Revealing anharmonic couplings and energy relaxation in DNA oligomers by ultrafast infrared Spectroscopy

The identification and characterization of NH2 hydrogen-bonded stretching vibrations [v(NH2)] in DNA oligomers is usually hampe red by the all-dominating absorption of the water stretching band in the spectral range of 3050-3600 cm(-1). Here, we use the two-color IR pump-probe technique to overcome the limitations of linear absorption spectroscopy by exciting adenine-thymme (A-T) oligomer vibrations in the fingerprint region and analyzing induced transient spectral changes in the v(NH2) spectral region. These transient changes are related to anharmonic couplings to the modes excited in the fingerprint region and to modes populated by intra- and intermolecular energy redistribution and relaxation. The combination of calculated anharmonic coupling parameters and experimental transient IR data allows the assignment of a transition at 3215 cm(-1) to the v(NH2) vibration of adenine in dA(20)-dT(20) DNA oligomers.