ANGULAR-DEPENDENCE OF DEUTERIUM SPIN-LATTICE RELAXATION RATES OF MACROSCOPICALLY ORIENTED DILAUROYLPHOSPHATIDYLCHOLINE IN THE LIQUID-CRYSTALLINE STATE

Deuterium (H-2) NMR relaxation plays a major role in the study of lipi d reorientational dynamics, with the angular dependence of the relaxat ion rates providing a novel and critical test of proposed motional mod els. Spin-lattice relaxation rates (R(1Z)) were measured for macroscop ically oriented bilayers of 2-diperdeuteriolauroyl-sn-glycero-3-phosph ocholine (DLPC-d(46)) in the liquid-crystalline (L(alpha)) phase. The results for different positions along the chain (index i) were depende nt on the angle theta between the macroscopic bilayer normal and the s tatic external magnetic field, and allowed the anisotropy of R(1Z)((i) ) to be determined for nine resolved quadrupolar resonances. Angular-d ependent relaxation data were evaluated using simple models of anisotr opic rotational diffusion within an odd or even potential of mean torq ue as a framework for describing (i) segmental reorientations of the c hains, or alternatively (ii) noncollective molecular motions within th e bilayer. Moreover, (iii) a simple quasi-hydrodynamic formulation inv olving collective fluctuations of a local director axis was considered (continuum model). For segmental motions the static electric field gr adient (EFG) tenser due to the electronic structure of the C-H-2 bond is averaged by local reorientations of the acyl chains, e.g., due to f rans-gauche rotational isomerizations and/or torsional oscillations. T he second and third formulations assume the static EFG tensor is preav eraged by local motions, yielding a residual EFG tensor which is furth er modulated by order fluctuations due to relatively slow motions of l arger amplitude; separation of time scales is implicit. The latter tre atments differ in that the molecular model allows for variation in bot h the principal values and principal axes of the residual EFG tensor, and includes the possibility of a nonzero effective asymmetry paramete r eta(eff). By contrast, the collective model considers an axially sym metric residual EFG tenser (eta(eff)=0), in which the relatively slow motions are described in terms of continuum fluctuations of a local di rector axis within the small-angle approximation. Each of the three mo dels can account for the observed angular anisotropy of the R(1Z)((i)) relaxation rates along the chains to a greater or lesser degree of su ccess, depending on the number of adjustable parameters. The collectiv e formulation has the fewest parameters and may be an oversimplificati on for description of the relaxation anisotropy. In addition, for each bilayer orientation, profiles of the relaxation rates R(1Z)((i)) and order parameters /S-CD((i))/ as a function of acyl position exhibited a square-law functional dependence within experimental error. This obs ervation points to an influence on the relaxation arising from relativ ely slow fluctuations in the order gradient set up along the chains by faster internal motions, viz. order fluctuations due to noncollective molecular motions or collective excitations of the bilayer. Finally, the rather small contribution from local internal motions suggests tha t the microviscosity of the bilayer interior corresponds to essentiall y liquid hydrocarbon. These new results illustrate the utility of R(1Z )((i)) angular anisotropies of phospholipids having perdeuterated acyl chains in experimental and theoretical investigations of molecular dy namics in liquid-crystalline bilayers. The implications of the finding s in relation to previous biophysical studies of membranes are discuss ed.