; Konnert-Hendrickson refinement ; phosphodiesterase ; protein structure ; enzyme mechanism ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The structure of a complex of staphylococcal nuclease with Ca2+ and deoxythymidine 3′,5′-biophosphate (pdTp) has been refined by stereochemically restrained leastsquares minimization to a crystallographic R value of 0.161 at Å resolution. The estimated root-mean-square (rms) error in the coordinates in 0.16 Å. The final model comprises 1082 protein atoms, onecalcium ion, the pdTp molecule, and 82 protein atoms, onecalcium ion, the pdTp molecule, and 82 solvent water molecules;it displays an rms deviation from ideality of 0.017 Å for bond distances and 1.8° for bond angles.The mean distance between corresponding α carbons in the refined and unrefined structures is 0.6 Å we observe small but significant differences between the refined and unrefined models in the turn between residues 27 and 30, the loop between residues 44 and 50, the first helix, and the extended strand between residues 112 and 117 which forms part of the active site binding pocket.The details of the calcium liganding and solvent structure in the activesite are clearly shown in the final electron density map. The structure ofthe catalytic site is consistent with mechanism that has been proposed for this enzyme. However, we note that two lysines from a symmetry-related molecule in the crystal lattice may play an important role in determining the geometry of inhibitor binding, and that only one of the two required calcium ions is observed in the crystal structure; thus, caution is advised in extrapolating from the structure of the complex of enzyme and inhibitor to that enzyme and substrate.
    Additional Material: 13 Ill.
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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: protein folding ; crystallographic data base ; structural analysis ; computer program system ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: PKB is a computer program system that combines a data base of three-dimensional protein structures with a series of algorithms for pattern recognition, data analysis, and graphics. By typing relatively simple commands the user may search the data base for instances of a structural motif and analyze in detail the set of individual structures that are found. The application of PKB to the study of protein folding is illustrated in three examples. The first analysis compares the conformations observed for a short sequential motif, sequences similar to the cell-attachment signal Arg-Gly-Asp. The second compares sequences observed for a conformational motif, a 16-residue βαβ unit. The third analysis considers a population of substructures containing ion-pair interaction, examining the relationship offrequency of occurrence to calculated electrostatic energy.
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  • 20
    ISSN: 0887-3585
    Keywords: subunit interactions ; icosahedral capsid ; electrostatic potential ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The role of the electrostatic interactions in the stability of the icosahedral β60 capsid of heavy riboflavin synthase from Bacillus Subtilis has been investigated using an approach based on the theory of Kirkwood and Tanford. The pH dependence of the electrostatic subunit interaction agrees well with experimental data. The electrostatic subunit interaction energy has a pronounced minimum at pH 8.2 for both the ligated and ligand-free capsid. The latter is characterized by a reduction of the magnitude and the pH range of the electrostatic attraction. It is found that only 8 charged groups, which form one cluster and two ion pairs, provide a significant contribution to the capsid stability. The analysis has shown that the aggregation/disaggregation equilibrium seems to be regulated by electrostatic interactions between β-subunits forming dimers, which connect the relatively stable pentamers in the β-60 capsid. The release of the ligand causesareduction of the electrostatic attraction of the dimers, which may induce disaggregation of the capsid. The electrostatic potential field due tothe titratable groups and α-helix macrodipoles has been calculated on the basic of the Coulomb relation. Two different values of the dielectric constant have been used for the protein and the surrounding solvent, respectively. The electrostatic potential shows a radially polardistribution with a positive pole at the inner capsid wall and a negative pole outside the capsid. An interesting feature of the electrostatic field is the formation of the positive potential “channels” that coincide with the channels constituted by the pentameric and trimeric β-subunit aggregates. It is supposed that the electrostatic potential field plays a role in enzyme-substrate recognition.
