Angular dependence of 2H NMR relaxation rates in lipid bilayers

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Abstract
The angular dependences of 2H Zeeman (T1Z) and quadrupolar (T1Q) 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. 79, 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−11Z values of both DMPC abd DPPC in the liquid crystalline (Lα) 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. 77, 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.