Flexible string model of unsaturated lipid bilayer

An analytically solvable model of unsaturated lipid bilayer is derived by introducing finite bending angle of the unsaturated bond relative to straight part of the lipid chain considered previously in our model of semiflexible strings. It is found that the lateral pressure profile of unsaturated lipids has a distinct maximum in the unsaturated bond region due to the enhanced excluded volume effect caused by the bent bond, leading to an increase of entropic repulsion between the lipid chains. Simultaneously, just away from the unsaturated bond, some parts of the neighboring lipid chains have less probability to collide for geometrical reasons, causing depletion of entropic repulsion relative to the saturated lipid chains case and resulting in the local minima of the lateral pressure profile surrounding the maximum at the bent unsaturated bond. The lipid chain order parameter, the lateral pressure profile, and the area per lipid are computed for palmitoyl-oleoyl-phosphatidylcholine and compared with those for dipalmitoylphosphatidylcholine lipid bilayer.