ESR/ENDOR Study of Guanine · HCl · 2H2O X-irradiated at 20K

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Abstract
Single crystals of guanine hydrochloride dihydrate, in which the guanine base is protonated at N7, were X-irradiated at 20K, 65K and 150K. Study with K-band ESR and ENDOR techniques indicated at least four radical species to appear in the temperature range 20–300K. Three of the radical species (Radicals 1, 2, and 3) were present immediately following irradiation at 20K. Radical 1 was identified as the molecular anion protonated at O6. Hyperfine couplings in Radical 1 to HC8, HN1, and HN7 were fully characterized with ENDOR spectroscopy. The data indicated this product to be trapped in four different conformations which coalesced into the most stable form as the sample temperature was raised to ca. 180K. Radical 2 was the C8 H-addition radical. For this radical, hyperfine couplings to HN7, HN9, and the two β-methylene protons were fully characterized with ENDOR spectroscopy. Radical 3 was the result of hydroxyl addition to C8 of the guanine base. For Radical 3, full characterization by ENDOR was possible for couplings to HN7, HN9 and the proton at C8. Annealing the samples beyond 250K for several hours led to disappearance of Radical 1, and appearance of Radical 4. The evidence indicated that Radical 4 was the result of net hydrogen abstraction from N9 of the guanine base. The ENDOR results permitted full characterization of hyperfine couplings to HN7 and the two amino protons (the HN10s). From these results, and those from two other systems containing N7-protonated guanine bases, reaction mechanisms are proposed to account for formation of the products.