Reactions of Oxidizing Radicals with 4,6-Dihydroxypyrimidines as Model Compounds for Uracil, Thymine, and Cystosine

Article abstract of DOI:10.1021/j100286a034

The reactions of the oxidizing radicals OH, O^-, SO4^-, and Br2^- with 4,6-dihydroxypyrimidine (4,6-DHP) and with 2- and 5-methyl-4,6-DHP were investigated by using in situ radiolysis and photolysis ESR and pulse radiolysis techniques.In the pH range 1-6 the OH radical reacts with these compounds -1 s^-1> selectively by addition at C-5 to produce radicals characterized by two pairs of magnetically equivalent nitrogens and exchangeable protons.On the basis of the coupling constants and from the increased pK_a value (8.3) of the radical from 4,6-DHP relative to that (5.3) of the parent compound, a diketo-type structure is suggested for the OH adducts.At pH 8-12 a base-catalyzed dehydration of the radical anions takes place which leads to the formation of species formally derived from the ionized parent compound by one-electron oxidation followed by deprotonation.These radicals can also be produced by reaction with SO4^- or Br2^-.Those from 4,6-DHP and from 2-Me-4,6-DHP were seen in two different protonation states: as a neutral radical between pH 3 and 5, and as the radical anion above pH ca. 5.Methyl groups on the 4,6-DHP molecule increase the rate of dehydration of the OH adducts.The O^- radical reacts with the methylated 4,6-DHP's by H abstraction from the methyl groups.The structures of all the radicals from the 4,6-DHP's are compared with those from naturally occuring dioxopyrimidines such as the uracils.