Solution structure of the DNA damage lesion 8-oxoguanosine from ultraviolet resonance Raman spectroscopy

UV radiation and reactive byproducts of cellular metabolism are constant threats to genomic stability. A frequent consequence is the oxidation of DNA nucleobases, especially guanine to 8-oxoguanosine. This highly mutagenic lesion can form base pairs with other nucleobases, does not significantly distort the DNA structure, and remains unnoticed by DNA polymerases. Detection and biophysical studies of modified nucleobases is challenging because they are not fluorescent and have broad electronic spectra that overlap with those of normal bases. The structure of 8-oxoguanosine and its anion in solution has been contentious in the literature. Using ultraviolet excitation in resonance with the nucleobase, we have obtained the Raman spectra of 8-oxoguanosine. The stable tautomer in solution is unequivocally identified as the diketone form. We show that, at high pH, 8-oxoguanosine gets deprotonated to form an anion through loss of the N1 proton from the pyrimidine ring. The enol form is never populated to a detectable level. Raman spectra are supported by density functional theoretical calculations and a complete normal-mode analysis to identify bands that can be used as reporters of protein-nucleobase interactions. We have demonstrated that UVRR spectra provide unprecedented information on the solution-state structures of modified nucleobases.