Effect of H₃O⁺ on the Structure and Dynamics of Water at the Interface with Phospholipid Bilayers

This study investigates the effect of hydronium ions (H₃O⁺) on the structure and dynamics of water at the interface of a phospholipid bilayer using molecular dynamics simulations of a POPC bilayer in the presence and absence of H₃O⁺ ions. From these simulations, the survival probability, hydrogen bond lifetimes, orientation relaxation, and angular distribution of interfacial water, at increasing distances from the membrane surface, were calculated. Simulations of POPC in the absence of H₃O⁺ ions reproduce previously reported deviations of interfacial water from the properties of bulk water. Our results show that in the presence of H₃O⁺, these deviations are even more pronounced with the strongest effects seen in the survival probability and orientation relaxation. To further investigate the effect of the H₃O⁺-induced reduction of area per lipid on interfacial water, we carried out simulations where H₃O⁺ ions were removed, but the area per lipid was fixed to the values seen in the presence of H₃O⁺. The combined findings from our study suggest that the presence of H₃O⁺ ions affects the properties of interfacial water, accentuates the deviation from bulk properties, and extends the long-range effect of these deviations further away from the membrane surface.