INCORPORATION OF SURFACE-TENSION INTO MOLECULAR-DYNAMICS SIMULATION OF AN INTERFACE - A FLUID-PHASE LIPID BILAYER-MEMBRANE

In this paper we report on the molecular dynamics simulation of a flui d phase hydrated dimyristoylphosphatidylcholine bilayer. The initial c onfiguration of the lipid was the x-ray crystal structure. A distincti ve feature of this simulation is that, upon heating the system, the fl uid phase emerged from parameters, initial conditions, and boundary co nditions determined independently of the collective properties of the fluid phase. The initial conditions did not include chain disorder cha racteristic of the fluid phase. The partial charges on the lipids were determined by ab initio self-consistent field calculations and requir ed no adjustment to produce a fluid phase, The boundary conditions wer e constant pressure and temperature, Thus the membrane was not explici tly required to assume an area/phospholipid molecule thought to be cha racteristic of the fluid phase, as is the case in constant volume simu lations, Normal to the membrane plane, the pressure was 1 atmosphere, corresponding to the normal laboratory situation. Parallel to the memb rane plane a negative pressure of -100 atmospheres was applied, derive d from the measured surface tension of a monolayer at an air-water int erface. The measured features of the computed membrane are generally i n close agreement with experiment, Our results confirm the concept tha t, for appropriately matched temperature and surface pressure, a monol ayer is a close approximation to one-half of a bilayer, Our results su ggest that the surface area per phospholipid molecule for fluid phosph atidylcholine bilayer membranes is smaller than has generally been ass umed in computational studies at constant volume. Our results confirm that the basis of the measured dipole potential is primarily water ori entations and also suggest the presence of potential barriers for the movement of positive charges across the water-headgroup interfacial re gion of the phospholipid.