The reactions of the hydroxymethyl radical with 1,3-dimethyluracil and 1,3-dimethylthymine

Hydroxymethyl radicals .CH2OH, generated by the radiolysis of methanol (0.5 mol dm-3) in N2O-saturated aqueous solutions, were reacted with 1,3-dimethyluracil or 1,3-dimethylthymine (10(-3) mol dm-3). The products were identified and their G values determined. It has been concluded that in 1,3-dimethyluracil .CH2OH attack occurs only at C(6) while in 1,3-dimethylthymine there is partitioning between addition (two-thirds) and H-abstraction from the C(5)-methyl group (one-third). A rate constant for CH2OH addition to 1,3-dimethyluracil of about 10(4) dm3 mol-1 s-1 is estimated. Complexities that may arise in the radiolysis of pyrimidines such as 1,3-dimethylthymine, apparently as a consequence of the formation of 5-alkylidenepyrimidines, are discussed. A value of 0.15 has been estimated for the disproportionation/combination ratio for the hydroxymethyl radical self-termination reaction.