Nuclease initial characterization

The observation of 4 -hydroxylated abasic site 23 allowed to propose the mechanism summarized in Fig. 7 in association to the observation of similar oxidation products previously characterized with Fe-bleomycin or neocarzi-nostatin (4,7). It results in the initial formation of a C4 radical 6 due to H4 abstraction by activated Cu(phen)2. Then a C4 -hydroxylated compound is probably formed. It allows the release of the nucleobase and the formation of 4 -hydroxylated abasic site 23 that is not associated to DNA cleavage. This site was trapped by the authors as a pyridazine after reaction with hydrazine followed by an enzymatic digestion to nucleosides. Oyushi and Sugiyama proposed that a C4 carbocation was involved as intermediate in the reaction, as for Cl -DNA oxidation performed by activated Cu(phen)2. This hypothesis needs, however, to be confirmed since other evolutions of the C4 radical 6, producing also C4 -hydroxylated site, have been proposed with other chemical nucleases (4). However, in the case of DNA oxidation by activated Cu(phen)2 this oxidation pathway seemed minor when compared to the pathway leading to the formation of phosphoglycolate fragment 11. 

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