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  • 21
    ISSN: 0887-3585
    Keywords: antifluorescyl monoclonal antibody ; high-affinity binding site ; effects of MPD on hapten binding ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The crystal structure of a fluorescein-Fab (4-4-20) complex was determined at 2.7 Å resolution by molecularreplacement methods. The starting model was the refined 2.7 Å structure of unliganded Fab from an autoantibody (BV04-01) with specificity for single-stranded DNA. In the 4-4-20 complex fluorescein fits tightly into a relatively deep slot formed by a network of tryptophan and tyrosine side chains. The planar xanthonyl ring of the hapten is accommodated at the bottom of the slot while the phenylcarboxyl group interfaces with solvent. Tyrosine 37 (light chain) and tryptophan 33 (heavy chain) flank the xanthonyl group and tryptophan 101 (light chain) provides the floor of the combining site. Tyrosine 103 (heavy chain) is situated near the phenyl ring of the hapten and tyrosine 102 (heavy chain) forms part of the boundary of the slot. Histidine 31 and arginine 39 of the light chain are located in positions adjacent to the two enolic groups at opposite ends of the xanthonyl ring, and thus account for neutralization of one of two negative charges in the haptenic dianion. Formation of an enol-arginine ion pair in a region of low dielectric constant may account for an incremental increase in affinity of 2-3 orders of magnitude in the 4-4-20 molecules relative to other members of an idiotypic family of monoclonalantifluorescyl antibodies. The phenyl carboxyl group of fluorescein appearsto be hydrogen bonded to the phenolic hydroxyl group of tyrosine 37 of the light chain. A molecule of 2-methyl-2,4-pentanediol (MPD), trapped in the interface of the variable domainsjust below the fluorescein binding site, may be partly responsible for the decrease in affinity for the hapten in MPD.
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  • 22
    ISSN: 0887-3585
    Keywords: computer simulation ; fluctuations in proteins ; secondary structural dynamics ; lysozyme ; protein-substrate complex ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The relation between protein secondary structure and internal motions was examined by using molecular dynamics to calculate positional fluctuations of individual helix, β-sheet, and loop structural elements in free and substrate-bound hen egg-white lysozyme. The time development of the fluctuations revealed a general correspondence between structure and dynamics; the fluctuations of the helices and β-sheets converged within the 101 psec period of the simulation and were lower than average in magnitude, while the fluctuations of theloop regions were not converged and were mostly larger than average in magnitude. Notable exceptions to this pattern occurred in the substrate-bound simulation. A loop region (residues 101-107) of the active site cleft had significantly reduced motion due to interactions withthe substrate. Moreover, part of a loop and a 310 helix (residues of 67-88) not in contact with the substrate showeda marked increase in fluctuations. That these differences in dynamics of free and substrate-bound lysozyme did not result simply from sampling errors was established by an analysis of the variations in the fluctuationsof the two halves of the 101 psec simulation of free lysozyme. Concerted transitions of four to five mainchain φ and ψ angles between dihedral wells were shown to be responsible for large coordinate shifts in the loops. These transitions displaced six or fewer residues and took place eitherabruptly, in 1 psec or less, or with a diffusive character over 5-10 psec. Displacements of rigid secondary structures involved longer timescale motions in bound lysozyme; a 0.5 Å rms change in the position of a helix occurred over the 55 psec simulation period. This helix reorientation within the protein appears to be a response to substrate binding. There was little correlation between the solvent accessible surface areaand the dynamics of the different structural elements.
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  • 23
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: long range truncation ; molecular dynamics ; myoglobin ; truncation effects ; protein electrostatics ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: This paper considers the effects of truncating long-range forces on protein dynamics. Six methods of truncation that we investigate as a function of cutoff criterion of the long-range potentials are (1) a shifted potential; (2) a switching function; (3) simple atom-atom truncation based on distance; (4) simple atom-atom truncation based on a list which is updated periodically (every 25 steps); (5) simple group-group truncation based on distance; and (6) simple group-group truncation based on a list which is updated periodically (every 25 steps). Based on 70 calculations of carboxymyoglobin we show that the method and distance of long range cutoff have a dramatic effect on overall protein behavior. Evaluation of the different methods is based on comparison of a simulation's rms fluctuation about the average coordinates of a no cutoff simulation and from the X-ray structure of the protein. The simulations in which long-range forces are truncated by a shifted potential shows large rms deviations for cutoff criteria less than 14 Å, and reasonable deviations and fluctuations at this cutoff distance or larger. Simulations using a switching function are investigated by varying the range over which electrostatic interactions are switched off. Results using a short switching function that switches off the potential over a short range of distances are poor for all cutoff distances. A switching function over a 5-9 Å range gives reasonable results for a distance-dependent dielectric, but not using a constant dielectric. Both the atom-atom and group-group truncation methods based on distance shows large rms deviations and fluctuation for short cutoff distance, while for cutoff distance of 11 Å or greater, reasonable results are achieved. Although comparison of these to distance-based truncation methods show surprisingly larger rms deviations for the group-group truncation, contrary to simulation studies of aqueous ionic solutions. The results of atom-atom or group-group list-based simulations generally appear to be less stable than the distance-based simulations, and require more frequent velocity scaling or stronger coupling to a heat bath.
