DETERMINING ETHANOL DISTRIBUTION IN PHOSPHOLIPID MULTILAYERS WITH MAS-NOESY SPECTRA

The location of an ethanol molecule within a membrane, an issue of con siderable controversy, was investigated directly by NMR with two-dimen sional NOESY. Lipid and ethanol H-1 NMR resonances of multilamellar li posomes were resolved by magic-angle spinning (MAS). We observed stron g proton lipid-ethanol crosspeaks in dispersions of saturated dimyrist oylphosphatidylcholine and monounsaturated stearoyloleoylphosphatidylc holine and in polyunsaturated stearoyldocosahexaenoylphosphatidylcholi ne. Crosspeak intensity has been interpreted in terms of an ethanol di stribution function over the lipid bilayer. Ethanol resides with the h ighest probability at the lipid water interface near the lipid glycero l backbone and upper methylene segments of lipid hydrocarbon chains. C hain unsaturation has only a minor influence on the ethanol distributi on function. In all cases, the ethanol concentration in the bilayer co re is significantly lower. At ambient temperature all. lipid-ethanol c rosspeaks are positive. Crosspeak intensity decreases with increasing water content and increasing temperature most likely because of shorte r correlation times of lipid and ethanol reorientation. This suggests a lifetime for specific lipid-ethanol contacts of about 1 ns. Lipid-et hanol and lipid-lipid crosspeaks reflect the high degree of motional d isorder of lipids and incorporated ethanol in membranes and the rather arbitrary nature of the location of the lipid-water interface.