New Aspects in the Free-Radical Chemistry of Pyrimidine Nucleobases

i) It has been known for some time that OH radicals and H atoms react with the pyrimidines by adding to the C(5)-C(6) double bond, but only the u.v.-spectra of the sum of these radicals have been reported so far. It will be shown how to arrive at the individual spectra of the C(5) and the C(6) adduct radicals. ii) alpha-Hydroxyalkyl radicals are known to inactivate biologically active DNA. In contrast to the electrophilic radicals H and OH they are nucleophilic and the hydroxymethyl radicals add exclusively at the C(6) position of 1,3-dimethyluracil (k approximately 10(4) dm3 mol-1 s-1). In the corresponding thymine derivative this reaction also occurs, but one third of the hydroxymethyl radicals abstract an H-atom from the C(5)-methyl group thereby forming an allylic radical. In the course of these reactions pyrimidines with an exocyclic double bond are formed. These products react much more rapidly with hydroxymethyl radicals than the starting material leading to highly hydroxymethylated material at very low doses. iii) The direct effect of ionizing radiation which would produce a pyrimidine base radical cation can be mimicked by reacting the pyrimidine with SO4-, a very good electron acceptor. In water, the radical cation of 1,3-dimethyluracil is rapidly (t1/2 less than 2 microseconds) converted into the C(5) OH adduct radical. In the presence of peroxodisulphate a chain reaction sets in which leads to the cis-glycol.