Ultraviolet Resonance Raman Spectroscopy Of Nucleic Acid Chromophores

Abstract

The four aromatic nitrogenous bases which are found in nucleic acids (adenine,guanine, cytosine, and uracil or thymine) are chromophoric residues which exhibit two absorption bands in the region 200-280nm. The ultraviolet resonance Raman (UVRR) spectra of several guanine or cytosine containing compounds and some isotopic derivatives have been taken with an incident ultraviolet laser beam at either 213nm, 218nm, or 266nm which is generated from the higher harmonics and stimulated Raman shifted frequencies of a YAG laser. Each of these wavelengths falls within either the first or second absorption bands of the chromophores. These spectra have been compared with the UVRR spectra which are calculated based on an earlier theory of one of the authors and various normal mode calculations which are found in the literature. Differences are observed and calculated in the intensities of the Raman bands in the spectra taken at different incident laser frequencies when they are resonant with different excited states of the chromophore. These different intensities must arise from different Franck-Condon overlap integrals between the molecular vibrations in the ground state and in each of the two excited states. An interesting observation is the existence of Raman bands which change in-tensity but do not change frequency upon isotopic substitution -- an effect ascribed to the change in Raman intensity with vibrational kinetic energy changes. It is suggested that the measurement and calculation of the intensity changes of the UVRR spectrum upon isotopic substitution should provide a method of improving the force fields of the purine and pyrimidine chromophores in nucleic acids as well as other aromatic ring compounds.