Oxidation of 2‘-deoxyguanosine dG with the chemical nuclease Mn-TMPyP/KHSO5 leads within one minute to a nearly quantitative amount of 2-amino-5-[(2-deoxy-β-d-erythro-pentofuranosyl)amino]-4H-imidazol-4-one (dIz or imidazolone for short) (90%) in a non-oxygen-dependent pathway and without formation of 8-oxo-dG. This new mechanism of dIz formation involves, as a crucial first step, the abstraction of two electrons from guanine by a high-valent porphyrin−Mn(V)O species, leading to the dG+ cation instead of the classical dG•+ radical cation. We describe also the oxidation of imidazolone into the corresponding imidazolone N-oxide in the presence of KHSO5.

The mechanism of oxidation of guanine residues on double-stranded oligonucleotides (ODNs) by the chemical nuclease Mn-TMPyP/KHSO5 is reported. By using HPLC coupled to an electrospray mass spectrometer (ESI/MS) the different oxidized ODN strands were directly analyzed, and labeling experiments in H2 18O allowed us to propose a two-electron oxidation mechanism for guanine residues engaged in double-stranded DNA. We found that the imidazolone derivative (dIz) was formed by trapping of a guanine-cation by a water molecule. Two reaction intermediates on the pathway of the formation of dIz were observed:  5,8-dihydroxy-7,8-dihydroguanine and an oxidized guanidinohydantoin intermediate. Furthermore, a secondary route of guanine oxidation leading to parabanic acid was also evidenced. The mechanism of the different routes of guanine oxidation in double-stranded DNA has been discussed in detail.

The cationic metalloporphyrin Mn-TMPyP activated by KHSO(5) has been used as cleaver of an oligonucleotide containing the four human telomere repeats of 5'-GGGTTA. This oligonucleotide formed an intramolecular quadruplex DNA under 200 mM KCl as probed by DMS footprinting and could fold into different quadruplex structures under 200 mM NaCl. We found that the oxo-metalloporphyrin was able to mediate efficient oxidative cleavage of the quadruplex. The location of damage showed that the metalloporphyrin was able to bind to the last G-tetrad of the quadruplex structure via an external interaction. This metalloporphyrin-G-tetrad interaction needs a relatively high flexibility of the single-stranded linker regions to allow the partial stacking of the metalloporphyrin with the last G-tetrad planar structure. The oxidative damage consisted of guanine oxidation within the interacting G-tetrad together with an 1'-carbon hydroxylation of deoxyribose residues of the thymidine residues located on the neighboring single-stranded loop. So the high-valent oxo-metalloporphyrin is able to mediate both electron-abstraction or H-abstraction on G or T residues, respectively, within the DNA quadruplex target.