New insights into the reaction paths of hydroxyl radicals with 2'-deoxyguanosine.

The reaction of HO(•) radical with 2'-deoxyguanosine is intensively studied as a model for DNA damage. Several aspects related to the reaction paths responsible for the most relevant lesions are not well understood. We have reinvestigated the reaction of HO(•) with 2'-deoxyguanosine by pulse radiolysis and extended our studies to a variety of substituted derivatives. The main path of hydrogen abstraction was confirmed to be from the exocyclic NH(2) group, followed by a water-assisted tautomerization. The rate constant (k = 2.3 × 10(4) s(-1)) obtained from the spectral changes at 620 nm is influenced by the substituent at the C8 position. When N1-H is replaced by N1-CH(3), the tautomerization does not occur. The spectral changes at 370 nm that correspond to a rate constant of 6.9 × 10(5) s(-1) were assigned to the cyclization of 2'-deoxyguanosin-5'-yl radical with formation of 5',8-cyclo-2'-deoxyguanosine as the product. When NEt(2) replaces the exocyclic NH(2), the spectral changes at all wavelengths follow second-order kinetics, suggesting a "slow" ring-opening of the 8-hydroxyl adduct of 2'-deoxyguanosine.