Ripple and Gel Phases of Saturated Phosphocholine Bilayers Investigated with Simulations

Lipid bilayers with saturated acyl chains typically are in a liquid crystalline (L α) phase, but can condense to a ripple (P β) or gel (L β) phase. We focused on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-phosphocholine (DPPC) lipid bilayers and their mixtures to characterize these lipid phases. MD simulations with the CHARMM36 lipid force field accurately predict the main transition temperature of DMPC and DPPC bilayers. Our MD simulations focused on the less-characterized P β phase and compared to x-ray experiments. The P β major arm has similar structure to that of the L β, while the thinner minor arm has interdigitated chains and the transition region between these two regions has large chain splay and disorder. The length of these major/minor arms are in reasonable agreement with x-ray experiments based on several μs of simulation. The L β phase form a chain tilt and lipid packing in good agreement with experiment. This work demonstrates that the CHARMM36 lipid force field can accurately represent more condensed phases and provides insight into the complex structure of the ripple phase