Rotational diffusion anisotropic

Another example comes from the work of Johnson, et a/.18 These workers studied spin labels dissolved in lipid bilayer dispersions of dipalmitoylphos-phatidylcholine and cholesterol (9 1 by weight) in the hope that anisotropic rotational diffusion of the spin label would mimic the motion of the bilayer components. In addition to 5-DS, which is sensitive to rotational motion about the NO bond, they used the steroidal nitroxide 8, which tends to rotate about an axis perpendicular to the N-O bond. ESR measurements were carried out at both 9 and 35 GHz and at temperatures ranging from 30 to 30 °C. Rather different results were obtained with the two spin labels, largely as a result of the different axes of rotation. Because the rotation rates were very slow, ESR spectra appeared as powder patterns rather than isotropic spectra and special methods were needed to extract the motional data. 

When determining QSC values of dimeric lithium phenolates in solution, the value has to be corrected for anisotropic rotational diffusion as the complexes have an oblate, non-spherical shape . The Li2A2S2 complex of 2,6-di-f-butylphenolate has a Li QSC value of ca 280 kHz in a number of different solvents. The 2,6-dimethylphenolate system is present as an Li2A2S4 complex in THF and apparently as an L12A2S2 complex in DEE. These complexes have QSC values of 170 and 270 kHz in the two solvents, respectively. 

The interest in alkaloids of the nicotine group continues. Convenient syntheses of 7V-CD3-labelled nicotine and nicotine analogues have been described.38 The 13C n.m.r. spin-lattice relaxation times of nicotine have been analysed in terms of anisotropic rotational diffusion constants. The results agree best with a dihedral angle of H(2 )-C(2 )-C(3)-C(2) of ca 0°.39 Dithiodinicotyrine , obtained... 

Zhang J, Jonas J. NMR Study of the geometric confinement effects of the anisotropic rotational diffusion of acetonitrile-dj. J Phys Chem 1993 97 8812-8815. 

Schurr, J. M., Babcock, H. R, Fujimoto, B. S. A test of the model free formulas effects of anisotropic rotational diffusion and dimerization. J. Magn. Reson. Ser. B 1994,105, 211-224. 

The three bond jump segmental motion description can also be combined with a description of restricted anisotropic rotational diffusion (13-14). In this case, the composite spectral density equation is... 

The new parameters for restricted anisotropic rotational diffusion are the angular amplitude over which rotation diffusion occurs, Z, and the rotational diffusion constant for restricted anisotropic rotational diffusion, D- r. 

This description of segmental motion can also be combined with restricted anisotropic rotational diffusion... 

TIME DOMAIN STUDIES OF ANISOTROPIC ROTATIONAL DIFFUSION... 

Residual dipolar coupling (RDC) partial average of a dipolar interaction between two nuclei in a molecule with anisotropic rotational diffusion resulting from its partial orientation in a magnetic field... 

Price et a/. have reported C Ti and NOE measurements on narrow fractions of poly(y-benzyl-L-glutamate) in the helix-supporting solvent dimethyl formamide. The strong molecular weight dependence of Tx and was satisfactorily interpreted on the basis of anisotropic rotational diffusion of a rigid helix. 

The ESR studies of local segmental dynamics of polymers benefited from the progress achieved in theoretical description of the effects of dynamics on line shape of nitroxide ESR spectra and in resulting software. The Schneider-Freed set of programs made possible the analysis of slow-motional spectra of nitroxides subjected to anisotropic rotational diffusion with rotation symmetry axis oriented quite arbitrarily with respect to the nitroxide axis system. The simulation of one such ESR spectrum with computers available in the 1980s required 20 min of computer time and the best fits to experimental spectra were found by visual comparison simulated spectra with experimental ones. 

The similarity exhibited here between the multi-variable and the memory hierarchy formulations of the orientational problem is obviously not a general result in fact, it derives from the choice of a variable and its time derivative in the multi-variable theory. Other variations on these themes can readily be devised for example, one could couple Q, and 4 (orthogonal ized to Qj, however) (8) or one could combine higher-order memory functions with a multi-variable theory or one could couple translational and rotational variables (9), The enormous flexibility of the G,L,E, means that the intuition of the user will play a particularly significant role in determining the success of the outcome. To illustrate this point, we now briefly recapitulate how the 6,L,E, applies to a quite different orientational problem namely, anisotropic rotational diffusion (10). 

For an anisotropic rotational diffusion process I(u)) will be the superposition of three Lorentzicins of the type equation (25) with three correlation times... 

Chuang TJ, Eisenthal KB (1972) Theory of fluorescence depolarizatirai by anisotropic rotational diffusion. J Chem Phys 57(12) 5094-5097. doi 10.1063/1.1678194... 

Burghardt TP (1983) Fluorescence depolarization by anisotropic rotational diffusion of a luminophore and its carrier molecule. J Chem Phys 78(10) 5913-5919. doi 10.1063/1. 444596... 

Third, the interpretation of polarization measurements normally requires assumptions about the nature of the motion leading to emission depolarization. Isotropic motion is easy to interpret. In some instances one can get a very detailed description of anisotropic rotational diffusion by combining nmr, light scattering and fluorescence depolarization measurements. 

CDCI3. A thorough study of the factors which can influence NOE measurements (anisotropic rotational diffusion, conformational averaging, and nuclear spin diffusion) was also performed. 

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