Fusion-relevant changes in lipid shape of hydrated cholesterol hemisuccinate induced by pH and counterion species

Cholesterol hemisuccinate (CHEMS) is a protonable lipid that is frequently used for the construction of pH-responsive delivery systems. Such systems have a stable, lamellar phase at pH 7, but can form a fusogenic, hexagonal phase at pH 5. This behavior can be explained by binding or release of counterions from the solvent and the related variations of the effective size of the polar lipid part. Here, we use MD simulations to study the ion recruitment to neutral or anionic bilayers of CHEMS in water. For deprotonated (anionic) CHEMS, we observed an almost complete decoration of the bilayer with sodium, potassium, or argininium cations, which challenges the previous hypothesis that the stability of bilayers from anionic CHEMS results from the electrostatic repulsion between the charged head groups. Protonated (neutral) bilayers of CHEMS did not bind sodium or potassium, but did adsorb argininium cations. Whereas the headgroup of protonated CHEMS is bent and strongly tilted away from the bilayer normal, the headgroup of the deprotonated CHEMS is found to become outstretched and significantly less tilted arising from the adsorption of the counterions. The tilt reduction is most pronounced upon adsorption of arginine which also leads to an increase in the otherwise constant area per lipid. In general, the cation binding to the deprotonated CHEMS acts to increase the effective headgroup volume. This change in the lipid shape may be one possible fact explaining the hexagonal-lamellar phase transitions for CHEMS known from experiments.