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  • 24
    ISSN: 0887-3585
    Keywords: Protein electrostatics ; protein kinases ; effector protein ; calciumbinding protein ; α-helix ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Calmodulin's calculated electrostatic potential surface is asymmetrically distributed about the molecule. Concentrations of uncompensated negative charge are localized near certain α-helices and calcium-binding loops. Further calculations suggest that these charge features of calmodulin can be selectively perturbed by changing clusters of phylogenetically conserved acidic amino acids in helices to lysines. When these cluster charge reversals are actually produced by using cassette-based site-specific mutagenesis of residues 82-84 or 118-120, the resulting proteins differ in their interaction with two distinct calmodulin-dependent protein kinases, myosin light chain kinase and calmodulin-ldependent protein kinase II. Each calmodulin mutant can be purified to apparent chemical homogeneity by an identical purification protocol that is based on conservation of its overall properties, including calcium binding. Although cluster charge reversals result in localized perturbations of the computed negative surface, single amino acid changes would not be expected to alter significantly the distribution of the negative surface because of the relatively high density of uncompensated negative charges in the region around residues 82-84 and 118-120. However, this does not preclude the possibility of single amino acid charge perturbations having a functional effect on the more intimate, catalytically active complex. The electrostatic surface of calmodulin described in this report may be a feature that would be altered only by cluster charge reversal mutations. Overall, the results suggest that the charge properties that are important for the efficient assembly of calmodulin-protein kinase signal transduction complexes in eukaryotic cells.
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  • 25
    ISSN: 0887-3585
    Keywords: retrovirus ; bacterial expression ; high-performance liquid chromatography ; NH2- and COOH-terminal sequence analysis ; kcat ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: A recombinant plasmid encompassing the human immunodeficiency virus type 1 (HIV 1) protease coding sequence and flanking regions (Ala-13 to Gly-185 of the pol open reading frame) has been expressed in two distinct strains of Escherichia coli, AR58 and AR68. In the first strain, AR58, the primary translation product, a 25 kilodalton (kDa) precursor protein, is short-lived and rapidly processes itself to the 11 kDa mature protease in vivo. In the second strain, AR68, the 25 kDa species isonly partially processed, and it, a 13 kDA intermediate, and the mature 11 kDA enzyme accumulate at a ratio of 3:4.5:2.5, respectively. The 11 kDa mature protease from AR58 and the 25 kDa precursor from AR68 have been purified to homogeneity. The yield of 11 kDa enzyme from AR58 is approximately 0.02 mg/g wet weight of E. coli cell pellet. The protease has both the expected NH2- and COOH-terminal sequences. The yield of 25 kDa enzyme from AR68 is approximately 0.1 mg/g wet weight of E. coli cell pellet. In vitro, the 25 kDa precursor enzyme rapidly (t1/2≅ 9 min) processes itself into a species with a mass of ∼13kDa and a species with a mass of ∼11 kDa. Both of these latter species can be separated by RP-HPLC, have the NH2-terminal sequence expected for the mature protease, and are active. The 11 kDa enzyme from AR58 comigrates with the 11 kDa enzyme from AR68 on RP-HPLC and SDS poly acrylamide gel electrophoresis. On extended incubation at 4°C at either neutral or acidic pH all species of the proteinexhibit further autodegradation at defined sequences. The availability of the mature, 11 kDa enzyme and the 25 kDa precursor will allow biochemical and physical studies on this critical viral enzyme.
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  • 26
    ISSN: 0887-3585
    Keywords: protein folding ; simulated annealing ; empirical potentials ; Monte Carlo dynamics ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: The current work describes a simplified representation of protein structure with uses in the simulation of protein folding. The model assumes that a protein can be represented by a freely rotating rigid chain with a single atom approximately the effect of each side chains. Potentials describing the attraction or repulsion between different types of amino acids are determined directly from the distribution of amino acids in the database of known protein structures. The optimization technique of simulated annealinghas been used to dynamically sample the conformations available to this sample model, allowing the protein to evolve from an extended, random coil into a compact globular structure. Many characteristics expected of true proteins, such as the sequence-dependent formation of secondary structure, the partitioning of hydrophobic residues, and specific disulfide, suggestion the model may accurately simulate the folding process.
