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RESEARCH PRODUCT

Fatty Acid Aggregates Simulated using Constant pH Molecular Dynamics with a Coarse-Grained Model

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Fatty acids are crucial biomolecules, important for lipid metabolism, signaling, models for protocell membranes, soaps, industrial applications, and drug delivery. Oleic acid has complex phase behavior with respect to the protonation state of the carboxylic head group, which depends on the pH of the solution. Oils form at low pHs, vesicles at intermediate pHs, and micelles at high pHs. We use constant pH molecular dynamics with the MARTINI coarse-grained model to investigate oleic acid aggregates at different pH conditions. We determine titration curves for the oleic acid monomers in different aggregates, and observe a shift in the microscopic pKa. In agreement with experimental results, the pKa of a monomer in bulk water is ca. 4 and shifts to ca. 5.5 in a small micelle and ca. 8-9 in a fatty acid bilayer. There is strong anti-cooperative protonation behavior between monomers within an aggregate. This work presents a proof of concept for using constant pH simulations with the MARTINI model, and provides a physiochemical basis for the phase behavior of fatty acids in different aggregate environments.