Solvation dynamics at aqueous lipid-membrane interfaces explored by temperature-dependent 3-pulse-echo peak shifts:: Influence of the lipid polymorphism

Three-pulse photon-echo peak-shift (3PEPS) experiments were performed to investigate structural relaxations relevant to solvation dynamics of a cyanine dye which is noncovalently anchored to aqueous interfaces of unilamellar phospholipid vesicles. For comparison, complementary 3PEPS measurements were performed on the same optical chromophore dissolved in bulk water. The influence of the lipid polymorphism is studied in great detail through the temperature dependence of the 3PEPS decay. As opposed to the main order/ disorder phase transition connected with packing phenomena in the hydrophobic membrane core, the rippled gel-to-gel pretransition strongly modifies the solvation response on time scales below 2 ps. This finding indicates that the pretransition is connected with structural modifications of the hydration shell surrounding the polar headgroups of the lipids and is consistent with a dehydration of the interface upon cooling the rippled-gel phases and entering the pure gel phases of the membranes. A comparison between the temperature dependencies of the 3PEPS decay and the linear absorption spectrum strongly suggests that a formal division of water above lipid membranes into "bound" and "free" species only is too simplistic.