Constant-pH MD Simulations of DMPA/DMPC Lipid Bilayers.

Current constant-pH molecular dynamics (CpHMD) simulations provide a proper treatment of pH effects on the structure and dynamics of soluble biomolecules like peptides and proteins. However, addressing such effects on lipid membrane assemblies has remained problematic until now, despite the important role played by lipid ionization at physiological pH in a plethora of biological processes. Modeling (de)protonation events in these systems requires a proper consideration of the physicochemical features of the membrane environment, including a sound treatment of solution ions. Here, we apply our recent CpHMD-L method to the study of pH effects on a 25% DMPA/DMPC bilayer membrane model, closely reproducing the correct lipid phases of this system, namely, gel-fluid coexistence at pH 4 and a fluid phase at pH 7. A significant transition is observed for the membrane ionization and mechanical properties at physiological pH, providing a molecular basis for the well-established role of phosphatidic acid (PA) as a key player in the regulation of many cellular events. Also, as reported experimentally, we observed pH-induced PA-PA lipid aggregation at acidic pH. By including the titration of anionic phospholipids, the current methodology makes possible to simulate lipid bilayers with increased realism. To the best of our knowledge, this is the first simulation study dealing with a continuous phospholipid bilayer with pH titration of all constituent lipids.