Publication costs assisted by the Petroleum Research Fund A detailed analysis of line shapes and relaxation is made for the perdeuterated 2,2,6,6-tetramethyl-4-piperidone AT-oxide (PD-Tempone) nitroxide radical covering the whole range from fast motional narrowing (tr ~10-12 sec) to the rigid limit (tr ~10-6 sec) in several deuterated solvents. The slow tumbling resultsshow particularly good agreement with the slow tumbling theory of Freed, et ai, for a reorientational model of moderate jumps (ca. 50°r ms), and essentially isotropic reorientation. It is shown that while such a model may not be readily distinguished, purely from its slow tumbling spectral predictions, from a free diffusion model including inertial effects, the latter model is incompatible with most other considerations. However, a simple analysis explicitly including the fluctuating (or random) torques indicates that more fundamental analysis of the dynamics may explain the slow tumbling results without necessarily involving substantial jumps. The spectral analysis is considerably enhanced by the increased resolution obtained from the use of the perdeuterated spin probe and deuterated solvents. Careful analysis of results at X-band and 35 GHz has shown that the nonsecular spectral densities exhibit significant deviations from a Debyelike spectral density yielding results very similar to those recently reported for the peroxylaminedisulfonate (PADS) radical. Related observations are discussed of apparent non-Debye-like spectral densities in the incipient slow tumbling spectra. These anomalies are also found to be amenable to a unified explanation in terms of the fluctuating torques. The analysis of much of the results in terms of a simple model yields an rms value for the fluctuating torques of ca, V6kT and tm ~tr where tm is the relaxation time of the torques. The high-temperature electron-spin flip processes are consistent with a Hubbard-type spinrotational mechanism, but the low-temperature results may be due to spin-rotational relaxation from intramolecular motions. The simple analysis of spin-rotational relaxation in terms of relaxation of the fluctuating torques is found to yield equivalent predictions to conventional treatments when the estimated values of rms torque and tm are used.

A stable smectic phospholipid bilayer phase aligned with the director parallel to the magnetic field can be generated by the addition of certain trivalent paramagnetic lanthanide ions to a bicellar solution of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC) in water. Suitable lanthanide ions are those with positive anisotropy of their magnetic susceptibility, namely Eu3+, Er3+, Tm3+, and Yb3+. For samples doped with Tm3+, this phase extends over a wide range of Tm3+ concentrations (6-40 mM) and temperatures (35-90 degrees C) and appears to undergo a transition from a fluid nematic discotic to a fluid, but highly ordered, smectic phase at a temperature that depends on the thulium concentration. As a membrane mimetic, these new, positively ordered phospholipid phases have high potential for structural studies using a variety of techniques such as magnetic resonance (EMR and NMR), small-angle x-ray and neutron diffraction, as well as optical and infrared spectroscopy.

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Measurements have been made on the ESR linewidths of vandyl acetylacetonate (VOAA) in a number of nonhydrogen-bonded solvents. The X-band spectra were taken over a wide range of temperatures and at pressures ranging up to 4.5 kbar. The high pressure measurements were carried out in a high pressure bomb containing a slow-wave helix, instead of a resonant cavity, and a pair of 100 kHz modulation coils; the sample was placed in a Teflon holder which also surrounded the helix and could be sealed after the sample was deoxygenated. The reorientation correlation time τθ, is written as τθ=43π r03κ(η/kT), where η is the coefficient of shear viscosity, T the absolute temperature, r0 the hydrodynamic radius determined by translational diffusion experiments, and κ the anisotropic interaction parameter. κ is proportional to the ratio of the mean square intermolecular torques to the mean square intermolecular force where the interaction is between VOAA and solvent molecules. The values of κ determined from these experiments are independent of pressure, temperature, and density. However, κ varies from solvent to solvent and an analysis of the data indicates, but not unambiguously, that κ decreases with solvent size and it increases with solvent dipole moment. The results are compatible with a quasirotational diffusion model, with the assumption that the autocorrelation functions for intermolecular forces and torques have the same time dependence, and with the assumption that the dependence of torques on intermolecular separation is nearly equivalent to that of forces. The single adjustable parameter, κ, is sufficient to explain all the spectra at low (η/T) for a given solvent regardless of pressure and temperature, but at high values of (η/T) unexplained discrepancies between theory and experiment occur. A discussion of anisotropic diffusion is also given; however, the anisotropic contributions are probably negligible for VOAA.