CHARACTERIZATION OF LIPID-MEMBRANE DYNAMICS BY SIMULATION .3. PROBINGMOLECULAR-TRANSPORT ACROSS THE PHOSPHOLIPID-BILAYER

Purpose, The goal of this study is to elucidate the role of the motion s of the hydrocarbon chains of a phospholipid bilayer in penetrant dif fusion. Penetrant size, as well as its position in the hydrocarbon cor e of the lipid bilayer, has also been explored regarding impact on the diffusion rate in a phospholipid bilayer. Method. Molecular dynamics, MD, simulations were carried out on a model dimyristoyl phosphatidylc holine (DMPC) membrane bilayer with and without methanol and propanol as penetrants. The MD trajectories were analyzed in terms of estimatin g time and space properties. Results. These simulations show that tors ion angle kink shifts in the hydrocarbon chains of phospholipids are n atural occurrences in a bilayer assembly. The diffusion coefficients o f methanol and propanol in a DMPC lipid bilayer, as calculated from th e MD simulations, agree with experimental measurements. Both methanol and propanol show different diffusion rates in different regions of th e hydrocarbon chain matrix of the lipid bilayer. Solute size has more impact on diffusion rate in the bilayer regions with high torsion angl e order parameters, as compared to the regions with low torsion angle order parameters. Conclusions. The simulated transport behavior sugges ts that a kink shift diffusion mechanism is more likely to occur in re gions with high torsion angle order parameters, and a free volume tran sport mechanism is more likely operative in the region with low torsio n angle order parameters, mainly the center core of the bilayer. A thr ee zone diffusion model is proposed for transport of a penetrant acros s a bilayer.