Optical birefringence of multilamellar gel phase of cholesterol/phosphatidylcholine mixtures

Abstract

A new method is proposed to determine optical birefringence of lipid membranes from measurements of dynamic light scattering and polarized light transmitted through a polydisperse system of lipid vesicles. By means of this method, effects of cholesterol on membrane molecular order were studied for the tilted gel phase (the L β′ phase) of dipalmitoylphosphatidylcholine with cholesterol concentrations ranging from 0–20 mol%. The birefringence increased gradually with increasing cholesterol content at concentrations from 6–15 mol%, indicating an increase in longitudinal order of molecular orientation in the multilamellar gel phase. This result provides evidence that the untilted gel phase appears even at low cholesterol concentrations. However, the gradual increase in the birefringence suggests the coexistence of the tilted and untilted phases in this concentration range of cholesterol, which does not accord with a recently proposed phase diagram of the binary mixtures [T.P.W. McMullen and R.N. McElhaney, Biochim. Biophys. Acta, 1234 (1995) 90–98].