Transient Species Produced in the Photochemistry of 5-Bromouracil and Its N-Methyl Derivatives

Abstract

The flash photolysis of aqueous solutions of 1-methyl-5-bromouracil, 3-methyl-5-bromouracil, and 1, 3-dimethyl-5-bromouracil has been studied as a function of pH, in the presence and absence of oxygen and nitrous oxide. In neutral solutions, the main primary photolytic process is the formation of Br atoms and the corresponding debrominated uracil radical. Two transient species with maxima at ∼ 320 and 420 nm are formed and are suggested to be the ketyl radicals of the corresponding barbituric acid at the C2 and C4 positions, respectively. These ketyl radicals have as precursor the debrominated uracil radical. Results obtained from pulse radiolysis are used in support of this assignment. In alkaline solutions, in addition to debromination, photoionization of the anions of 5-bromouracil and of 3-methyl-5-bromouracil at the C2 position occurs with the formation of eaq− and the C2–alkoxy radical. The anion of 1-Me-5-BU at the C4 position and 1, 3-dimethyl-5-BU do not photoionize. Transient spectra obtained in the flash photolysis of 5-bromouracil atpH 13.0 are given and discussed.