Radiation chemistry of purines in the solid state: ESR and ENDOR studies of x-irradiated xanthosine dihydrate single crystals

By means of ESR and ENDOR spectroscopy, we were able to detect and characterize four products of the purine riboside xanthosine in x-irradiated xanthosine dihydrate single crystals. Product I was identified (at 65 K) as the π cation of the xanthine base deprotonated at N3. Product II was identified (at 65 K) as the π anion of the xanthine base protonated at O6. Product III was identified as the result of the hydrogen abstraction from C5′ of the sugar group (probably from a primary cation of the ribose). Product IV was identified as the result of hydrogen addition at C8 of the purine ring. Although studied in detail at 65 K, products I and II were present after irradiation and study at 15 K. Slow warming of the crystal beyond 100 K led to gradual and irreversible decay of the signals from products I and II. No clear-cut evidence for new products appeared until the crystals were warmed beyond 250 K, where spectra from the C8 H adduct (product IV) appeared. At 65 K, signals from product III were present, but obscured by the more intense ones from products I and II. For that reason, product III was studied in detail at about 210 K. At 295 K, signals from product III decayed to nil in about 2 h.