  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 semi-flexible
strings. It is found that lateral pressure profile of unsaturated lipids has
distinct maximum in the unsaturated bond region due to 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 neighbouring lipid chains have less probability to collide by
geometrical reasons, causing depletion of entropic repulsion relative to
saturated lipid chains case and resulting in the local minima of the lateral
pressure profile surrounding the maximum at the bent unsaturated bond. Lipid
chain order parameter, lateral pressure profile, and area per lipid are
computed for POPC and compared with those for DPPC lipid bilayer.
