The correlation between molecular orientational order and reorientational dynamics of probe molecules in lipid multibilayers

Abstract

The behaviour of probe molecules in lipid systems can be characterized in terms of molecular order parameters and rates of reorientational motion. The correlation between these parameters has been investigated in oriented lipid multibilayer systems on changing the degree of unsaturation of the chains and the cholesterol content.

Angle-resolved fluorescence depolarization experiments were carried out as a function of two angles: (1) the angle of incidence of the excitation beam and (2) the angle between the directions of incidence and observation. The e.s.r. spectra from nitroxide spin labels were simulated numerically with a general stochastic Liouville equation formalism. The probe molecules were assumed to undergo stochastic small-step reorientational diffusion subject to an orienting potential.

Both techniques indicate that the introduction of unsaturation into the lipid chains lowers both the order parameters and the diffusion coefficients of the probe molecules. The incorporation of cholesterol reverses this trend. These findings are at odds with current ideas about the fluidity of membrane systems.