Indirect radiation effects on thymine in the glassy state of its dilute aqueous solution

Comparable amounts of OH and HO2 radicals are trapped in hyperquenched glas sy water gamma-irradiated at 77 K in a dose range up to about 20 kGy. Addit ion of thymine, in concentration as low as 0.005 M, changes their relative contributions, Thymine radicals (TH) are seen to be formed by addition of a tomic hydrogen already at 77 K at the expense of the relative contribution of HO2 radicals. This is not due to the scavenging of once-formed HO2 radic als by thymine, but it occurs concurrently with HO2 radical formation. Ther efore, it is rationalized by a concerted process involving the primary inte rmediate H2O+. Because of the large water-to-thymine ratio of approximate t o 10(4) in homogeneous glassy solution, the radiation effect on thymine mus t be indirect. In contrast, it is known that DNA in freeze-concentrated sol ution is exposed to either direct or "water-mediated" radiation effects whe re TH radical formation occurs at much higher temperature.