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

Abstract

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.