Molecular Dynamics Simulation Study Of The Positioning And Dynamics Of A-Tocopherol In Phospholipid Bilayers

Using molecular dynamics simulations, we investigate the interaction of alpha-tocopherol (alpha-toc) with dipalmitoylphosphatidylcholine (DPPC), dimyristoylphosphatidylcholine (DMPC), palmitoyloleoylphosphatidylcholine (POPC), and palmitoyloleoylphosphatidylethanolamine (POPE) lipid bilayers. The goal is to develop a better understanding of the positioning and orientation of alpha-toc inside the bilayers; properties of significant relevance to alpha-toc anti-oxidant activity. We investigated bilayer systems with 128 lipids in the presence of either single or 14 alpha-toc molecules. The single alpha-toc bilayer systems were investigated via biased MD simulations in which the potential of mean force (PMF) and diffusivity were obtained as functions of the distance between alpha-toc head group and bilayer center. The higher alpha-toc concentration systems were investigated with unbiased MD simulations. For all four bilayers at both concentrations, the simulations show that the most probable location of the alpha-toc hydroxyl group is just below the lipid carbonyl group. Overall, the simulation results are in good agreement with existing experimental data except for the DMPC bilayer system for which some experiments predict alpha-toc to be located closer to bilayer center. The flip-flop frequency calculated shows that the alpha-toc flip-flop rate is sensitive to bilayer lipid type. In particular, alpha-toc has a much lower flip-flop rate in a POPE bilayer compared to the three PC lipid bilayers due to the smaller area per lipid in the POPE bilayer. For DMPC and POPC, the alpha-toc flip-flop rates are significantly higher at higher alpha-toc concentration and this appears to be related to the local structural disruption caused by alpha-toc clusters spanning the bilayer.