Cytochrome P450 monooxygenases (CYP450s) involved in critical biological processes catalyze the incorporation of one atom of molecular oxygen into biological substrates. The catalytic cycle of CYP450s includes key intermediates: oxy-heme (i.e., the ferric-superoxide), peroxide, hydroperoxide and high-valent ferry-oxo (i.e., Compounds I and II). Over the years, the generation and spectroscopic characterization of these analogs have been of great interest. In this presentation, we describe the generation of superoxide, peroxide and hydroperoxide analogs using the F8 (F8 = tetrakis(2,6-difluorophenyl)porphyrinate) heme framework through electron reduction and protonation processes and characterization with UV-vis, EPR and resonance Raman spectroscopies. The peroxide complex [(F8)FeIII(O22–)]– converts to [(F8)FeIII(O2–)] using a strong oxidant, Magic Blue. When Cr(η-C6H6)2 is used as a reductant added to the superoxide complex, an equilibrium mixture of superoxide and peroxide complexes is formed; a redox potential for the [(F8)FeIII(O2–)]/[(F8)FeIII(O22–)]– couple could then be determined. The hydroperoxide complex, [(F8)FeIII(OOH)], is deprotonated to the peroxide analog using a derivatized phosphazene base, PtE2(dma), giving an equilibrium mixture of hydroperoxide and peroxide complexes; thus, a pKa for the hydroperoxide analog could be determined. With these thermodynamic parameters, the above-mentioned reduction potential and pKa, an O–H bond dissociation free energy (BDFE) of [(F8)FeIII(OOH)] is calculated using a thermodynamic square scheme and the Bordwell relationship. The BDFE thus determined data defines or estimates the oxidative capability of the ferric heme superoxide complex. Consistent with our calculated BDFE, [(F8)FeIII(O2–)] reacts with TEMPO–H (2,2′-6,6′-tetramethylpiperidine-1-ol) to give a TEMPO radical and hydroperoxide complex via net hydrogen atom transfer.

Thermochemistry of a ferric heme hydroperoxide complex: Hydrogen atom abstraction reactivity of the superoxide partner complex

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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