A Coarse-Grained\nSPICA Makeover for Solvated and Bare\nSodium and Chloride Ions

Aqueous ionic solutions are pivotal in various scientific\ndomains\ndue to their natural prevalence and vital roles in biological and\nchemical processes. Molecular dynamics has emerged as an effective\nmethodology for studying the dynamic behavior of these systems. While\nall-atomistic models have made significant strides in accurately representing\nand simulating these ions, the challenge persists in achieving precise\nmodels for coarse-grained (CG) simulations. Our study introduces two\noptimized models for sodium and chloride ions within the nonpolarizable\nsurface property fitting coarse-grained force field (SPICA-FF) framework.\nThe two models represent solvated ions, such as the original FF model,\nand unsolvated or bare ions. The nonbonded Lennard-Jones interactions\nwere reparameterized to faithfully reproduce bulk properties, including\ndensity and surface tension, in sodium chloride solutions at varying\nconcentrations. Notably, these optimized models replicate experimental\nsurface tensions at high ionic strengths, a property not well-captured\nby the ions of the original model in the SPICA-FF. The optimized unsolvated\nmodel also proved successful in reproducing experimental osmotic pressure.\nAdditionally, the newly reparameterized ion models capture hydrophobic\ninteractions within sodium chloride solutions and show qualitative\nagreement when modeling structural changes in phospholipid bilayers,\naligning with experimental observations. For aqueous solutions, these\noptimized models promise a more precise representation of the ion\nbehavior.