E.S.R. Spectra of Free Radicals Induced in Nucleic Acid Purine and Pyrimidine Bases and Selected Analogues by Exposure to Hydrogen Atoms

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Abstract
Purine and pyrimidine bases, selected to permit determination of effects of structure, were exposed to hydrogen or deuterium atoms within an E.S.R. cavity. Spectra were recorded at intervals during the exposure. Conclusions about pyrimidines are: atoms add to all pyrimidines tested; addition is to the 5–6 double bond of 1,3-dideuterouracil; methyl groups on carbons 5 or 6 exert a dominant directive effect on addition; dihydropyrimidinyl radicals appear to have a near planar conformation at room temperature, but 1,3-dimethyldihydropyrimidinyl radicals are non-planar; steric blocking of H atom addition appears to be unimportant; and oxo and amino groups on carbon 4 have slightly different effects on the electronic structure of the radicals. Reaction of H atoms with 5,6-dihydropyrimidines gives the same radicals by abstraction as are formed by addition to the corresponding pyrimidine. All purines tested give radicals with nearly identical electronic structures.