Under the influence of alcohol: The effect of ethanol and methanol on lipid bilayers

Extensive microscopic molecular dynamics simulations have been performed to study the effects of short-chain alcohols, methanol and ethanol, on two different fully hydrated lipid bilayer systems in the fluid phase at 323 K. It is found that ethanol has a stronger effect on the structural properties of the membranes. In particular, the bilayers become more fluid and permeable: Ethanol molecules are able to penetrate through the membrane in typical time scales of about 200 ns whereas for methanol that time scale is considerably longer, at least of the order of microseconds. We find good agreement with NMR and micropipette studies. We have also measured partitioning coefficients and the rate of crossing events for alcohols, i.e., typical time scale it takes for a molecule to cross the lipid bilayer and to move from one leaflet to the other. For structural properties, two-dimensional centre of mass radial-distribution functions indicate the possibility for quasi long-range order for ethanol-ethanol correlations in contrast to liquid-like behaviour for all other combinations.