Molecular dynamics simulation of cation-phospholipid clustering in phospholipid bilayers: Possible role in stalk formation during membrane fusion

In this study, we performed all-atom long-timescale molecular dynamics simulations of phospholipid bilayers incorporating three different proportions of negatively charged lipids in the presence of K ⁺, Mg ^{2 +}, and Ca ^{2 +} ions to systemically determine how membrane properties are affected by cations and lipid compositions. Our simulations revealed that the binding affinity of Ca ^{2 +} ions with lipids is significantly stronger than that of K ⁺ and Mg ^{2 +} ions, regardless of the composition of the lipid bilayer. The binding of Ca ^{2 +} ions to the lipids resulted in bilayers having smaller lateral areas, greater thicknesses, greater order, and slower rotation of their lipid head groups, relative to those of corresponding K ⁺- and Mg ^{2 +}-containing systems. The Ca ^{2 +} ions bind preferentially to the phosphate groups of the lipids. The complexes formed between the cations and the lipids further assembled to form various multiple-cation-centered clusters in the presence of anionic lipids and at higher ionic strength-most notably for Ca ^{2 +}. The formation of cation-lipid complexes and clusters dehydrated and neutralized the anionic lipids, creating a more-hydrophobic environment suitable for membrane aggregation. We propose that the formation of Ca ^{2 +}-phospholipid clusters across apposed lipid bilayers can work as a "cation glue" to adhere apposed membranes together, providing an adequate configuration for stalk formation during membrane fusion.