Improved Salt Bridge Interactions in CHARMM36

Another refinement in the C36m FF concerns improved description of salt bridge interactions involving guanidinium and carboxylate functional groups with a pair-specific non-bonded LJ parameter (NBFIX term in CHARMM) between the guanidinium nitrogen in arginine and the carboxylate oxygen in glutamate, aspartate as well as the C terminus. This salt bridge interaction was found to be too favorable in the CHARMM protein force fields as indicated by the overestimation of the equilibrium association constant of a guanidinium-acetate solution ,^{33 , 34} as well as the underestimation of its osmotic pressure (personal communication, Benoit Roux). The added NBFIX term increases the R_min from the 3.55 Å based on the Lorentz-Berthelot rule to a larger value of 3.637 Å (Shen and Roux, personal communication), which we subsequently showed to improve the agreement with the experimental osmotic pressure of guanidinium acetate solutions (Supplementary Figure 19). We noted that the NBFIX approach employed here differs from Piana et al’s work²⁷ where the CHARMM22 charges of the Arg, Asp and Glu side chains were reduced in magnitude, with both approaches leading to weaker and more realistic salt-bridge interactions. The NBFIX term makes sure only the specific interaction between Arg and Asp/Glu is modified, while the interaction of these residues with other amino acids, water, or ions are kept the same as in the C36 FF. Again, our aim is to improve the C36 FF with minimal changes in the model.