Ultrafast memory loss and relaxation processes in hydrogen-bonded systems

Structural memory of aqueous systems, such as neat water and biomolecules, in an aqueous environment is strongly influenced by hydrogen bond dynamics. Vibrational spectroscopy in the femtosecond (fs) time domain is applied to map structural dynamics in real-time and identify underlying molecular interactions. Neat liquid water displays an ultrafast loss of structural memory with the fastest decay of structural correlations occurring in the sub-100 fs regime. Both OH stretching and bending excitations of water molecules decay on a subpicosecond time scale, followed by dissipation of excess energy in the hydrogen bond network within a few picoseconds. Water shells around fully hydrated DNA show similar although slightly slower dynamics. A detailed study of hydration shells around ionic phosphate groups in the DNA backbone demonstrates a strong phosphate-water coupling and a subpicosecond rearrangement of hydrogen bonds upon energy disposal. Hydration shells serve as primary heat sinks for excess energy.