Publication costs assisted by the Petroleum Research Fund A detailed study of anisotropic ordering, line shapes, and relaxation is reported for the perdeuterated 2,2,6,6-tetramethyl-4-piperidone N-oxide (PD-Tempone) nitroxide radical in several liquid crystal solvents. The line width results are analyzed in terms of the Polnaszek, Bruno, and Freed (PBF) theory appropriately modified for anisotropic ordering both in the motional narrowing and slow tumbling region. The motional narrowing results are usually consistent with isotropic rotational diffusion, but under a weak (asymmetric) ordering potential, (Doo) 88 -0.1, and activation energies characteristic of the twist viscous properties of the liquid crystal. Anomalous line shape behavior in the incipient slow tumbling region is observed, which is not explained by the extrapolation of the appropriate parameters from the motional narrowing region. This anomaly is discussed in terms of anisotropic viscosity and director fluctuations. The latter is predicted to be of negligible importance for the weakly ordered spin probe, as well as qualitatively of the wrong behavior. Anisotropic viscosity, while apparently able to "explain" the anomaly, leads to physically untenable conclusions. The anomaly is then discussed in terms of slowly fluctuating intermolecular torques, leading to a frequency-dependent diffusion coefficient. While this latter may in part offer an explanation (when one distinguishes between torque components parallel and perpendicular to the director), the implied slowness of the fluctuating torques suggests, from general theory, a new model based upon a local solvent structure around the spin probe which may persist over longer periods than the reorientation time of the spin probe. A simple model calculation of the effects on the ESR relaxation is given. This limiting model is also appropriate for highly structured isotropic liquids. It is shown that such a model could have the same formal spectral effects as anisotropic rotational diffusion, and it would yield non-Debye-like spectral densities of the type that could potentially "explain" the observed incipient slow tumbling anomaly. More general theoretical approaches for the analysis of these effects are briefly discussed.

A careful study is described of the ESR lineshapes for the peroxylamine disulfonate (PADS) radical dissolved in 85% glycerol solution and in frozen water and D2O. In the frozen media, spectra characteristic of rotational correlation times τR ranging from 1.0 × 10−11sec to > 10−6sec are obtained, while the range in glycerol is from 1 × 10−10sec to >10−6sec. The very rapid rotational motion in frozen water is taken to imply that PADS is rotating in a clathrate cage. The activation energies in ice and 85% glycerol are 14.7 ± 0.1 and 11.3 ± 0.1 kcal/mole, respectively, (from motional-narrowing data). The value for ice is very similar to that obtained for other rate processes in ice. The lineshapes for τR ≲ 10−9sec are analyzed in terms of the familiar spin-relaxation theories valid in the motionally narrowed region. These results are well fitted by the model of axially symmetric rotational diffusion with the symmetry axis in the plane of the N, O, and S atoms and parallel to a line passing through the two S atoms. Diffusion about this axis is found to be 2.9 ± 1 and 4.7 ± 1 times faster for frozen water and glycerol solvent, respectively than about the other axes over a wide range of values of average τR̄. It was possible to obtain these results, because accurate measurements of the g and A tensors for PADS in these media could be made from the well-resolved rigid spectra at X band and 35 GHz; the intrinsic widths in D2O are only about 1.5 G. The spectra in the slow-motional region τ R¯ >10−9sec were simulated utilizing the slow tumbling formulation of Freed, Bruno, and Polnaszek appropriately generalized to include completely asymmetric g and A tensors. The simulated spectra are found, in general, to be in quite good agreement with experimental observations. The agreement is clearly improved by introducing axially symmetric rotational diffusion, as found for the motional-narrowing region, into the simulations. Spectra are simulated for Brownian rotational diffusion as well as for simplified models of free diffusion, which includes inertial effects, and for diffusion by jumps of substantial angle. Improved agreement with experiment is found with some of these latter models. What appears to be a surprisingly small nonsecular linewidth contribution in the motional-narrowing region is briefly discussed in terms of these models.

