Comparison of negative and positive ion electrospray tandem mass spectrometry for the liquid chromatography tandem mass spectrometry analysis of oxidized deoxynucleosides

Oxidized deoxynucleosides are widely used as biomarkers for DNA oxidation a nd oxidative stress assessment. Although gas chromatography mass spectromet ry is widely used for the measurement of multiple DNA lesions, this approac h requires complex sample preparation contributing to possible artifactual oxidation. To address these issues, a high performance liquid chromatograph y (HPLC)-tandem mass spectrometric (LC-MS/MS) method was developed to measu re 8-hydroxy-2'-deoxyguanosine (8-OH-dG), 8-hydroxy-2'-deoxyadenosine (8-OH -dA), 2-hydroxy-2'-deoxyadenosine (2-OH-dA), thymidine glycol (TG), and 5-h ydroxymethyl-2'-deoxyuridine (HMDU) in DNA samples with fast sample prepara tion. In order to selectively monitor the product ions of these precursors with optimum sensitivity for use during quantitative LC-MS/MS analysis, uni que and abundant fragment ions had to be identified during MS/MS with colli sion-induced dissociation (CID). Positive and negative ion electrospray tan dem mass spectra with CID were compared for the analysis of these five oxid ized deoxynucleosides. The most abundant fragment ions were usually formed by cleavage of the glycosidic bond in both positive and negative ion modes. However, in the negative ion electrospray tandem mass spectra of 8-OH-dG, 2-OH-dA, and 8-OH-dA, cleavage of two bonds within the sugar ring produced abundant S-1 type ions with loss of a neutral molecule weighing 90 u, [M - H - 90](-). The signal-to-noise ratio was similar for negative and positive ion electrospray MS/MS except in the case of thymidine glycol where the si gnal-to-noise was 100 times greater in negative ionization mode. Therefore, negative ion electrospray tandem mass spectrometry with CID would be prefe rred to positive ion mode for the analysis of sets of oxidized deoxynucleos ides that include thymidine glycol. Investigation of the fragmentation path ways indicated some new general rules for the fragmentation of negatively c harged oxidized nucleosides. When purine nucleosides contain a hydroxyl gro up in the C-8 position, an S-1 type product ion will dominate the product i ons due to a six-membered ring hydrogen transfer process. Finally, a new ty pe of fragment ion formed by elimination of a neutral molecule weighing 48 (CO2H4) from the sugar moiety was observed for all three oxidized purine nu cleosides. (C) 2001 American Society for Mass Spectrometry.