Lipopolysaccharide Simulations are Sensitive to Phosphate Charge and Ion Parameterization

The high proportion of lipopolysaccharide (LPS) molecules in the outer membrane of Gram-negative bacteria make it a highly effective barrier to small molecules, antibiotic drugs, and other antimicrobial agents. Given this vital role in protecting bacteria from potentially hostile environments, simulations of LPS bilayers and outer membrane systems represent a critical tool for understanding the mechanisms of bacterial resistance and the development of new antibiotic compounds that circumvent these defenses. The basis of these simulations are parameterizations of LPS, which have been developed for all major molecular dynamics force fields. However, these parameterizations differ in both the protonation state of LPS as well as how LPS membranes behave in the presence of various ion species. To address these discrepancies and understand the effects of phosphate charge on bilayer properties, simulations were performed for multiple distinct LPS chemotypes with different ion parameterizations in both protonated or deprotonated lipid A states. These simulations show that bilayer properties, such as the area per lipid and inter-lipid hydrogen bonding, are highly influenced by the choice of phosphate group charges, cation type, and ion parameterization, with protonated LPS and monovalent cations with modified nonbonded parameters providing the best match to experiments. Additionally, alchemical free energy simulations were performed to determine theoretical pKa values for LPS, and subsequently validated by 31P solid-state NMR experiments. Results from these complementary computational and experimental studies demonstrate that the protonated state dominates at physiological pH, contrary to the deprotonated form modeled by many LPS force fields. In all, these results highlight the sensitivity of LPS simulations to phosphate charge and ion parameters, while offering recommendations for how existing models should be updated for consistency between force fields as well as to best match experiments.