Structure determination of isomeric hexadeoxynucleotide photoproducts by high-field NMR and fast atom bombardment/tandem mass spectrometry

UV irradiation of a T-rich hexadeoxynucleotide, d(TATTAT), produced four photoproducts, which can be cleanly separated from starting material and each other by reverse-phase HPLC. The structures of the photoproducts were investigated by using 1D ¹H-NMR, which allows the nature, but not the location, of the photoproduct between adjoining Ts or between T and A to be determined. Both fast atom bombardment (FAB) and matrix-assisted laser desorption ionization (MALDI) were able to confirm the molecular weights of the starting material and the four photoproducts. Furthermore, the results of tandem mass spectrometry experiments, with a four-sector spectrometer, of the [M + H]⁺ ions of the various materials were consistent with structure and showed the location of the photoproduct in the d(TATTAT) molecule. All the isomers gave distinct product-ion spectra that are consistent with structure and sufficient for locating the site of the modification.