Acetone-sensitized photolysis of 5-iodouracil and 5-iodouridine in aqueous solution in the presence of alcohols: free radical chain mechanism

Abstract

The photolysis of 5-iodouracil (IU) and 5-iodouridine (IUd) in deoxygenated aqueous solution was studied as a function of pH and in the presence of alcohols. UV and high performance liquid chromatography (HPLC) analysis shows that uracil and I − are the photoproducts of IU. The quantum yield Φ IU→HU of uracil formation from IU in the presence of 2-propanol (1.4 M), using continuous 254 nm radiation, increases with pH from 0.058 in neutral solution to 0.4 at pH 13. The conversion is significantly enhanced by acetone (0.12 M); for an intensity of I 254=2.2 mW cm -2, Φ IU→HU-0.9 and 10 at pH 7 and 12 respectively. Linear dependences of Φ IU→HU vs. I 254 − 1 2 were obtained and, at fixed pH and I 254, Φ IU→HU values decrease with decreasing concentrations of acetone or 2-propanol. The chain reaction is initiated by hydrogen-atom abstraction by triplet acetone from 2- propanol. The suggested propagation sequence is electron transfer from (CH 3) 2 COH/(CH 3) 2 CO − radicals to IU, thereby yielding acetone, I − and the uracilyl radical, which regenerates the alcohol radical by hydrogen- atom abstraction from 2-propanol. The increase in Φ IU→HU with increasing pH is mainly ascribed to a higher efficiency of electron transfer from the (CH 3) 2 .CO − radical to the IU anion than from the (CH 3) 2 .COH radical and to the slower termination reaction of (CH 3) 2 .CO −. Similar dependences and comparable Φ values were obtained for IUd. The chain reaction is suppressed by traces of oxygen. An indication for a chain reaction also in the absence of acetone is based on the linear dependence of Φ IU→HU vs. I 254 − 1 2 at pH 13.