Guanine oxidation in double-stranded DNA by Mn-TMPyP/KHSO5: 5,8-dihydroxy-7,8-dihydroguanine residue as a key precursor of imidazolone and parabanic acid derivatives

The mechanism of oxidation of guanine residues on double-stranded oligonucl eotides (ODNs) by the chemical nuclease Mn-TMPyP/KHSO5 is reported. By usin g HPLC coupled to an electrospray mass spectrometer (ESI/MS) the different oxidized ODN strands were directly analyzed, and labeling experiments in (H 2O)-O-18 allowed us to propose a two-electron oxidation mechanism for guani ne residues engaged in double-stranded DNA. We found that the imidazolone d erivative (dIz) was formed by trapping of a guanine-cation by a water molec ule. Two reaction intermediates on the pathway of the formation of dIz were observed: 5,8-dihydroxy-7,8-dihydroguanine and an oxidized guanidinohydant oin intermediate. Furthermore, a secondary route of guanine oxidation leadi ng to parabanic acid was also evidenced. The mechanism of the different rou tes of guanine oxidation in double-stranded DNA has been discussed in detai l.