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  • 1
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0091-7419
    Keywords: Life Sciences ; Molecular Cell Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: According to current models of muscle contraction (Huxley, H. E., Science 164: 1356-1366 [19691]), motion of flexible myosin crossbridges is essential t o the contractile cycle. Using a spin-label analog of iodoacetamide bound to the subfragment # 1 (S1) region of myosin, we have obtained rotational correlation times (τ2) for this region of the molecule with the ultimate goal of making quantitative measurements of the motion of the crossbridges under conditions comparable to those in living, contracting muscle. We used the newly developed technique of saturation transfer electron paramagnetic resonance spectroscopy (Hyde, J.S., and Thomas, D.D., Ann. N.Y. Acad. Sci. 222:680-692 [1973]), which is uniquely sensitive t o rotational motion in the range of 10-7-10-3 sec. Our results indicate that the spin label is rigidly bound to S1 (τ2 for isolated S1 is 2 × 10-7 sec) and that the motion of the label reflects the motion of the S1 region of myosin. The value of τ2 for the S1 segment of myosin is less than twice that for isolated S1, while the molecular weights differ by a factor of 4, indicating flexibility of myosin in agreement with the conclusions of Mendelson et aL (Biochemistry 12:2250-2255 [1973]). Adding F-actin increases τ2 in either myosin or isolated S1 by a factor of nearly 103, indicating rigid immobilization of S1 by actin. Formation of myosin filaments (at an ionic strength of 0.15 or less) increases τ2 by a factor of 10-30, depending o n the ionic strength, indicating a decrease of the rotational mobility of S1 in these aggregates. The remaining motion is at least a factor of 10 faster than would be expected for the filament itself, suggesting motion of the S1 region independent of the filament backbone but slower than in a single molecule. F-actin has a strong immobilizing effect on labeled S l in myosin filaments (in 0.137 M KC1), but the immobilization is less complete than that observed when F-actin is added t o labeled myosin monomers (in 0.5 M KC1). A spin-label analog of maleimide, attached to the SH-2 thiol groups of S1, is immobilized to a much lesser extent by F-actin than is the label on SH-1 groups. The maleimide label also was attached directly to F-actin and was sufficiently immobilized to suggest rigid binding to actin.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Irradiation of a single homogeneous electron paramagnetic resonance (EPR) transition by two microwave sources separated by Δf, where Δf(very-much-less-than) the linewidth, results in the production of intermodulation sidebands at f0±(k+1/2)Δf, where f0 is the mean of the two irradiating frequencies and k is an integer ≥1. These sidebands can be seen directly with a microwave spectrum analyzer. Any one of the sidebands can be observed by suitable phase sensitive detection and displayed as a function of the polarizing magnetic field H0, resulting in a multiquantum EPR spectrum. The widths of the EPR lines decrease as k increases, resulting in improved resolution. The two k=1 transitions can be superimposed, resulting in a signal of double intensity. At saturating microwave power, this double-intensity line is two times less intense than an ordinary EPR line. No magnetic field modulation needs to be used when detecting multiquantum transitions, and pure absorption or pure dispersion signals are obtained with good baseline stability. Considerable attention is paid to the technical problem of irradiating the sample with just two microwave frequencies. In the apparatus described, spurious instrumental sidebands are reduced by 60 dB or more.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The g-tensor components associated with tetrachloro-, tetrabromo-, and tetraiodo-benzosemiquinones in frozen solutions of dimethylsulfoxide and ethanol-D at 135 K have been measured by ESR spectroscopy at Q band (35 GHz). Spin-orbit interactions on the halogens make a significant contribution to the g tensor; components in the plane of the aromatic ring, (gx and gy) are enhanced, whereas the component perpendicular to the ring (gz) is decreased. Related solvent effects have been observed in the spectrum of tetraiodobenzosemiquinone; in going from dimethylsulfoxide to ethanol gx and gy were found to increase, whereas gz decreased. It is argued that the positive contribution to gx and gy arises from the participation of halogen pz orbitals in the molecular π orbitals and that the negative contribution to gz in part reflects spin density on the halogens due to σ–π interaction induced configurational admixture in the wave function. Possible participation of halogen d orbitals is discussed. The dominant part of the solvent dependence of the g tensor components is suggested to result from changes in the energy separation between the orbital of the unpaired electron and lone-pair orbitals on the halogens.