Effects of phospholipid unsaturation on the membrane/water interface: A molecular simulation study

Molecular dynamics (MD) simulations of fully hydrated bilayers in the liqui d-crystalline state made of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) or 1-palmitoyl-2-elaidoyl-phosphatidylcholin (PEPC) were carried out to in vestigate the effect of the incorporation of a double bond in the phosphati dylcholine (PC) beta -chain (cis or trans) on the membrane/water interface. The bilayers reached thermal equilibrium after 3 and 1 ns of MD simulation s, respectively, and productive runs were carried out for 3 ns for each bil ayer. As reference systems, the 1,2-dimyristoyl-phosphatidylcholine (DMPC) bilayer (M. Pasenkiewicz-Gierula, Y. Takaoka, H. Miyagawa, K. Kitamura, and A. Kusumi, 1999, Biophys. J. 76:1228-1240) and DMPC-cholesterol (Chol) bil ayer containing 22 mot % Chol (M. Pasenkiewicz-Gierula, T. Rog, K. Kitamura , A. and Kusumi, 2000, Biophys. J. 78:1376-1389) were used. The study shows that at the interface of POPC, PEPC, and DMPC-Chol bilayers, average numbe rs of PC-water and PC-PC interactions are similar and, respectively, greate r and smaller than in the DMPC bilayer. The average area/PC in mono-unsatur ated bilayers is similar to4 Angstrom (2) larger than in the DMPC bilayer; nevertheless, a strong correlation was found between a single molecular are a (SMA) of a PC and the number of interactions this PC makes; i.e., PCs (ei ther saturated or unsaturated) with the same SMA form similar numbers of in termolecular links. The numbers and corresponding SMAs are distributed abou t averages pertinent to each bilayer. No significant difference between cis and trans bonds was found.