Ultrafast Energy Redistribution in Local Hydration Shells of Phospholipids: A Two-Dimensional Infrared Study

Structural and functional properties of phospholipids are strongly influenced by dynamics of their hydration shells. Here, we show that local water pools as small as three water molecules around the polar headgroups in phospholipid reverse micelles (dioleoylphosphatidylcholine, DOPC) serve as efficient sinks of excess energy released during vibrational relaxation. Transient two-dimensional (2D) infrared spectra of OH stretching excitations of H2O shells demonstrate a subpicosecond buildup of a hot water ground state, in which excess energy is randomized in low-frequency modes. An analysis of center line slopes of the 2D spectra reveals kinetics of energy dissipation that are significantly faster than structural fluctuations of the water pool and remain unchanged at intermediate hydration levels between three and eight water molecules per polar headgroup. Our results suggest that confined small water pools in biomolecular systems are sufficient to dissipate excess energy originating from the decay of electronic or vibrational excitations.