Free-energy surfaces of ionic adsorption in cholesterol-free and cholesterol-rich phospholipid membranes

Free energy surfaces associated to the adsorption of metal cations ( Na +, K +, Ca2+, andMg2+) in biological environments have been computed by metadynamics simulations. In all cases, the systems were modelled using the CHARMM36 force field. The free- energy landscapes unveil specific binding behaviour of metal cations. So, Na + and K + are more likely to stay in the aqueous solution, and can easily bind to a few lipid oxygens by overcoming low free- energy barriers. Differently, Ca2+ is most stable when bound to four lipid oxygens of the membranes, rather than being hydrated in the aqueous solution. Finally, Mg2+ is tightly hydrated, and can hardly lose a hydration water and bind directly to the membranes. When cholesterol is included inside the membrane at concentration up to 50%, the resulting free- energy landscapes reveal the competition between binding of sodium to water and to lipid head groups, although the binding competitiveness of lipid head groups is diminished by cholesterol contents. Whencholesterol concentration is greater than 30%, the ionic binding is significantly reduced, which coincides with the phase transition point of DMPC- cholesterol membranes from a liquid- disordered phase to a liquid- ordered phase.