The Peroxidase Activity of a Hemin−DNA Oligonucleotide Complex: Free Radical Damage to Specific Guanine Bases of the DNA

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Abstract
A specific DNA oligonucleotide−hemin complex (PS2.M−hemin complex) that exhibits DNA-enhanced peroxidative activity was studied by EPR and UV−visible spectroscopy and by chemical probing analysis. EPR data obtained from low-temperature experiments on the PS2.M−hemin complex showed both a low-field g ∼6 and a high-field g ∼2 signal. These EPR signals are typical of high-spin ferric heme with axial symmetry as judged by the EPR spectrum of six-coordinate heme iron in acidic Fe(III)-myoglobin. This similarity 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 analyses of the acid−base transition for the hemin complex yielded a pKa value for the water ligand of 8.70 ± 0.03 (mean ± SD). Low-temperature EPR analysis coupled with parallel spin-trapping investigations following the reaction of the PS2.M−hemin complex and hydrogen peroxide (H2O2) indicated the formation of a carbon-centered radical, most likely on the PS2.M oligonucleotide. Chemical probing analysis identified specific guanine bases within 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 might contribute to the superior peroxidative activity of the PS2.M−hemin complex.