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  • 27
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0887-3585
    Keywords: Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
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  • 28
    ISSN: 0887-3585
    Keywords: substrate-assisted catalysis ; serine protease ; fusion proteins ; site-directed mutagenesis ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: A combination of protein engineering and substrate optimization was used to create variants of the serine protease, subtilisin BPN′, which efficiently and specifically cleave a designed target sequence in a fusion protein. The broad substrate specificity of wildtype subtilisin BPN′ is greatly restricted by substitution of the catalytic histidine 64 with alanine (H64A) so that certain histidine-containing substrates are preferentially hydrolysed (Carter, P., Wells, J. A. Science 237:394-399, 1987). The catalytic efficiency, (kcat/Km), of this H64A variant was increased almost 20-fold by judicious choice of substrate and by installing three additional mutations which increase the activity of wild-type subtilisin. The most favorable substrate sequence identified was introduced as a linker in a fusion protein between a synthetic IgG binding domain of Staphylococcus aureus protein A and Escherichia coli alkaline phosphatase. The fusion protein (affinity purified on an IgG column) was cleaved by the prototype H64A enzyme and its improved variant, efficiently and exclusively at the target site, to liberate an alkaline phosphatase product of the expected size and N-terminal sequence. Several features of H64A variants of subtilisin make them attractive for site-specific proteolysis of fusion proteins: they have exquisite substrate specificity on the N-terminal side of the cleavage site and yet are broadly specific on the C-terminal side; they can be produced in large quantities and remain highly active even in the presence of detergents, reductants (modest concentrations), protease inhibitors, at high temperatures, or when specifically immobilized on a solid support.
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  • 29
    ISSN: 0887-3585
    Keywords: mutagenesis ; structure-function relationships ; enzymatic catalysis ; Chemistry ; Biochemistry and Biotechnology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Medicine
    Notes: Site saturation mutagenesis has been carried out at Ala-237 in RTEM-1 β-lactamase to assess the role of this site in modulating differences in specificity of β-lactamases for penams vs. cephams as substrates. (An Ala-237 Thr mutation had previously been shown to increase activity on cephems by about 30-80%.1,2) Screening of all 19 possibles mutants on penams and cephems revealed the even more active Ala-237 Asn mutant. Detailed kinnetic analysis showns that this mutant has about four times the activity toward cephalothin and cephalosporin C as the wild-type enzyme. Both mutations reduce the activity toward penams to about 10% that of RETM-1 β-lactamase and lower by about 5°C the tempreature at which the enzyme denatures. Functional properties of the other mutants have also been surveyed. The most intresting aspect of these results is that two quite disparate amino acids, theronine and asparagine, when intorduced for Ala-237, cause such similar changes in enzyme specificity while more similar residues do not alter the catalytic properties of the enzyme to such a significant degree.
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  • 30
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acrylonitrile (AN) has been graft copolymerized onto isotactic polypropylene (IPP) by the preirradiation method using Co60 as the source of gamma rays in the presence of ethanol, n-propanol, isopropanol, n-butanol, tert. butyl alcohol, and n-pentanol. Alcohols vary in their ability to influence grafting of AN onto IPP and the following reactivity order was found: n-pentanol 〉 n-butanol 〉 tert.-butanol 〉 n-propanol 〉 ethanol 〉 isopropanol. An attempt has been made to explain the observed reactivity pattern shown by different alcohols. Thermal behavior of the graft copolymer has been compared with that of IPP and irradiated IPP and it was observed that grafted IPP is thermally more stable than IPP and irradiated IPP.
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  • 31
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rate constants of hydrolysis of the enantiomers of amino acid nitrophenyl esters by catalytic domains composed of cationic surfactant aggregates and dipeptide catalysts or Ndecanoyl-L-histidine have been determined at pH 7.30. The dipeptide catalysts shows large rate enhancement and stereoselectivity in aggregate domains. The surfactant structural effects are examined by investigation of the rate constants and stereoselectivities, and the nature of stereoselective catalysis is discussed.
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  • 32
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The catalytic activity of a series of [Rh L-L chel]X complexes, in which we have varied the unsaturated ligand [L-L = cis, cis-cycloocta 1,5-diene(cod) or 2,5-norbornadiene(nbd) the nitrogen chelating ligand [chel = 2,2′-bipyridine(bipy), 2,2′-dipyridylamine(dipyam), 2,2′-bipyrazine (bipz), 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me2bipy)] and the counter ion [X = PF6, ClO4, BPh4], has been examined in reactions with phyenylacetylene (PA). The catalytic behaviour of the [Rh(cod)Cl2],tmeda (tmeda = N,N,N′,N′tetramethylethylendiamine), [Rh(cod)Cl2],teda] (teda = triethylendiamine), of the dimer [Rh(cod)Cl]2, and the use of NaOH as cocatalyst in different reaction conditions was also examined. The influence of the ligands on the catalytic activity of these RhI complexes is discussed. 1H and 13C NMR spectra have shown that highly stereoregular polyphenylacetilene can be obtained. Conditions for homogeneous doping of PPA, to obtain materials whose conductivity varies over 10-11 magnitude orders, are proposed. The stability of the doped polymers is also discussed.
