pSPICA: A Coarse-Grained Force Field for Lipid Membranes Based on a Polar Water Model.

We present a coarse-grained (CG) force field (FF), pSPICA, for lipid membranes that incorporates a CG polar water model, which guarantees a reasonable dielectric response for water. Using a relatively simple functional form for the interaction, the CG parameters were systematically optimized to reproduce surface/interfacial tension, density, solvation or transfer free energy, as well as distribution functions obtained from all-atom molecular dynamics trajectory generated with the CHARMM FF, following the scheme used in the SPICA FF. Lipid membranes simulated using the present CG FF demonstrate reasonable membrane area and thickness, elasticity, and line tension, which ensure that the simulated lipid membranes exhibit proper mesoscopic morphology. The major advantages of the pSPICA FF with a polar water model were its ability to simulate membrane electroporation and its superior performance in the morphological characterization of charged lipid aggregates. We also demonstrated that the pSPICA can better describe the membrane permeation of hydrophilic segments involving a water string formation.