Molecular dynamics study of binary POPC bilayers: molecular condensing effects on membrane structure and dynamics

Molecular dynamics (MD) simulations of binary 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers containing cholesterol (CHOL), ceramide (CER), diacylglycerol ( DAG), or sphingomyelin (SM) were carried out to investigate effects of these molecules on the structure and dynamics of membranes. Clear condensing effects were observed in all POPC binary bilayers, and those molecular condensing effects were ranked in the order of POPC/CHOL, POPC/CER, POPC/DAG, and POPC/SM bilayer. The differences of condensing effects are attributable to the factors such as molecular shapes, type of polar head groups and hydrogen bond networks in the binary lipid bilayers. The rigid sterol ring of CHOL is due to the ordering of acyl chains of POPC in the membrane. The lack of bulky PC head group of lipids results in the lipids condition in which DAG, CER and CHOL are buried in the hydrophobic region of the bilayer, leading to the condensed membranes. CHOL and CER are closely arranged in each binary bilayer due to the better intermolecular affinities of these lipids. The hydrogen bond network can also be a factor for the condensing of membranes and the decrease of lateral diffusion of lipids in the binary lipid bilayers.