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  • 33
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interaction of benzil and thioxanthone triplets with triethylamine leads to the production of free radicals that are able to initiate the polymerization of methylmethacrylate and vinyl acetate. The process is mediated by a charge transfer complex. The initiation efficiency is higher in benzene than in acetonitrile, a result explained in terms of competition between back electron transfer, production of free ions and ketyl radical formation. The initiation efficiency decreases when the carbonyl concentration increases, because of the occurrence of a radical molecule reaction such as The α-hydroxy radicals produced in this reaction are unable to initiate the polymerization chain.
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  • 34
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The seeded polymerizations of vinyl acetate, using monodisperse poly(vinyl acetate) latex particles prepared in the absence of emulsifiers with potassium persulfate, have been investigated at 70°C with potassium persulfate as an initiator. New small particles were formed in the system containing a small amount of seed particles, but were not observed in the system containing a large amount of seed particles. The size of the secondary particles increased, and their number decreased, with an increase in the seed particle number. The minimum diameter of PVAc particles, which are stabilized by the sulfate ion groups bound at the end of polymer chains during polymerization, was determined to be 0.12 μm diameter from the limiting total surface area of seed particles which prevented further secondary nucleation. The minimum diameter of the particles increased as the speed of the stirrer increased. The new small particle number calculated using this value agreed well with that formed in the seeded polymerization.
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  • 35
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The donor-acceptor interaction of acrylonitrile (AN) with indene (In) has been investigated by means of ultraviolet spectroscopy and dielectric polarization measurements. The latter method yielded a value of 0.5 for the association constant of the charge-transfer-complex (CTC) and a 4.02 D for its dipole moment. The copolymerization of the two monomers was characterized by a tendency towards alternation when conducted in the bulk. This might be due to the participation of a weak CTC in the process of copolymerization. However, this tendency disappeared in polar solvents such as DMF.
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  • 36
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The role of the cosurfactant (hexadecane) in the miniemulsion copolymerization of 50 : 50 molar ratio vinyl acetate-butyl acrylate monomer mixture is analyzed from an experimental point of view. The main factor responsible for the different kinetic behavior between the miniemulsion and conventional emulsion copolymerization processes was found to be the different particle nucleation mechanism operating in each process. Experimental evidence is presented indicating that in the miniemulsion copolymerization particle nucleation takes place in the preformed stable submicron monomer droplets.
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  • 37
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of 4-substituted-1,2,4-triazoline-3,5-diones(4R-TD's), i.e., MeTD(4-methyl substituted) and PhTD(4-Phenyl substituted) with electron rich aromatic compounds were investigated. N,N-Dimethylaniline undergoes reaction instantaneously with MeTD and PhTD. Electrophilic aromatic substitution occurred at room temperature at the para position without use of any catalyst. N,N,N′,N′-tetramethyl-m-phenylenediamine (TMPDA) undergoes reaction with 2 mol of PhTD and MeTD which lead to the formation of 2:1 adducts in high yields. These compounds were fully characterized by IR, 13C-NMR, 1H-NMR and elemental analysis and were used as model compounds. The reaction of bistriazolinediones with TMPDA was performed in dimethylformamid at room temperature. The reactions are exothermic, fast, and gave novel polymer structures via electrophilic aromatic substitution. Some physical properties and structural characterization of these new polymers have been studied, and will be reported.
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  • 38
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    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The polymerization of metal alkoxides was carried out in organic solvents and light responsive polymers were obtained. As starting materials for polymerization, Ti(OBu)4, Zr(OBu)4, Nb(OEt)5, and Ta(OEt)5 were used. The polymerization yield roughly corresponds to the formation of TiO2, ZrO2, NbO2(OEt), and TaO2(OEt), respectively. The polymers obtained are paramagnetic and all give similar (dark) ESR spectra. When illuminated by visible light, a new ESR absorption is detected in the vicinity of g = 2.00, or 330-340 mT (for 9.450 GHz) region. The formation and decay rates of this ESR absorption was measured. The decay rate constants obtained from the formation curves were larger than those obtained from the decay curves, indicating relatively short lived species are formed during light illumination. The polymers thus prepared have photocatalytic activity and can photolyze 1 : 1 methanol/water system with visible light. Other possible uses of these polymers as paramagnetic or photochromic materials are discussed.
