Site of H atom attack on uracil and its derivatives in aqueous solution

Hydrogen atoms from the radiolysis of water at pH 1.6 add to the 5,6-double bond of pyrimidines. The preferential site of attack is the C(5) position (values in brackets) in the case of 6-methyluracil (87%), 1,3-dimethyluracil (71%), uracil (69%) and poly(U) (60%). This reaction yields a radical of reducing properties which can be monitored by its reaction with tetranitromethane in a pulse radiolysis experiment. In thymine (37%), thymidine (32%) and 1,3-dimethylthymine (25%) H-addition no longer preferentially occurs at C(5), but addition is now mainly at C(6). Hydrogen abstraction from the methyl groups or the sugar moiety is negligible (less than or equal to 5.5%). A comparison is made with literature values for the equivalent reactions of OH radicals.