Influence of cholesterol on dynamics of dimyristoylphosphatidylcholine bilayers as studied by deuterium NMR relaxation

Investigation of the deuterium (H-2) nuclear magnetic resonance (NMR) relax ation rates of lipid bilayers containing cholesterol can yield new insights regarding its role in membrane function and dynamics. Spin-lattice (R-1Z) and quadrupolar order (R-1Q) H-2 NMR relaxation rates were measured at 46.1 and 76.8 MHz for macroscopically oriented bilayers of 1,2-diperdeuteriomyr istoyl-sn-glycero-3-phosphocholine (DMPC-d(54)) containing cholesterol (1/1 molar ratio) in the liquid-ordered phase at 40 degrees C. The data for var ious segmental positions along the DMPC-d(54) acyl chain were simultaneousl y fitted to a composite membrane deformation model, including fast segmenta l motions which preaverage the coupling tensor along the lipid acyl chain, slow molecular reorientations, and small-amplitude collective fluctuations. In contrast to pure DMPC-d(54) in the liquid-crystalline (L-alpha) phase, for the DMPC-d(54): cholesterol (1/1) system a linear square-law functional dependence of the relaxation rates on the order parameter (quadrupolar spl itting) does not appear evident. Moreover, for acyl segments closer to the top of the chain, the angular anisotropy of the H-2 R-1Z and R-1Q relaxatio n rates is more pronounced than toward the chain terminus. The residual (pr eaveraged) coupling tensor has its greatest effective asymmetry parameter n ear the polar groups, decreasing for the groups closest to the end of the c hain. The results suggest that axial rotations of the phospholipid molecule s occur at a somewhat higher rate than in pure bilayers, as a consequence o f the higher ordering and reduction of chain entanglement. On the other han d, the rigid cholesterol molecule appears to undergo somewhat slower axial rotation, possibly due to its noncylindrical shape. Collective motions are found to be less predominant in the case of DMPC-d(54): cholesterol than fo r pure DMPC-d(54), which may indicate an increased dynamical rigidity of li pid bilayers containing cholesterol versus pure lipid systems. (C) 1999 Ame rican Institute of Physics. [S0021-9606(99)00510-3].