Rapid flip-flop motions of diacylglycerol and ceramide in phospholipid bilayers

We have investigated flip-flop motions of diacylglycerol and ceramide in phospholipid bilayers using coarse-grained molecular dynamics simulations. In the simulations, flip-flop motions of diacylglycerol and ceramide in the DAPC membrane are slower than cholesterol. Rates correlate with the number of unsaturated bonds in the membrane phospholipids and hence with fluidity of membranes. These findings qualitatively agree with corresponding experimental data. Statistical analysis of the trajectories suggests that flip-flop can be approximated as a Poisson process. The rate of the transverse movement is influenced by depth of the polar head group in the membrane and extent of interaction with water.