Ultrafast dynamics of N-H and O-H stretching excitations in hydrated DNA oligomers

DNA oligomers containing 23 alternating adenine-thymine base pairs are studied at different hydration levels by ultrafast two-color pump-probe spectroscopy. N-H stretching excitations of the base pairs and O-H stretching excitations of water are discerned in DNA-surfactant complex films. In a 0% relative humidity (r.h.) atmosphere, lifetimes of approximately 0.5 ps are found for N-H and O-H stretching modes. The transient pump-probe anisotropies demonstrate the coupling of different N-H stretching modes. DNA films at 92% r.h. show similar N-H stretching bands as at 0% r.h. and two types of O-H stretching responses. An O-H stretching band with maximum at 3500 cm−1, limited spectral diffusion and population kinetics similar to 0% r.h. is attributed to water molecules directly interacting with the DNA oligomers. O-H stretching excitations with a ∼200 fs lifetime, broad absorption and a hot ground state formed by vibrational relaxation are assigned to water molecules weakly interacting with DNA.