ACETONE-SENSITIZED PHOTOLYSIS OF 5-IODOURACIL AND 5-IODOURIDINE IN AQUEOUS-SOLUTION IN THE PRESENCE OF ALCOHOLS - FREE-RADICAL CHAIN MECHANISM

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 phi(IV-->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 I254 = 2.2 mW cm-2, phi(IU-->HU) = 0.9 and 10 at pH 7 and 12 respectively. Linear dependences of PHI(IU-->HU) vs. I254(-1/2) were obtained and, at fixed pH and I254, phi(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 (CH3)2.COH/(CH3)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 PHI(IU-->HU) with increasing pH is mainly ascribed to a higher efficiency of electron transfer from the (CH3)2.CO- radical to the IU anion than from the (CH3)2.COH radical and to the slower termination reaction Of (CH3)2.CO-. Similar dependences and comparable PHI 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 PHI(IU-->HU) vs. I254(-1/2) at pH 13.