Update of the cholesterol force field parameters in CHARMM

A modification of the CHARMM36 lipid force field (C36) for cholesterol, henceforth, called C36c, is reported. A fused ring compound, decalin, was used to model the steroid section of cholesterol. For decalin, C36 inaccurately predicts the heat of vaporization (~10 kJ/mol) and molar volume (~10 cc/mol), but C36c resulted in near perfect comparison with experiment. MD simulations of decalin and heptane at various compositions were run to estimate the enthalpy and volumes of mixing to compare to experiment for this simple model of cholesterol in a chain environment. Superior estimates for these thermodynamic properties were obtained with C36c versus C36. These new parameters were applied to cholesterol, and quantum mechanical calculations were performed to modify the torsional potential of an acyl chain torsion for cholesterol. This model was tested through simulations of DMPC/10% cholesterol, DMPC/30% cholesterol, and DOPC/10% cholesterol. The C36 and C36c results were similar for surface areas per lipid, deuterium order parameters, electron density profiles, and atomic form factors and generally agree well with experiment. However, C36 and C36c produced slightly different cholesterol angle distributions with C36c adopting a more perpendicular orientation with respect to the bilayer plane. The new parameters in the C36c modification should enable more accurate simulations of lipid bilayers with cholesterol, especially for those interested in the free energy of lipid flip/flop or transfer of phospholipids and/or cholesterol.