A Stable Radical in the Ribose Moiety of X-Irradiated 3′-Cytidylic Acid and Its Relation to a Similar Radical in Cytidine: An ESR-ENDOR Study

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Abstract
A stable free radical, labeled 3.alpha.H, is formed in 3''-cytidylic acid (3''CMP) single crystals in X-irradiating at 77.degree. K and then warming to above room temperature. It remains stable up to 150.degree. C. ESR and ENDOR [electron nuclear double resonance] measurements explicitly indicate a structure of the form H2.ovrhdot.C-CX.dbd.CHY. The same free radical structure is assigned to a radical in cytidine (whose spectral parameters were analyzed by Hampton and Alexander). The chemical structures of groups X and Y are tentatively assigned for the 3.alpha.H radical in both 3''CMP and cytidine. Measurements on the formation of the 3.alpha.H radical in 3''CMP indicate that the precursor radical is due to hydrogen abstraction at C5''. The activation energy of the conversion is about 30 kcal/mol. A mechanism is postulated for the formation of the 3.alpha.H radical in both 3''CMP and cytidine. It accounts for the decay of the 3.alpha.H radical in cytidine and the presence of yet another hitherto unassigned radical product observed by Allison and Alexander. The postulated mechanism provides a reaction scheme for arriving at various end products identified in aqueous radiolysis studies of DNA and its components.

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