Solvent accessible surface area (ASA) of simulated phospholipid membranes

The membrane-solvent interface has been investigated through calculations of the solvent accessible surface area (ASA) for simulated membranes of DPPC and POPE. For DPPC at 52 degrees C we found an ASA of 126+/-8 A(2) per lipid molecule, equivalent to twice the projected lateral area. The ASA was dominated by the contribution from the quaternary ammonium ion which accounts for about two thirds of the total value. The remaining ASA is primarily the phosphate moiety (22+/-1 A(2)), and this contribution almost entirely represents the two 'out-of-chain' oxygen atoms. Interestingly, even the most exposed parts of the PC head-group show average ASAs of less than half of its maximal or 'fully hydrated' value. The average ASA of a simulated POPE membrane was 96+/-7 A(2) per lipid. The smaller value than for DPPC reflects much lower ASA of the ammonium ion, which is partially compensated by increased exposure of the ethylene and phosphate moieties. The ASA of the polar moieties of (PO(4), NH(3) and COO) constitutes 65% of the total accessible area for POPE, making this interface more polar than that of DPPC. It is suggested that ASA information can be valuable in attempts to rationalize experimental investigations, particularly molecular interpretations of thermodynamic information.