Probing solvation dynamics in liquid water and at phospholipid/water interfaces with femtosecond photon-echo spectroscopies

Three-pulse photon-echo peak-shift (3PEPS) experiments have been performed to explore the time scales and the nature of structural relaxations relevant to solvation dynamics of a cyanine dye non-covalently anchored to phospholipid/water interfaces. For comparison, equivalent 3PEPS data are presented for the same chromophore dissolved in neat water. In the bulk liquid, solvation includes at least two distinct time scales. A fast component dominating the solvation response below 3 ps is connected to restricted intermolecular translational degrees of freedom of the hydrogen-bonded liquid network in qualitative agreement with aqueous solvation dynamics of other chromophores previously reported in the literature. However, on longer time scales above 3 ps, an additional exponential tail can be found in the 3PEPS decay which has not been observed previously. Its time constant is in good agreement with the dielectric relaxation time of bulk water indicating that a significant fraction of the solvation energy is also relaxed by single-molecule rotational diffusion of water. At the membrane interface, the 3PEPS data indicate that already on sub-picosecond time scales, solvation is considerably perturbed in comparison to bulk water. This finding indicates a substantial disruption of the hydrogen-bonded network of the bulk liquid. Diffusive single-molecule reorientation of water seems to contribute to solvation at the interface in the same manner as it does in the bulk phase. These findings are in qualitative agreement with recent molecular dynamics simulations on the structure and the dynamics of water at phospholipid membrane interfaces.