Reactions of Water Radiolysis Intermediates with Pyrimidine and Purine Bases in Aqueous BeF₂ Glasses: An EPR Study

Summary

Free radical formation by reaction of water radiolysis intermediates produced by X-irradiation in aqueous glasses containing 7 m BeF₂ at 77 K with pyrimidine and purine constitutents of DNA (10 mm) was studied by electron paramagnetic resonance (EPR) spectroscopy. Reactions of electrons and ^·OH radicals were observed; H^· form a minority and their contribution was difficult to establish. The electrons form substrate radicals at 77 K, while ^·OH radicals, stabilized in the matrix at 77 K, become mobile at about 140 K and in part react with solute molecules. The radicals formed by both reactions were characterized after isolation of the corresponding components by thermal annealing up to about 190 K and spectra simulation using literature parameters whenever possible. The spectra from cytosine gave strong evidence for heteroatom protonation following electron addition while those from adenine were somewhat less clear. The spectra of uracil and all methylated pyrimidines gave no evidence for the protonation state of the electron adduct. For the ^·OH radical, the reaction with uracil and cytosine was found to be addition to the 5,6-double bond, in line with studies using aqueous solutions. For all methylated pyrimidines, however, H-abstraction from the methyl group was dominant. ^·OH addition to adenine was found to take place at C₂.