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  • 39
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Isotactic poly(2-hydroxyethyl methacrylate) (PHEMA) is soluble and has the compact random coil structure in water solution. The effect of six ureas (thiourea, urea, methylurea, ethylurea, 1,3-dimethylurea, and tetramethylurea) on the viscometric behavior of aqueous solutions of isotactic PHEMA as a function of urea concentrations at 25°C has been investigated. The compact random coil structure is found to disappear as ureas are added. The result is discussed in terms of water structure breaking and making effect of solutes, and hydrophobic interactions. In addition, experiments were performed with aqueous isotactic PHEMA solutions at four different temperatures. Isotactic PHEMA in pure water has the most compact conformation at 20°C.
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  • 40
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spontaneous copolymerization of cyclic ketene acetal, 2-methylene-4-phenyl-1,3-dioxolane (I) with common electrophilic vinyl monomers, such as methyl α-cyanoacrylate (MCA), acrylonitrile (AN), and methyl methacrylate (MMA) were investigated to further explore zwitterion polymerization method with cyclic ketene acetals. In the reaction of I with MCA and AN, spontaneous copolymerization took place at ambient temperature. The copolymers of I with MCA gave low molecular weight polymers, but copolymers obtained with I and AN were high molecular weight polymers. In the reaction of I and MMA, high molecular weight copolymer was obtained only at temperatures above 80°C. Thus, obtained polymers were not the alternating copolymers and possessed high I content in all the cases. From the above results, macrozwitterionic mechanism was suggested as discussed.
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  • 41
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of a monomer bearing polymerizable acrylic and bicyclic ortho ester groups is described. The monomer 1-ethyl-4-[(2-methacryloxy)ethyl carbamoyloxymethyl]-2,6,7-trioxabicyclo[2.2.2]octane is characterized by elemental analysis, 1H- and 13C-NMR spectra, mass spectra, and heat of polymerization. The free-radical and ionic polymerization of 4 is probed as is its copolymerization with several vinyl monomers.
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  • 42
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The formation mechanism of microporous film which was obtained by casting a methyl ethyl ketone (MEK)/N,N-dimethlformamide (DMF) solution of an electron beam (EB)-reactive urethane-acrylamide prepolymer was discussed. The porosity and crystallinity increased with increasing DMF content in the solvent mixture. This correlation was explained by a progressive slight coagulation and nucleation of the prepolymer due to the difference in volatility between MEK and DMF. As the result, when the DMF content reached 25 vol%, micropores were formed throughout the film. Such a mechanism was supported by the localization of the acrylamide groups at the surface which was induced by the crystallization and by the interaction of amide groups with DMF. The polymerization of acrylamide groups by EB could fix the microporous morphology.
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  • 43
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 2 Ill.
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  • 44
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Copolymerization of styrene (ST) with maleic anhydride (MAH) initiated by thiol compound was investigated under various conditions. The kinetics of copolymerization of ST with MAH initiated by p-toluenethiol (pTT) was studied in dioxane in the temperature range of 25-60°C, and the rates (Rp) of copolymerization and activation energy were determined. Rp was found to depend on [pTT],0.6 ([ST] + [MAH])2.7. The overall energy of activation was 10.8 kcal/mol in the temperature range of 25-60°C. A mechanism involving the formation of a complex between MAH and pTT the decomposition of which yields the initial radical is suggested.
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  • 45
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 46
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: When PVC films containing zinc(II) chloride were heated in the temperature range 65-95°C, broad absorptions developed in the 250-900 nm region with maxima around 610 nm. The development of the color, the rate of which was proportional to the ZnCl2 content, was preceded by an induction period, the length of which depended on the temperature. The activation energy of the process decreased as the ZnCl2 content of the film increased. Color developments in air and in vacuum were compared.
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  • 47
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    Electronic Resource
    Bognor Regis [u.a.] : Wiley-Blackwell
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Copolyamides of nylon-4,6 and nylon-4,T were prepared by a two-step method: (1) a prepolymerization in an autoclave (40 min at 210°C) and (2) a postcondensation in the solid state (4 h, 260°C). On these materials was studied the melting behavior with DSC, the crystalline structure with WAXS, the water absorption, and the mechanical properties with a torsion pendulum. In these copolyamides the order was found to remain high, but the crystalline structures of -4,6 and -4,T were not isomorphous. The torsion moduli increased with -4,T content both at RT and at 140°C.
