Extending a Highly Coarse-Grained Lipid Model to Asymmetric Membranes for MD Simulations

Abstract

Lipid leaflet asymmetry is an essential aspect of biomembranes that leads to numerous novel properties not present in ordinary symmetric membranes. To investigate these systems through molecular dynamics simulation over large length and time scales usually necessitates coarse-grained models to reduce computational cost. However, highly coarse-grained models, such as the Cooke model, exhibit artificially accelerated dynamics, in particular lipid flip-flop. This may cause a membrane's asymmetry to decay before statistically significant measurements of its consequences can be made. We investigate a remedy for this problem proposed by Wang et al. (Commun. Comput. Phys. 13), which energetically discourages lipids from entering the opposite leaflet. We measure the efficiency of this modification in maintaining asymmetry in normal unstressed membranes as well as bilayers with varying degrees of stress imbalance between leaflets. We propose further changes to improve the remedy under extenuating circumstances, and we report changes to basic membrane characteristics caused by the modification as well as possible ways by which one can compensate for them.