Spin anisotropic rotation

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. 

Fig. 3. Magic angle spinning and its effect on anisotropic interactions. The dashed vector, denoting one particular dipolar interaction, or some other measure of molecular orientation, is shown on the right in terms of the geometry of sample spinning. By rotating the sample rapidly about an appropriate axis, the anisotropy or broadening experienced by the nucleus can be reduced to zero.

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... 

Membrane fluidity is determined by following anisotropic rotation of fluorescent or spin probes. Liquid-crystalline (or fluid) to gel thermotropic phase transition of lipids (Figure 1) (cf. Section 3.1.1 l)in liposomes or intact biomembranes can be followed by Fourier transform infrared (FTIR) spectroscopy or differential scanning calorimetry (DSC). 

Anisotropic interactions, orientational dependence, 26f Anisotropic rotation, polyformal spin relaxation, local motion, 70,78t Anisotropy, chemical shift (CSA), solid sample NMR, 22 Antiphase components, undesirable INEPT properties, 106 Areas, integrated, quantitative NMR studies, 1371... 

Carbon-13 NMR has been used to study anisotropic rotational motion in liquids, as have combinations of techniques. Gillen and Griffiths (1972) have obtained the two reorientational relaxation times for benzene (a symmetric top) by combining reorientation relaxation times obtained from Raman band shapes and deuterium spin-lattice NMR relaxation times. The most extensive series of measurements probing anisotropic molecular reorientations have been made by Pecora and co-workers (Alms et al., (1973a, b)), who combined Carbon-13 spin lattice relaxation times with those obtained from depolarized Rayleigh spectra. 

Proton, deuteron and carbon spin relaxation measurements of liquid crystals have provided detailed information about the molecular motions of such anisotropic liquids (anisotropic rotation and translation diffusion of individual molecules), and about a peculiar feature of liquid crystalline phases, namely collective molecular reorientations or order fluctuations. Spin relaxation in liquid crystalline mesophases has challenged NMR groups since the early 1970s, shortly after the publication of theoretical predictions that order fluctuations of the director (OFD, OF), i.e. thermal excitations of the long-range orientational molecular alignment (director), may play an important unusual role in nuclear spin relaxation of ordered liquids. Unique to these materials, which are composed of rod-like or disc-like (i.e. strongly anisotropic molecules), it was predicted that such thermal fluctuations of the director should, at the frequencies of these fluctuation modes, produce rather peculiar Ti(p) dispersion profiles. For example in the case of uniaxial nematic... 

Freed et al. [3] evaluated the relaxation rate of polymer chains by the analysis of the anisotropic spectra of nitroxide spin labels. The main triplet spectrum was due to hyperfine coupling caused by the nitrogen nucleus. It narrowed with an increase in mobility of the radicals because of motional averaging of the anisotropic hyperfine interaction. The rotational correlation time of nitroxide spin labels can be estimated using the procedure of Freed et al. by taking into account the anisotropic rotational motion. The ESR line width, AHmsi of the spectrum can be expressed as follows ... 

The spin probes 5- and 16-DSA are localized in the different regions of membrane lipids 5-DSA in surface lipids at depth of 8A° and hydrophobic nitroxyl radical 16-DSA penetrates in lipid bilayer to the depth more than 20A° [14, 16], A motion of 16-DSA can be characterized by rotation correlation time (x) by which a microviscosity value is estimated and anisotropic rotation of 5-DSA by order parameter (S) [15,16],... 

I here are numerous problems associated with the determination of nucleic acid structures in solution using NMR methods. These include spin diffusion, anisotropic rotation, conformational averaging, low density of experimental constraints and lack of long-range information. 

Correlation time, rotational (tr)> 427,487,489 Coupling, nuclear spin-spin anisotropic, 400 calculations of, 49-60... 

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

Fig. 1 Illustration of the sensitivity of X-band EPR line shapes to the motions and orientational constraints of the nitroxide spin probe, (a) Isotropic rotation with different correlation times (b) anisotropic rotations along z and y principle magnetic axes (c) orientation of magnetic field (Z laboratory axis) relative to the magnetic axes of the nitroxide. Euler angles defining relative orientation of laboratory and magnetic frames are indicated.

Because of the rotation of the N—N bond, X-500 is considerably more flexible than the polyamides discussed above. A higher polymer volume fraction is required for an anisotropic phase to appear. In solution, the X-500 polymer is not anisotropic at rest but becomes so when sheared. The characteristic viscosity anomaly which occurs at the onset of Hquid crystal formation appears only at higher shear rates for X-500. The critical volume fraction ( ) shifts to lower polymer concentrations under conditions of greater shear (32). The mechanical orientation that is necessary for Hquid crystal formation must occur during the spinning process which enhances the alignment of the macromolecules. 

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