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  • 48
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and radical polymerization of a novel series of acrylates and methacrylates containing p-cyanophenyl-p-hydroxybenzyl ether, p-methoxyphenyl-p-hydroxybenzyl ether, and 1-(p-cyanophenyl)-2-(p-hydroxyphenyl) ethane groups attached to the polymerizable group through aliphatic spacers containing 11 and 6 methylenic units is described. The resulting polymers were characterized by differential scanning calorimetry and optical polarization microscopy. All polymers exhibited enantiotropic mesomorphism. Low molar mass compounds based on benzyl ether or diphenyl ethane units exhibit only monotropic or virtual transitions. This demonstrates that the “polymer effect” stabilizes mesophases obtained from “mesogenic units” which do not contain rigid interconnecting groups.
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  • 49
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: By reacting poly(methacryloyl chloride) (PMKC) with allyl amine, a multiallyl monomer in PMKC matrix has been obtained. Free-radical polymerization of multiallyl monomer in diluted solutions at a concentration of 12 g/L multiallyl monomer occurs partly along ordered allyl units in the matrix and results in ladder-type branched polymers. The polymers obtained are soluble in alcohols, DMF, DMSO and have unreacted allyl double bonds. The structures of multiallyl monomer and homopolymer have been found on the basis of elemental analysis, IR and 1H-NMR spectra and an examination of the products of hydrolysis. The effect of the reaction of degradative chain transfer on the structure of the polymer obtained has been discussed.
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  • 50
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Polymerizable carbamates were synthesized from 3-vinylphenyl and 2-methacryloyloxyethyl isocyanates and perfume and herbicide alcohols, such as 2-phenethyl alcohol, citronellol, geraniol, 1-menthol, borneol, and 2-(2,4-dichlorophenoxy)- and 2-(2,4,5-trichlorophenoxy)ethyl alcohols. Copolymerization of these carbamate monomers and N-vinyl-2-pyrrolidone with AIBN in dioxane gave respective copolymers. Hydrolyses of both monomers and copolymers, however, required severe acid conditions, although different chemical structures gave different hydrolytic behaviors.
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  • 51
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    Bognor Regis [u.a.] : Wiley-Blackwell
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The wavelength sensitivity of unpigmented 100 mil thick ABS exposed to sunlight and filtered xenon are radiation was determined by the sharp cut filter technique based on three types of photochemical changes: bleaching, yellowing and loss in impact strength. Bleaching of the yellow-colored species formed in the processed material is caused by wavelengths between 380 and 525 nm with maximum color change by the 475-485 nm region. Photochemical yellowing is due to wavelengths between 300 and 380 nm with all wavelengths being almost equally effective. The spectral sensitivity based on change in impact strength shifts from the UV to the visible region as photochemical yellowing progresses. Addition of two stabilizers, a benzotriazole ultraviolet absorber and a hindered amine stabilizer, shifts the wavelength sensitivity based on yellowing to wavelengths shorter than 330 nm, but has no influence on the spectral effects based on impact strength. It is postulated that the rate of yellowing is reduced mainly by the ultraviolet absorber and stabilization against loss in impact strength is due largely to the hindered amine. Differences in rates and spectral response of the three types of photochemical changes indicate that they are due to different initiating mechanisms and thus require different types of stabilization. The significance to stability testing is discussed.
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  • 52
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    Bognor Regis [u.a.] : Wiley-Blackwell
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 3 Ill.
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  • 53
    Electronic Resource
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    Bognor Regis [u.a.] : Wiley-Blackwell
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 54
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 4 Ill.
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  • 55
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 3 Ill.
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  • 56
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New polymers containing benzimidazole in the polymeric backbone were obtained from the reaction of bis(oxazolones) with known and with a novel aromatic tetraamine. The polymers have inherent viscosities in the range of 0.1-0.4 dL/g (in DMF 30°C).
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  • 57
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The copolymerization of vinyl acetate with methyl acrylate in the presence of Et2AlCl, Et1.5AlCl1.5, and Et2AlCl-benzoyl peroxide systems has been investigated. The influence of monomer ratios and organoaluminium compound concentration on the copolymer yield and composition have been determined and discussed. The monomer sequences distribution has been studied by means of 13C-NMR. It was found that organoaluminium compounds in the studied systems catalyze not only the alternating copolymerization, but also the homopropagation of both monomers. An alternating copolymer was obtained in reactions carried out at -78°C, when a large excess of vinyl acetate was used in the monomer feed.
