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ABSTRACT
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 (POPC and DPPC) 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 timescales of ~200 ns, whereas for methanol that timescale is considerably longer, at least of the order of microseconds. A closer examination exposes a number of effects due to ethanol. Hydrogen-bonding analysis reveals that a large fraction of ethanols is involved in hydrogen bonds with lipids. This in turn is intimately coupled to the ordering of hydrocarbon chains: we find that binding to an ethanol decreases the order of the chains. We have also determined the dependence of lipid-chain ordering on ethanol concentration and found that to be nonmonotonous. Overall, we find good agreement with NMR and micropipette studies.
INTRODUCTION
It is well known that even small changes in the composition of cell membranes can strongly affect the functioning of intrinsic membrane proteins, such as ion and water channels, which regulate the chemical and physical balance in cells (1,2). Such changes may occur due to the introduction of short-chain alcohols, or other anesthetics, at membrane surfaces. Although anesthetics are being used every single day in hospitals around the world, the molecular level mechanisms of general anesthesia remain elusive (see e.g., (3-5)). The same applies to the effect of alcohols on biological systems. Klemm (6) provides a good review of the topic.
Another aspect to the effect of alcohols appears in a more applied context. In the process of producing alcoholic beverages, wine in particular, yeasts like Saccharomyces cerevisiae have to sustain high ethanol concentrations without losing their viability. However, in ~10% of all wine fermentations, the industry encounters so-called stuck fermentations (7,8). There is no satisfactory understanding of this effect. Some models propose that an effect very similar to general anesthesia is responsible for rendering the yeast cells dormant (9). It has been suggested that high alcohol concentrations change the membrane structure and force transmembrane proteins into unfavorable conformations. In these...