Effect of Ion Binding in Palmitoyl-Oleoyl Phosphatidylserine Monolayers

Molecular dynamics simulations of palmitoyl-oleoyl phosphatidylserine (POPS) monolayers at the air-water interface were performed with different ionic strengths with the aim of determining the specific organization and dynamics of counterion binding events. Na$+$ ions penetrated the monolayers into both the ester carbonyl and carboxylate regions of the phospholipids. The~binding events increase with the addition of salt. Differences in lipid order parameter, headgroup orientation, and~prevalence of inter- and intramolecular hydrogen bonding events between the amine group of the lipid and oxygen groups are observed depending on whether the Na$+$ is binding near the carboxylate or ester region of the lipid. The observed changes are explained in terms of the salting-out effect.