ESI-MS/MS for the differentiation of diastereomeric pyrimidine glycols in mononucleosides

Pyrimidine glycols, or 5,6-dihydroxy-5,6-dihydropyrimidines, are primary lesions in DNA induced by reactive oxygen species. In this article, we report the preparation and tandem mass spectrometry (MS/MS) characterization of the two cis diastereomers of the glycol lesions of 2′-deoxyuridine, 5-methyl-2′-deoxycytidine, and thymidine. Our results show that collisional activation of the [M + Na]⁺ ions of all the three pairs of cis isomers and that of the [M + H]⁺ ions of the 2′-deoxyuridine glycols and 5-methyl-2′-deoxycytidine glycols give a facile loss of a water molecule. Interestingly, the water loss occurs more readily for the 6S isomer than for the 6R isomer. Likewise, product ion spectra of the [M - H]_- ions of the two cis isomers of the 2′-deoxyuridine glycols and thymidine glycols show more facile loss of water for the 6S isomer than for the 6R isomer. MS/MS acquired at different collisional energies gave similar results, which establishes the reproducibility of spectra.