A DNA Oligonucleotide−Hemin Complex Cleavest-Butyl Hydroperoxide through a Homolytic Mechanism

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Abstract
Both electron paramagnetic resonance (EPR) and electronic absorption spectroscopy have been employed to investigate the reaction of a guanine-rich DNA nucleotide−hemin complex (PS2.M−hemin complex) and organic peroxide (t-Bu-OOH). Incubation of the PS2.M−hemin complex with t-Bu-OOH resulted in the time-dependent decrease in the heme Soret with concomitant changes to the visible bands of the electronic absorbance spectrum for the PS2.M−hemin complex. Parallel EPR studies using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) combined with spectral simulation demonstrated the presence of tert-butyloxyl, carbon-centered methyl, and methyl peroxyl radicals as well as a simple nitroxide (triplet) signal. Experiments, performed by maintaining a constant ratio of t-Bu-OOH/PS2.M−hemin complex (∼35 mol/mol) while varying DMPO concentration, indicated that the relative contributions of each radical adduct to the composite EPR spectrum were significantly influenced by the DMPO concentration. For example, at DMPO/PS2.M−hemin of 10−50 mol/mol, a complex mixture of radicals was consistently detected, whereas at high trapping efficiency (i.e., DMPO/PS2.M−hemin of ∼250 mol/mol) the tert-butyloxyl−DMPO adduct was predominant. In contrast, at relatively low DMPO/PS2.M−hemin complex ratios of ≤5 mol/mol, a simple nitroxide three-line EPR signal was detected largely in the absence of all other radicals. Together, these data indicate that tert-butyloxyl radical is the primary radical likely formed from the homolytic cleavage of the O−O peroxy bond of t-Bu-OOH, while methyl and methyl peroxyl radicals result from β-scission of the primary tert-butyloxyl radical product.

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