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 1896-1904 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A bimodal loop-gap resonator for use in electron paramagnetic resonance (EPR) spectroscopy at S band is described. It consists of two identical one-loop–one-gap resonators in coaxial juxtaposition. In one mode, the currents in the two loops are parallel and in the other antiparallel. By introducing additional capacitors between the loops, the frequencies of the two modes can be made to coincide. Details are given concerning variable coupling to each mode, tuning of the resonant frequency of one mode to that of the other, and adjustment of the isolation between modes. An equivalent circuit is given and network analysis carried out both experimentally and theoretically. EPR applications are described including (a) probing of the field distributions with DPPH, (b) continuous wave (cw) EPR with a spin-label line sample, (c) cw electron–electron double resonance (ELDOR), (d) modulation of saturation, and (e) saturation-recovery (SR) EPR. Bloch induction experiments can be performed when the sample extends half way through the structure, but microwave signals induced by Mx and My components of magnetization cancel when it extends completely through. This latter situation is particularly favorable for SR, modulation of saturation, and ELDOR experiments, which depend on observing Mz indirectly using a second weak observing microwave source. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe the design, construction, and characterization of an X-band multiquantum electron paramagnetic resonance (MQEPR) microwave bridge, with MQ electron–electron double resonance and MQ electron–nuclear double resonance capabilities. The main feature of the bridge is the use of double-balanced mixers as double sideband modulators to generate multiple irradiation fields with variable frequency separation. The microwave source is a low phase noise Gunn diode oscillator, the frequency of which is translated by a nominal 300±Δf MHz. This approach, called double sideband/fixed filter (DSB/FF), allows the use of fixed bandpass microwave filters to reduce incident spurious products to at least −70 dBc. Each frequency is amplified separately to avoid system-generated intermodulation (IM) sidebands in the incident irradiation. As a result, the dominant source of system intermodulation is the nonlinearity in the receiver system, consisting of a low noise amplifier (LNA) and a double-balanced signal mixer. A detailed analysis of receiver-generated IM products is presented. The use of the loop-gap resonator with a high resonator efficiency parameter, Λ, and low Q is essential to achieve a balance between microwave power and system IM sidebands. It is shown that even at maximum incident power, the levels of these sidebands can be reduced to 51 dB below the MQEPR response by switching out the LNA. This permits the extension of MQEPR applications into systems where high power is required. The operation modes of the bridge are briefly described. Alternative bridge designs are considered and compared with the DSB/FF design. It is found that the DSB/FF approach gives the best overall performance with greater flexibility and compatibility with multiple operation modes. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A Varian Q-band E-110 microwave bridge for electron paramagnetic resonance (EPR) spectroscopy has been modified by addition of a low-phase noise Gunn diode oscillator of our own design, a low-noise GaAs field-effect transistor microwave signal amplifier, and a balanced mixer requiring high input power (10 mW) at the local oscillator port. The oscillator has previously been found to have −129 dBc/Hz phase noise, 22 dB lower than for the original klystron. Noise measurements indicate that the microwave amplifier and mixer reduce the overall receiver noise figure by 24.6 dB, a very significant improvement. It is shown that reduction of both phase noise and receiver noise are required in order to achieve full improvement in signal-to-noise ratio over the full range of available microwave power. Spectra of 1.6×10−6 M 15N-perdeutero TEMPONE (1-oxyl-2,2,6,6-tetramethyl-4-piperidone) and of 10−6 M Mn2+ are shown in order to demonstrate sensitivity.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 58 (1987), S. 1879-1886 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A continuous and stopped flow EPR spectrometer based on a new loop gap resonator operating at X band is described. The important features of the instrument are: (1) very small amounts of material are consumed in both stopped and continuous flow modes of operation, (2) dead times on the order of 4 ms for stopped flow and 1 ms for continuous flow are realized with a very narrow age distribution in the sample compartment, and (3) the dead times and, hence, sample age are highly reproducible and independent of viscosity due to the use of a positive displacement syringe ram. The performance of the instrument is evaluated using the decay of ascorbate radical generated by Ce(iv) oxidation and reduction of nitroxides by ascorbic acid.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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