MOLECULAR-DYNAMICS SIMULATION OF A PHOSPHOLIPID MEMBRANE

We present the results of molecular dynamics (MD) simulations of a pho spholipid membrane in water, including full atomic detail. The goal of the simulations was twofold: first we wanted to set up a simulation s ystem which is able to reproduce experimental results and can serve as a model membrane in future simulations. This goal being reached it is then further possible to gain insight in to those properties that are experimentally more difficult to access. The system studied is dipalm itoylphosphatidylcholine/water, consisting of 5408 atoms. Using origin al force field parameters the membrane turned out to approach a gel-li ke state. With slight changes of the parameters, the system adopted a liquid-crystalline state. Separate 80 ps runs were performed on both t he gel and liquid-crystalline systems. Comparison of MD results with r eliable experimental data (bilayer repeat distance, surface area per l ipid, tail order parameters, atom distributions) showed that our simul ations, especially the one in the liquid-crystalline phase, can serve as a realistic model for a phospholipid membrane. Further analysis of the trajectories revealed valuable information on various properties. In the liquid-crystalline phase, the interface turns out to be quite d iffuse, with water molecules penetrating into the bilayer to the posit ion of the carbonyl groups. The 10-90% width of the interface turns ou t to be 1;3 nm and the width of the hydrocarbon interior 3.0 nm. The h eadgroup dipoles are oriented at a small angle with respect to the bil ayer plane. The resulting charge distribution is almost completely can celled by the water molecules. The electron density distribution shows a large dip in the middle of the membrane. In this part the tails are more flexible. The mean life time between dihedral transitions is 20 ps. The average number of gauche angles per tail is 3.5. The occurrenc e of kinks is not a significant feature.