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  • 58
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cyclotrimerization of model aliphatic and cycloaliphatic isocyanates (butyl and cyclohexyl isocyanate) was carried out using an ammonium carboxylate and a salicylaldehyde-potassium complex as catalysts. The kinetics of the cyclotrimerization of butyl isocyanate in both 2-ethoxyethyl acetate and dimethylformamide (DMF) using the 2-ethylhexanoate salt of trimethylaminopropanol-2 was found to be of first order with respect to the isocyanate and also of first order with respect to the catalyst. The reaction rate in DMF was considerably greater than in 2-ethoxyethyl acetate, as could be expected. Employing the salicylaldehyde-potassium catalyst, the cyclotrimerization of butyl isocyanate followed second-order kinetics with respect to the isocyanate and first order with regard to the catalyst. Due to the fact that the cyclotrimerization of cyclohexyl isocyanate was found to be slower than that of butyl isocyanate, the cyclotrimerization of this isocyanate was carried out only in DMF using the 2-ethylhexanoate salt of trimethylaminopropanol-2 as the catalyst. The kinetics of this reaction was found to follow second order with respect to the isocyanate and first order with regard to the catalyst. The products of the reactions were identified by IR, 1H-NMR, and mass spectrometry.
    Additional Material: 11 Ill.
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  • 59
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure of macronet styrene polymer in original, partially sulfonated and fully sulfonated form was studied using inverse steric exclusion chromatography. By alternative measurements of the partially sulfonated sample in water and in tetrahydrofuran, it was possible to characterize separately sulfonated and unsulfonated domains. It was found that the polymer contains 10-15% of highly expandable polymer mass and the rest is formed by much more dense polymer skeleton.
    Additional Material: 3 Tab.
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  • 60
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The surface of low density polyethylene has been grafted with glycidyl acrylate and glycidyl methacrylate by photoinitiation. ESCA measurements on the grafted surface showed a 72% coverage for glycidyl acrylate and 52% for glycidyl methacrylate after 10 min of grafting with UV irradiation. ATR-IR showed a 10 times more extensive grafting for glycidyl acrylate than for glycidyl methacrylate after 10 min of grafting, indicating reaction to deeper layers. Acetone and ethanol were used as solvents: acetone yielded slightly more grafting at the surface. The grafted surfaces were reacted with 2M solutions of aniline and propylamine in ethanol. After 4 h reaction at 60°C, with aniline 52% of the epoxy groups while for propylamine 96% of the groups were consumed, as measured with ATR-IR.
    Additional Material: 8 Ill.
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  • 61
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Graft copolymerization of a bicycloorthoester (BOE) with polymer-supported sulfonium salts was studied. Several polymer-supported sulfonium salts were prepared by the homopolymerizations of p-vinylbenzyl tetramethylenesulfonium hexafluoroantimonate (2) and 4-(p-vinylphenyl)butyl tetramethylenesulfonium hexafluoroantimonate (3), and by the copolymerizations of 2 with some vinyl monomers (n-butyl vinyl ether, styrene, acrylonitrile, and p-styrenesulfonic acid potassium salt). These sulfonium salts could initiate the polymerization of BOE to give grafted polymers. Temperature dependences of the catalytic activity of them were not so dramatic as that of benzyl tetramethylenesulfonium hexafluoroantimonate (1), but the activities of them were higher than that of 1 at temperatures lower than 80°C. The conversion of BOE in the polymerizations with these polymer initiators was ca. 30-70% at 120°C for 7 h. An effect of the comonomer structure on the catalytic activity was observed and styrene was the best comonomer for 2 in terms of the reactivity of the copolymer. The spacer-modified sulfonium salt (homopolymer of 3) was slightly lower than polymer-supported benzyl type sulfonium salt (homopolymer of 2) in the catalytic activity.
    Additional Material: 4 Ill.
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  • 62
    ISSN: 0887-624X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The grafting of functional polymers onto carbon black surface by the reaction of epoxide groups introduced onto the surface was investigated. The introduction of epoxide groups was achieved by the reaction of the phenolic hydroxyl and carboxyl groups on carbon black with chloromethyloxirane in the presence of sodium hydroxide. The amount of epoxide groups introduced onto the channel black Neospectra II was equal to 0.40 meq/g. The epoxide groups were reacted with polypropylene glycol, silicone diol (SDO), silicone diamine (SDA), and polyethyleneimine to give polymer-grafted carbon blacks. For example, the percentage of grafting of SDO (Mn = 1.0 × 103) and SDA (Mn = 3.9 × 103) reached to 23.6 and 67.2%, respectively. The percentage of grafting increased with the molecular weight (Mn) of the polymers, while the number of grafted chain (Gn) decreased. In the case of SDO, the relationship between Mn and Gn was found to be Gn = 4.68 × 10-2 Mn-0.77. The carbon black obtained from the reaction gave stable colloidal dispersions in tetrahydrofuran and the stability of dispersion increased with an increase in the percentage of grafting.
    Additional Material: 7 Ill.
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