Measurement of the Main Photooxidation Products of 2′-Deoxyguanosine Using Chromatographic Methods Coupled to Mass Spectrometry

Analytical methods were developed for the measurement of the main photooxidation products of 2′-deoxyguanosine (dGuo), arising from either the type I (electron transfer) or the type II (singlet oxygen) photosensitization mechanism. Oxidation of dGuo by a type I mechanism leads to the predominant formation of 2,2-diamino-5-[2-deoxy-β-d-erythro-pentofuranosyl)amino]-5(2H)-oxazolone. On the other hand, the two 4R and 4S diastereomers of 4-hydroxy-8-oxo-4,8-dihydro-2′-deoxyguanosine are the main singlet oxygen-mediated dGuo oxidation products. The modified nucleosides were measured by either gas chromatography coupled to mass spectrometry after silylation of the sample or by high-performance liquid chromatography associated to tandem mass spectrometry. In order to improve the accuracy of the assays, isotopically labeled internal standards were synthesized for an isotope dilution mass spectrometry quantitation. The methods were successfully applied to the measurement of methylene blue- and riboflavin-mediated 2′-deoxyguanosine photooxidation reactions. The advantages of the two above-mentioned methods are discussed on the basis of comparative sensitivity and accuracy.