Headgroup hydration and mobility of DOTAP/DOPC bilayers: A fluorescence solvent relaxation study

The biophysical properties of liposome surfaces are critical for interactions between lipid aggregates and macromolecules. Liposomes formed from cationic lipids, commonly used to deliver genes into cells in vitro and in vivo, are an example of such a system. We apply the fluorescence solvent relaxation technique to study the structure and dynamics of fully hydrated liquid crystalline lipid bilayers composed of mixtures of cationic dioleoyltrimethyl-ammoniumpropane (DOTAP) and neutral dioleoylphosphatidylcholine (DOPC). Using three different naphthalene derivatives as fluorescent dyes (Patman, Laurdan and Prodan) allowed different parts of the headgroup region to be probed. Wavelength-dependent parallax quenching measurements resulted in the precise determination of Laurdan and Patman locations within the DOPC bilayer. Acrylamide quenching experiments were used to examine DOTAP-induced dye relocalization. The nonmonotonic dependence of dipolar relaxation kinetics (occurring exclusively on the nanosecond time scale) on DOTAP content in the membrane was found to exhibit a maximum mean solvent relaxation time at 30 mol % of DOTAP. Up to 30 mol %, addition of DOTAP does not influence the amount of bound water at the level of the sn(1) carbonyls, but leads to an increased packing of phospholipid headgroups. Above this concentration, elevated lipid bilayer water penetration was observed.