BIOPHYSICS: It's Crowded Inside

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Science  28 Nov 2003:
Vol. 302, Issue 5650, pp. 1480b
DOI: 10.1126/science.302.5650.1480b

The lipid bilayer structure of biological membranes is well defined conceptually—hydrophilic and charged head groups on the outside and nonpolar hydrocarbon tails on the inside—but not as well described at the atomic scale. Marrink and Mark have carried out computer simulations of vesicle formation and vesicle fusion in a system holding about a thousand dipalmitoyl phosphatidylcholine (DPPC) molecules and hundreds of thousands of water molecules. In general terms, the lipids behave pretty much as expected, coalescing first into micelles and then aggregating into a vesicle, with a higher initial lipid concentration yielding two micelles and a hemifused vesicle. Replacing a quarter of the DPPC with dipalmitoyl phosphatidylethanolamine produced vesicles with an excess of the smaller ethanolamine head group in the inner monolayer, consistent with its known preference for negative curvature domains. Close examination of the nonpolar core revealed that some of the hydrocarbon tails of the outer lipids extended across the midway point, whereas others folded back on themselves and occupied the extra space available in the head group region of the outer layer. Although these vesicles are small and thus more curved than most cellular membranes, these packing modes may nevertheless be relevant to the lipid gymnastics during membrane fusion. — GJC

J. Am. Chem. Soc. 10.1021/ja0352092 (2003).

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