Coarse-grained molecular dynamics study of permeability enhancement in DPPC bilayers by incorporation of lysolipid.

The enhanced permeability of flat lipid bilayer membranes at their gel to liquid-crystalline (LC) phase transition has been explored using coarse-grained molecular dynamics. The phase transition temperature, T(m), is deduced by monitoring the area per lipid, the lipid lateral diffusion constant, and the lipid-lipid radial distribution function. We find that a peak in the permeability coincides with the phase transition from the gel to LC state when lysolipid is present. This peak in permeability correlates with a jump in the area per lipid near the same temperature as well as increased fluctuations in the lipid bilayer free volume. At temperatures above T(m), the permeability is only slightly dependent on the amount of lysolipid present. The increased free volume due to the "missing tail" of the lysolipid is partially compensated for by a decrease in area per lipid as the amount of lysolipid increases. We also found that in the coarse-grained model a small amount (< or = 15 mol %) of lysolipid stabilizes the gel phase and increases the phase transition temperature, while a larger amount of lysolipid (20 mol %) reduces T(m) back to that for pure DPPC, and bilayers consisting of > or = 30 mol % lysolipid did not form a gel phase but still exhibited a peak in permeability near T(m) for pure DPPC.