Driving force for crystallization of anionic lipid membranes revealed by atomistic simulations

Crystalline vesicles are promising nanomaterials due to their mechanical stability in various environments. To control their fabrication, it is essential to understand the effects of different experimental conditions on crystallization. Here we perform atomistic molecular dynamics simulations of anionic lipid membranes of 1,2-dilauroyl-sn-glycero-3-phosphol-l-serine. In the presence of Na⁺ monovalent counterions, we access the phase transition from the liquid-like disordered liquid-crystalline phase to the ordered gel phase by lowering the temperature of the system. The phase transition is conclusively evidenced by the scattering structure factor. Quantitative calculations show that the enhancement of the intertail van der Waals interaction (about -6 k_BT) plays a dominant role in driving the phase transition rather than the increase in the cohesive interaction (-0.5 k_BT) between lipids and counterions. Meanwhile, in the presence of multivalent counterions of Zn²⁺ or La³⁺ the gel phase is found throughout the temperature range investigated. Moreover, the van der Waals interaction per hydrocarbon group is ∼20% stronger in the gel phase (∼ -1.8 k_BT regardless of the counterions) than in the liquid-crystalline phase (-1.5 k_BT).