Anisotropic 2H NMR spin-lattice relaxation in L alpha-phase cerebroside bilayers

A series of 2H NMR inversion recovery experiments in the L alpha phase of the cerebroside N-palmitoylgalactosylsphingosine (NPGS) have been performed. In these liquid crystalline lipid bilayers we have observed substantial anisotropy in the spin-lattice relaxation of the CD2 groups in the acyl chains. The form and magnitude of the anisotropy varies with position in the chain, being positive in the upper region, decreasing to zero at the 4-position, and reversing sign at the lower chain positions. It is also shown that addition of cholesterol to the bilayer results in profound changes in the anisotropy. These observations are accounted for by a simple motional model of discrete hops among nine sites, which result from the coupling of two modes of motion--long-axis rotational diffusion and gauche-trans isomerization. This model is employed in quantitative simulations of the spectral line shapes and permits determination of site populations and motional rates. These results, plus preliminary results in sphingomyelin and lecithin bilayers, illustrate the utility of T1 anisotropy measurements as a probe of dynamics in L alpha-phase bilayers.