Combined Monte Carlo and molecular dynamics simulation of fully hydrated dioleyl and palmitoyl-oleyl phosphatidylcholine lipid bilayers

We have applied a new equilibration procedure for the atomic level simulati on of a hydrated lipid bilayer to hydrated bilayers of dioleyl-phosphatidyl choline (DOPC) and palmitoyl-oleyl phosphatidylcholine (POPC). The procedur e consists of alternating molecular dynamics trajectory calculations in a c onstant surface tension and temperature ensemble with configurational bias Monte Carlo moves to different regions of the configuration space of the bi layer in a constant volume and temperature ensemble. The procedure is appli ed to bilayers of 128 molecules of POPC with 4628 water molecules, and 128 molecules of DOPC with 4825 water molecules. Progress toward equilibration is almost three times as fast in central processing unit (CPU) time compare d with a purely molecular dynamics (MD) simulation. Equilibration is comple te, as judged by the lack of energy drift in 200-ps runs of continuous MD, After the equilibrium state was reached, as determined by agreement between the simulation volume per lipid molecule with experiment, continuous MD wa s run in an ensemble in which the lateral area was restrained to fluctuate about a mean value and a pressure of 1 atm applied normal to the bilayer su rface. Three separate continuous MD runs, 200 ps in duration each, separate d by 10,000 CBMC steps, were carried out for each system. Properties of the systems were calculated and averaged over the three separate runs, Results of the simulations are presented and compared with experimental data and w ith other recent simulations of POPC and DOPC. Analysis of the hydration en vironment in the headgroups supports a mechanism by which unsaturation cont ributes to reduced transition temperatures. In this view, the relatively ho rizontal orientation of the unsaturated bond increases the area per lipid, resulting in increased water penetration between the headgroups, As a resul t the headgroup-headgroup interactions are attenuated and shielded, and thi s contributes to the lowered transition temperature.