Phase Behavior of a Lipid Bilayer System Studied by a Replica-Exchange Molecular Dynamics Simulation

A replica-exchange molecular dynamics (REMD) simulation of a lipid bilayer system has been performed in order to study the sol--gel phase transitions. The REMD method enhances conformational sampling efficiency, and one can study the system in a wide temperature range at once. A coarse-grained model MARTINI was used. The results show sudden changes in enthalpy, bilayer thickness, and area of bilayer around 296 K. A peak in heat capacity was also observed around this temperature. These results suggest sol--gel phase transitions. We also investigated temperature dependences of composite energy terms. According to potential of mean force maps and tilt angle distributions, the bilayer is found to have two states in the gel phase. One is a tilted gel state and the other is an un-tilted gel state. A previous work with MARTINI force field reported only the un-titled gel state. Another previous work with MARTINI force field reported titled gel state only with externally applied tension. This suggests that enhanced conformational sampling is important even with a coarse-grained model which has smoother energy landscapes and reach longer time scales than atomistic models. The lipid bilayer in the un-tilted gel state is a little thicker than that in the tilted gel state, while the area of the lipid bilayer is quite similar. Tilt angle distributions are also analyzed, and the tilt angle of the tilted gel phase is around 30 \circ .