REACTIVITY OF 5,6-DIHYDRO-5-HYDROXYTHYMID-6-YL GENERATED VIA PHOTOINDUCED SINGLE-ELECTRON TRANSFER AND THE ROLE OF CYCLOHEXA-1,4-DIENE IN THE PHOTODEOXYGENATION PROCESS

The major reactive species formed via reaction of hydroxyl radical and the pyrimidine nucleoside thymidine, 5,6-dihydro-5-hydroxythymid-6-yl (1), is generated photochemically under anaerobic conditions from 6 v ia photoinduced single electron transfer. Under the conditions in whic h it is generated, 1 is trapped by hydrogen atom donors to form thymid ine C5-hydrate (7), and undergoes oxidation, resulting in the formatio n of thymidine glycol (8). Isotopic (H-2, O-18) labeling experiments i ndicate that dehydration of 1 is not competitive with intermolecular h ydrogen atom donation by 3,3,6,6-tetradeuteriocyclohexa-1,4-diene. Ext rapolation of the known rate constants for hydrogen atom donation by c yclohexa-1,4-diene to alkyl radicals suggests that intramolecular hydr ogen atom abstraction and dehydration are <2 s(-1), and are not kineti cally competent to be involved in nucleic acid strand scission that ar ises from 1. Relative quantum yields for disappearance of 6 in the pre sence and absence of cyclohexa-1,4-diene suggest that the diene reduce s the N-methylcarbazole cation radical, preventing back electron trans fer. Labeling studies using 3,3,6,6-tetradeuteriocyclohexa-1,4-diene a nd 1,2,3,4,5,6-hexadeuteriocyclohexa-1,4-diene suggest that the result ing olefin cation radical, or other reactive species derived from the trap, competes with cyclohexa-1,4-diene for 1.