It is shown how the analysis of Freed et al. for ESR lineshapes in the slow tumbling region may be generalized to include anisotropic liquids. Particular emphasis is given to the case of nitroxide radicals in liquid crystal environments with cylindrically symmetric restoring potentials U (13). It is found that when IU(I3)I:5 kT, spectral appearances are qualitatively (but not quantitatively) similar to those for isotropic liquids. In particular, the spectra are sensitive to the model of reorientation. They are also predicted to be a very sensitive indicator of effects of anisotropic viscosity. The analysis given for the motional narrowing region yields analytic expressions for the needed spectral densities, where previously only numerical results had been obtained. The analytic expressions are valid when IU(I3)I.::;kT. Analytic solutions to the rotational diffusion equation appropriate for I U (13)1> kT are given and it is outlined how they may be applied to magnetic resonance.

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Deuterium (2H) nuclear magnetic resonance (NMR) quadrupole splittings and relaxation times have been measured for a variety of specifically deuterated lipids intercalated in lamellar-multibilayer dispersions and single-bilayer vesicles of egg lecithin and lecithin-cholesterol mixtures. The deduced order parameters and relaxation times vary with position of deuteration, acyl chain length, unsaturation, and temperature. The order parameters and spinlattice relaxation times T1 indicate rapid intramolecular motions of restricted amplitude in both the choline head group and hydrocarbon chains. The ordering profile for the acyl chains is similar to that predicted by statistical-mechanical theory. The order parameters yield estimates of the bilayer thickness and linear coefficient of expansion in close agreement with the x-ray determinations. A comparison of the deuterium and electron spin resonance spinprobe order parameters demonstrates the perturbation of the bilayer by the bulky nitroxide probe. The transverse relaxation time T2 for single-bilayer vesicles is quantitatively accounted for by a simple modification of classical relaxation theory which takes into account the modulation of the static quadrupole interaction by rapid local molecular motions and the modulation of the residual quadrupole interaction by the slower overall tumbling of the vesicle. It is unambiguously demonstrated that molecular motion and order in single-bilayer vesicles are very similar to those in lamellar multibilayers. Significant differences occur only for a few segments near the terminal methyl groups of the acyl chains, where the order parameters for vesicles are 10-30% smaller than those found for lamellae. The incorporation of cholesterol in lecithin bilayers is shown to increase the degree of orientational order in vesicles and lamellae, and to increase the hydrodynamic radius of vesicles. Thus, single-bilayer vesicles and multilamellar dispersions of phospholipids are equally useful models for biological membranes. They yield equivalent information about the internal organization and mobility of lipid bilayers, when the spectral manifestations of overall vesicle motion are correctly taken into account.

Solvent induced vibrational relaxation in diatomics. I. Derivation of a local relaxation rateThe field dependence of solvent proton nuclear magnetic relaxation is shown to provide a test of whether the relaxation is dominated by translational or rotational motions. When a paramagnetic center is present with a sufficiently long electron relaxation time, the dominance of translational contributions to the correlation time for the electron-nucleus interaction permits characterization of the translational diffusion of the solvent in local environment of the paramagnetic center. For bovine serum albumin covalently labeled with nitroxide radicals the approach yields a value of (3 ± 1) X 10-6 cm 2 s -I for water within the first 10 A ofthe protein surface. The method is general for the measurement of localized solvent diffusion coefficients.4038

In this work the nonaxial ordering and spin relaxation of PD-Tempone spin probe in several liquid crystalline solvents exhibiting smectic A and B phases were studied utilizing methods previously employed by Polnaszek and Freed for the study of nematic liquid crystals. The results reported here for the isotropic and nematic phases are generally in accord with those obtained previously. An analysis of isotropic hyperfine shifts, changes in the ordering tensor, and anomalous relaxation behavior in the smectic phases suggest a model in which the PD-tempone probes are partially expelled from the dipolar region of the liquid crystalline molecules toward the more flexible hydrocarbon end chains as a result of the packing of the smetic layers, and concomitantly the probes increasingly experience a slowly relaxing local structure (SRLS) in a cavity-like location. Differences in observations from different types of smectic liquid crystals are interpreted in terms of their differing structures based on X-ray studies. It is shown that the angular dependent line widths in the smectic phases are significantly affected by the size and shape of the sample. These inhomogeneous broadening effects are discussed in detail in terms of static distortions of the smectic layering induced by wall effects and magnetic-field induced torques, and are in reasonable agreement with predictions of a simple model. The residual homogeneous widths are discussed in terms of combined models of anisotropic rotation and anisotropic viscosity as well as associated SRLS models. For the former case, the problem of defining the rotational diffusion tensor, which must be time dependent in any axis system, is discussed in some detail.