Spatial and energetic-entropic decomposition of surface tension in lipid bilayers from molecular dynamics simulations

The spatial and groupwise distribution of surface tension in a fully hydrat ed 256 lipid dipalmitoylphosphatidylcholine (DPPC) bilayer is determined fr om a 5 ns molecular dynamics simulation by resolving the normal and lateral pressures in space through the introduction of a local virial. The resulti ng surface tension is separated into contributions from different types of interactions and pairwise terms between lipid headgroups, chains and water. By additionally performing a series of five simulations at constant areas ranging from 0.605 to 0.665 nm(2) (each of 6 ns length), it is possible to independently resolve the energetic contributions to surface tension from t he area dependence of the interaction energies. This also enables us to cal culate the remaining entropic part of the tension and the thermal expansivi ty. Together with the total lateral pressures this yields a full decomposit ion of surface tension into energetic and entropic contributions from elect rostatics, Lennard-Jones and bonded interactions between lipid chains, head groups and water molecules. The resulting total surface tension in the bila yer is found to be a sum of very large terms of opposing signs, explaining the sensitivity of simulation surface tension to details in force fields. H eadgroup and headgroup-water interactions are identified as attractive on a verage while the chain region wants to expand the bilayer. Both effects are dominated by entropic contributions but there are also substantial energet ic terms in the hydrophobic core. The net lateral pressure is small and rel atively smooth compared to the individual components, in agreement with exp erimental observations of DPPC lipids forming stable bilayers. (C) 2000 Ame rican Institute of Physics. [S0021-9606(00)50333-X].