Cyclohexadienylperoxyl radicals

Using basic pH leads to higher plateau rate constants, indicating that the ratedetermining step is reaction 18. Reaction 17 must be at least as fast as the rate of 02 addition in the highest 02 concentration used, kn 8 x 105 s 1, which is the limit of the instrument measurement. The G of benzene in pulse radiolysis was found to be equal to that of the nitroform anion (1.6 x 10-7 molJ-1) as can be expected from reactions 17-19. Since the yield of the cyclohexadienylperoxyl radical is 2.9 x 10-7 mol. 1 1 it means that only a fraction (ca 60%) of the cyclohexadienylperoxyl radicals eliminates HO2. The H02 elimination occurs by H-transfer of the allylic hydrogen to the oxygen... 

In this context it is worth considering that in the free-radical chemistry of DNA the C(6)- OH-adducts radicals are certainly of a greater importance than the C(6)-H-adduct radicals investigated here. If the benzene system is a good guide H02--elimination from hydroxycyclohexadienylperoxyl radicals is noticeably slower than that from cyclohexadienylperoxyl radicals (Pan et al. 1993a), i.e., the rate of H02--elimination from 5,6-dihydro-6-hydroxy-thymidine-5-per-oxyl radicals may even be slower. 

Since the yield of the cyclohexadienylperoxyl radical is 2.9 x 10 molJ it means that only a fraction (ca 60%) of the cyclohexadienylperoxyl radicals eliminates HO2. The HO2 elimination occurs by H-transfer of the allylic hydrogen to the oxygen... 

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