CHOLESTEROL AT DIFFERENT BILAYER CONCENTRATIONS CAN PROMOTE OR ANTAGONIZE LATERAL SEGREGATION OF PHOSPHOLIPIDS OF DIFFERING ACYL-CHAIN LENGTH

Fourier-transform infrared-spectroscopic and fluorescence measurements have been combined to examine the effect of cholesterol on the interm ixing of short-chain dilauroyl phosphatidylcholine (DLPC) and its brom o-substituted derivative (12BrPC) with longer-chain (C16- or C18-) pho sphatidylcholines (PCs) in hydrated lipid bilayers. infrared spectrosc opy of mixtures combining protonated DLPC or 12BrPC with chain-perdeut erated dipalmitoyl PC reveals that cholesterol at lower concentrations in the bilayer modifies the resolved thermal melting profiles for bot h phospholipid components and, at high bilayer concentrations, produce s a convergence of the thermal transitions for the two PC species. Flu orescence-quenching measurements using a short-chain fluorescent PC (1 -dodecanoyl-2-[8-[N-indolyl]octanoyl] PC) in ternary mixtures combinin g 12BrPC, dipalmitoyl or distearoyl PC, and cholesterol confirm that v ery high cholesterol levels (50 mol %) abolish the lateral segregation of the PC components at 25 degrees C, a temperature where the phospho lipids extensively phase-separate in the absence of sterol. By contras t, under these same conditions cholesterol at lower concentrations in the bilayer is found to enhance the tendency of the PC components to e xhibit lateral segregation. We show that these seemingly contradictory effects of cholesterol can be readily explained in the light of a ter nary phase diagram that is fully consistent with our current understan ding of the nature of cholesterol-phospholipid interactions in binary mixtures.