How water models influence the interfacial organization of oxysterol epimers: A comparative simulation study using...

Water is a critical determinant of interfacial behavior, therefore the choice of water model in molecular dynamics (MD) simulations can profoundly influence the accuracy of predicted monolayer properties. In this work, we compare the impact of two water models-three-point TIP3P and four-point OPC-on the interfacial organization, hydration, and hydrogen bonding of epimeric 22-hydroxycholesterols [22(R)-OH and 22(S)-OH] in model membranes. All-atom MD simulations, complemented by Langmuir monolayer experiments, demonstrate that the water model selection strongly affects monolayer behavior: the OPC model produces sharper density gradients and reduced water penetration at the interface, whereas TIP3P permits deeper water ingress and yields a more diffuse interfacial region. As a result, stereochemical differences are more pronounced with the OPC model, particularly for the hydration-prone 22(R)-OH. Specifically, 22(S)-OH forms more condensed monolayers with the side-chain hydroxyl group buried within the interface, whereas 22(R)-OH generates more expanded monolayers characterized by enhanced hydration in close agreement with experimental observations. These stereochemistry-dependent effects arise from distinct hydrogen-bonding patterns: 22(S)-OH preferentially engages in sterol-sterol hydrogen bonds, while 22(R)-OH remains more hydrated through hydrogen bonding with water molecules. Collectively, these findings underscore the importance of accurate water model selection for reliable representation of biointerface properties and highlight that even subtle stereochemical modifications can exert disproportionate effects on interfacial organization and hydration.