Free radical chain mechanism in the acetone-sensitized photolysis of 5-bromouracil and derivatives in alcohol-water mixtures

Abstract

The acetone-sensitized photolysis of 5-bromouracil (BrU), 1-methyl-5-bromouracil (MeBrU) and 5-bromouridine (BrUd) in deoxygenated aqueous solution in the presence of alcohols was studied on irradiation at 254 nm. BrU is converted to uracil with the release of Br −, as shown by UV analysis and high performance liquid chromatography. The quantum yield of the BrU → uracil photoconversion, using 0.1 M acetone and approximately 1 M 2-propanol, increases linearly with the inverse square root of the intensity ( I 254) and approaches values of 15 at pH 13 for I 254=0.06 mW cm −2. Comparable results were obtained for MeBrU and BrUd; the maximum Φ values are 12 and 4 respectively. Φ(−BrU) decreases significantly with decreasing pH and decreasing concentrations of either acetone or 2-propanol (at a given pH). No evidence for the involvement of the triplet state of BrU in the photoreduction was found. Φ(−BrU) is reduced when 2-propanol is replaced by methanol and significantly reduced in the presence of tert-butanol. To account for the chain reaction in neutral solution, it is suggested that the (CH 3) 2·COH radical is initially formed by H-atom abstraction by triplet acetone from 2-propanol; propagation occurs by electron transfer to BrU, Br − release and H-atom abstraction by the uracil-5-yl radical from 2-propanol. The enhancement of Φ(−BrU) in the alkaline pH range is ascribed to a lower reactivity of the termination reaction with respect to the competing electron transfer from the (CH 3) 2·CO − radical to BrU.