The SPICA Coarse-Grained Force Field for Proteins and Peptides

A coarse-grained (CG) model for peptides and proteins was developed as an extension of the SPICA (Surface Property fItting Coarse grAined) force field (FF). The model was designed to examine membrane proteins that are fully compatible with the lipid membranes of the SPICA FF. A preliminary version of this protein model was created using thermodynamic properties, including the surface tension and density in the SPICA (formerly called SDK) FF. In this study, we improved the CG protein model to facilitate molecular dynamics (MD) simulation with a reproduction of multiple properties from both experiments and all-atom (AA) simulations. The side chain analogs reproduced the transfer free energy profiles across the lipid membrane and demonstrated reasonable dimerization free energies in water compared to those from AA-MD. A series of peptides/proteins adsorbed or penetrated into the membrane simulated by the CG-MD correctly predicted the penetration depths and tilt angles of peripheral and transmembrane peptides/proteins comparable to those in the orientation of protein in membrane (OPM) database. In addition, the dimerization free energies of several transmembrane helices within a lipid bilayer were comparable to those from experimental estimation. Application studies on a series of membrane protein assemblies, scramblases, and poliovirus capsids demonstrated a good performance of the SPICA FF.