MOLECULAR-DYNAMICS IN LIPID BILAYERS - ANISOTROPIC DIFFUSION IN AN ODD RESTORING POTENTIAL

Recent H-2 nuclear magnetic resonance spin relaxation studies have que stioned the influence of restoring potential parity on the description of lipid or molecular reorientational dynamics. In biomembranes the p olar head groups of lipid and sterol constituents are expected to asso ciate with the aqueous interface; therefore, realistic descriptions of molecular reorientation in bilayer systems should use an odd restorin g potential. The multiexponential correlation functions and related sp ectral density functions for small-step anisotropic diffusion in a pse udo-restoring potential of the form U(beta) = - lambda cos beta are ev aluated as a function of molecular ordering [P2(cos beta)]. From analy sis of these results the single exponential approximation used in prev ious investigations is found to overestimate the decay rate at lower o rder, but is reliable for [P2(cos beta)] > 0.6. The exception to this trend is the decay constant for the G11(OMEGA; t) correlation function , which is not accurately portrayed by the first-order approximations. A second-order single exponential approximation is presented, and is shown to be equivalent to the weighted sum of the multidecay constants . In general, the potential parity is found to have only minor effects on the spin relaxation rates obtained to describe molecular reorienta tion in lipid bilayers.