Angular dependence of 2H NMR relaxation rates in lipid bilayers

Abstract

The angular dependences of ²H Zeeman (T_1Z) and quadrupolar (T_1Q) relaxation times have been used to assess the relative contribution of order director fluctuations (ODF) to the spin‐lattice relaxation rate of saturated phospholipid acyl chains. In order to circumvent orientational averaging due to the effects of rapid lateral diffusion [Brown and Davis, Chem. Phys. Lett. 7 9, 431 (1981)], relaxation measurements were performed on macroscopically oriented multibilayers of 1,2‐dimyristoyl‐sn‐glycero‐3‐phosphocholine (DMPC) and 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC). ²H T⁻¹_1Z values of both DMPC abd DPPC in the liquid crystalline (L_α) phase are relatively insensitive to the orientation of the bilayer normals with respect to the magnetic field. From a comparison of the observed angular dependences of both spin‐lattice relaxation rates with those predicted if ODF were the only relaxation mechanism, we conclude that although ODF cannot be ruled out as a relaxation mechanism, they do not provide the dominant relaxation pathway in the MHz regime, as previously suggested [Brown, J. Chem. Phys. 7 7, 1576 (1982)]. While the frequency dispersion of relaxation rates may in principle provide a critical test for models of molecular dynamics in these systems, the angular dependence of ²H spin‐lattice relaxation rates can provide a useful alternative test, particularly when the ²H relaxation time data cover less than a decade of Larmor frequencies.