Electron Gain and Electron Loss Radicals Stabilized on the Purine and Pyrimidine of a Cocrystal Exhibiting Base-Base Interstacking: ESR-ENDOR of X-Irradiated Adenosine:5-Bromouracil

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Abstract
The predominant free radicals trapped in cocrystals of adenosine:5-bromouracil X-irradiated at 12.degree. K were identified by ESR-ENDOR [electron nuclear double resonance] spectroscopy and the radical reactions were followed upon annealing to 480.degree. K. The dominant electron abstraction and electron addition products stabilized on the bases at 12.degree. K are observed to be the bromouracil .pi.-cation and the adenine .pi.-cation and .pi.-anion. The formation of an anion on bromouracil is inferred from the presence of a radical formed by deuterium addition to C6 of bromouracil at higher temperatures. Above 40.degree. K the bromouracil .pi.-cation appears to decay by recombination and is reduced to undetectable levels at .apprx. 170.degree. K. Both adenine .pi.-ions are also observed to decay within the same temperature range. Above 200.degree. K hydrogen adducts are stabilized on the bases. Experiments using partially deuterated cocrystals indicate that the H-adducts are formed via both H addition and protonation of the respective anions. Two H abstraction radicals stabilized on the sugar residue are detectable at temperatures above 200.degree. K, but these may be present at much lower temperatures. In the presence of purine:pyrimidine stacking interactions, holes are predominantly transferred to the purines while electrons are predominantly transferred to the pyrimidines.