In solid-state 2H NMR of fluid lipid bilayers, quasielastic deformations at MHz frequencies are detected as a square-law dependence of the nuclear spin-lattice (R(1Z)) relaxation rates and order parameters (S(CD)). The signature square-law slope is found to decrease progressively with the mole fraction of cholesterol and with the acyl chain length, due to a stiffening of the membrane. The correspondence to thermal vesicle fluctuations and molecular dynamics simulations implies that a broad distribution of modes is present, ranging from the membrane size down to the molecular dimensions.
Abstract:
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 (Lα) phase have been employed. The 2H spin-lattice relaxation times (T1) and C-2H bond segmental order parameters (SCD) 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 SCD(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. © 1985 American Chemical Society.
PMID: 266718;PMCID: PMC431056;
