The peroxidase activity of a hemin-DNA oligonucleotide complex: Free radical damage to specific guanine bases of the DNA

A specific DNA oligonucleotide-hemin complex (PS2.M-hemin complex) that exh ibits DNA-enhanced peroxidative activity was studied by EPR and UV-visible spectroscopy and by chemical probing analysis. EPR data obtained from low-t emperature experiments on the PS2.M-hemin complex showed both a low-field g similar to6 and a high-field g similar to2 signal. These EPR signals are t ypical of high-spin ferric heme with axial symmetry as judged by the EPR sp ectrum of six-coordinate heme iron in acidic Fe(III)-myoglobin. This simila rity is consistent with the presence of two axial ligands to the heme iron within the PS2.M-hemin complex, one of which is a water molecule. Optical a nalyses of the acid-base transition for the hemin complex yielded a pK(a) v alue for the water ligand of 8.70 +/- 0.03 (mean +/- SD). Low-temperature E PR analysis coupled with parallel spin-trapping investigations following th e reaction of the PS2.M-hemin complex and hydrogen peroxide (H2O2) indicate d the formation of a carbon-centered radical, most likely on the PS2.M olig onucleotide. Chemical probing analysis identified specific guanine bases wi thin the PS2.M sequence that underwent oxidative damage upon reaction with H2O2 These and other experimental findings support the hypothesis that the interaction of specific guanines of PS2.M with the bound hemin cofactor mig ht contribute to the superior peroxidative activity of the PS2.M-hemin comp lex.