Vibrational dynamics of the water shell of DNA studied by femtosecond two-dimensional infrared spectroscopy

Vibrational dynamics of the water shell around DNA oligomers are addressed by femtosecond two-dimensional (2D) infrared spectroscopy in a wide range of water concentrations. In the 2D spectra, NH stretching excitations of DNA are clearly separated from OH stretching excitations of water. For full hydration, OH stretching excitations display spectral diffusion on a time scale of 500 fs. Vibrational relaxation results in the build-up of a hot water ground state on a 1–2 ps time scale. A comparison of center line slopes of the 2D spectra with the frequency–time correlation function of bulk water suggests a decay of frequency correlations distinctly slower than in bulk H2O. We attribute this behavior to local DNA–water interactions which reduce structural fluctuations and to the smaller rate of resonant OH stretch energy transfer. In contrast, the kinetics of excess energy dissipation are similar in the two systems.