In Vivo Bypass Efficiencies and Mutational Signatures of the

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5-Cyanoimino-4-Oxomethylene-4,5-Dihydroimidazole and 5-Cyanoamino-4-Imidazolecarboxylic Acid Intermediates in Nitrosative Guanosine Deamination: Evidence from ¹⁸O-Labeling Experiments

The nitrosative deaminations (37 °C, NaNO2, NaAc buffer, pH 3.7) of guanosine 1r in (18O)-water (97.6%) and of [6-18O]-1r in normal water were studied. [6-18O]-1r was prepared from 2-amino-6-chloropurine riboside using adenosine deaminase. The reaction products xanthosine 3r and oxanosine 4r were separated by HPLC and characterized by LC/MS analysis and 13C NMR spectroscopy. The 18O-isotopic shifts on the 13C NMR signals were measured and allowed the identification of all isotopomers formed. The results show that oxanosine is formed via 5-cyanoimino-4-oxomethylene-4,5-dihydroimidazole, 5, and its 1,4-addition product 5-cyanoamino-4-imidazolecarboxylic acid, 6. This hydration of 5 to 6 leads to aromatization and greatly dominates over water addition to the cyanoimino group of 5 to form 5-guanidinyliden-4-oxomethylene-4, 5-dihydroimidazole, 7. 5-Guanidinyl-4-imidazolecarboxylic acid, 8, the product of water addition to 6, is not involved