Reactions of G -H Radicals with Nucleophiles

The combination of G(-H) and N02 radicals was directly monitored by the appearance of the characteristic absorption band of nitroguanine products near 380 nm that correlates with the decay of the transient absorbance of G(-H) radicals 

The 2AP radical cations, 2AP +, formed together with hydrated electrons by the photoionization process, rapidly deprotonate to the neutral radicals, 2AP(-H)- [31]. The latter radicals selectively oxidize nearby G bases to form G(-H) radicals - a process that is completed within 100gs after the actinic laser flash. Combination of the G(-H) and N02 radicals occurs on millisecond time scales with similar rate 

Another interesting example of radical-radical reactions is the combination of G(-H) and 02 radicals [29], In healthy tissues, the concentrations of 02 radicals is small and is regulated by SODs [70, 71]. These enzymes rapidly deactivate 07 radicals by catalytic dismutation to oxygen and hydrogen peroxide  

The rate of combination of the 02 and G(-H) radicals positioned in single- and double-stranded oligonucleotides can be determined from the decay kinetics of the characteristic narrow absorption band of G(-H) radicals at 315 nm [29]. The rate constant of this bimolecular combination reaction was found to be around 4.7 x 108M 1s 1. The Cu,Zn-SOD reacts with 02 radicals with nearly diffusion-controlled rates [74, 75] and thus dramatically enhances the lifetimes of the DNA-bound G(-H) radicals from 4— 7ms to 0.2-0.6 s in the presence of micromolar concentrations of Cu,Zn-SOD (around 5gM) [29]. Thus, radical-radical combination reactions can play important roles in shortening the lifetimes of guanine radicals in DNA, as mentioned earlier. 

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