Aa. Nevzorov et al., LIPID BILAYER DYNAMICS FROM SIMULTANEOUS ANALYSIS OF ORIENTATION AND FREQUENCY-DEPENDENCE OF DEUTERIUM SPIN-LATTICE AND QUADRUPOLAR ORDER RELAXATION, Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 58(2), 1998, pp. 2259-2281
Simultaneous analysis of the deuterium (H-2) NMR spin relaxation rates
of lipid bilayers as a function of both frequency and sample orientat
ion may be decisive in evaluating different models for the dynamics of
membranes. Angular dependent H-2 spin-lattice (R-1Z) and quadrupolar
order (R-1Q) H-2 relaxation rates have been measured at 46.1 and 76.8
MHz for macroscopic ally oriented bilayers of 1, diperdeuteriomyristoy
l-sn-glycero-3-phosphocholine (DMPC-d(54)), with perdeuterated acyl ch
ains, in the liquid-crystalline (L-alpha) state. The data have been si
multaneously fitted to various dynamical models, together with frequen
cy dependent H-2 R-1Z data for vesicles of specifically H-2-labeled DM
PC. The same mechanism for the nuclear spin relaxation in lipids has b
een assumed for both oriented bilayers and vesicles, except for the pr
esence of orientational averaging and a possible contribution from ves
icle tumbling in the latter case. A noncollective model describing ind
ividual molecular reorientations in the presence of a potential of mea
n torque is able to adequately account for the orientation dependence;
however, the quality of the fits to the frequency dispersion is less
satisfactory. By contrast, a three-dimensional director fluctuation mo
del accounts for the frequency dispersion for DMPC vesicles, but does
not fit the orientation dependence of the R-1Z and R-1Q relaxation dat
a. Higher-order director fluctuations have also been included, which d
o not significantly improve the quality of the fits to the collective
model. Therefore, a composite membrane model is proposed including bot
h noncollective molecular motions and director fluctuations. The model
adequately describes both the frequency and orientation dependent dat
a along the entire acyl chain simultaneously, which suggests that both
dynamical processes can be detected by analyzing the H-2 NMR relaxati
on rates in the MHz range. Quantitative information about the bilayer
dynamics including lipid reorientation rates, degree of molecular orde
ring, relative contributions from collective and noncollective motions
, and director-frame spectral densities of motion has been obtained. T
he results suggest the bilayer dynamics in the MHz regime reflect mole
cular reorientations that are superimposed onto nematiclike deformatio
ns of the membrane interior in the liquid crystalline state.