Ultraviolet resonance Raman excitation profiles of nucleic acid bases with excitation from 200 to 300 nanometers

Raman spectra are presented for dilute aqueous solutions of the four ribonucleotides AMP, GMP, UMP, and CMP obtained with laser excitation at 299, 266, 253, 240, 229, 218, 209, and 200 nm. Distinct evidence of strong, selective resonance enhancement is obtained. Low-resolution excitation profiles have been constructed for the strongest bands by using the phosphate band at 994 cm^-1 as an internal reference. The excitation spectra for many of the vibrational bands are dominated by a peak corresponding to the lowest-energy electronic transition near 260 nm. Smaller peaks are seen for higher-energy electronic transitions. For some modes, the resonance enhancement is dominated by the higher-energy transitions. It is clear from these new data that a full description of the resonance Raman profiles of the nucleic acids will have to include several excited electronic states. Two examples are given of cases where ionic species can be distinguished easily by using far-UV excitation, but these species are indistinguishable with 266-nm excitation. This demonstrates the utility of far-UV resonance Raman spectroscopy for obtaining structural information.