Dependence of Deuterium Spin-Lattice Relaxation Rates of Multilamellar Phospholipid Dispersions on Orientational Order

²H NMR studies of a homologous series of 1,2-diacyl-sn-glycero-3-phosphocholines with perdeuterated saturated chains, ranging in length from C12:0 to C16:0, have been performed with use of quadrupolar echo techniques at a resonance frequency of 55.4 MHz. Randomly oriented, multilamellar dispersions containing 50 wt % H₂O in the liquid crystalline (L_α) phase have been employed. The ²H spin-lattice relaxation times (T₁) and C-²H bond segmental order parameters (S_CD) of each of the resolved quadrupolar splittings have been obtained from the powder-type spectra, corresponding to a random distribution of orientations, as well as from the 0 °C oriented subspectra obtained by numerical deconvolution (de-Pakeing). Evidence that the spin-lattice relaxation rate profiles as a function of chain position T₁^-1(i) are related to the corresponding order profiles S_CD(i) by a square-law functional dependence has been obtained, indicative of a contribution from relatively slow fluctuations in the local bilayer ordering to the relaxation. The results suggest that two broad classes of motions influence the ²H spin-lattice relaxation rates of lipid bilayers: rapid local motions, most likely due to bond rotational isomerizations and long-axis rotational diffusion of the lipid chains, as well as slower director fluctuations as found in other liquid crystalline mesophases.