Probing the distribution of water molecules hydrating lipid membranes with ultrafast Förster vibrational energy transfer.

We determine the relative positioning of water molecules in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) membranes by measuring the rate of vibrational resonant (Förster) energy transfer between the water hydroxyl stretch vibrations. The rate of Förster energy transfer is strongly distance dependent and thus gives detailed information on the relative positioning of the water molecules. We determine the rate of intermolecular Förster energy by measuring the anisotropy dynamics of excited O-D stretch vibrations of HDO and D(2)O molecules with polarization-resolved femtosecond mid-infrared spectroscopy. We study the dynamics for deuterium fractions between 0.1 and 1 and for hydration levels between 2 and 12 water molecules per DOPC lipid molecule. We find that most of the water molecules hydrating the membrane are contained in nanoclusters and have an average intermolecular distance of 3.4 Å. The density of these nanoclusters increases with increasing hydration level of the DOPC membranes.