Ferric Heme Superoxide Reductive Transformations to Ferric Heme (Hydro)Peroxide Species: Spectroscopic Characterization and Thermodynamic Implications for H‐Atom Transfer (HAT)

• Published in: Angewandte Chemie International Edition 2021-03, Vol.60 (11), p.5907-5912
• Main Authors: Kim, Hyun, Rogler, Patrick J., Sharma, Savita K., Schaefer, Andrew W., Solomon, Edward I., Karlin, Kenneth D.
• Format: Article
• Language: English
• Subjects: bond-dissociation free energy; end-on peroxides; Energy of dissociation; Ferric Compounds - chemistry; Ferric Compounds - metabolism; Free energy; Heme; Heme - chemistry; Heme - metabolism; Hydrogen - chemistry; Hydrogen - metabolism; Hydrogen bonds; hydrogen-atom transfer; Hydroxylamine; Molecular Structure; Oxidation; Oxidation-Reduction; Peroxide; Peroxides - chemistry; Peroxides - metabolism; Spectrophotometry, Ultraviolet; square scheme; Substrates; Superoxide; Superoxides - chemistry; Superoxides - metabolism; thermodynamic parameters; Thermodynamics
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202013791

A new end‐on low‐spin ferric heme peroxide, [(PIm)FeIII−(O22−)]− (PIm‐P), and subsequently formed hydroperoxide species, [(PIm)FeIII−(OOH)] (PIm‐HP) are generated utilizing the iron‐porphyrinate PIm with its tethered axial base imidazolyl group. Measured thermodynamic parameters, the ferric heme superoxide [(PIm)FeIII−(O2⋅−)] (PIm‐S) reduction potential (E°′) and the PIm‐HP pKa value, lead to the finding of the OO−H bond‐dissociation free energy (BDFE) of PIm‐HP as 69.5 kcal mol−1 using a thermodynamic square scheme and Bordwell relationship. The results are validated by the observed oxidizing ability of PIm‐S via hydrogen‐atom transfer (HAT) compared to that of the F8 superoxide complex, [(F8)FeIII−(O2.−)] (S) (F8=tetrakis(2,6‐difluorophenyl)porphyrinate, without an internally appended axial base imidazolyl), as determined from reactivity comparison of superoxide complexes PIm‐S and S with the hydroxylamine (O‐H) substrates TEMPO‐H and ABNO‐H. Thermodynamic comparisons for O2‐derived iron‐porphyrinate interrelated ferric heme superoxide, peroxide, and hydroperoxide analogues in the presence and absence of an appended imidazolyl axial base are presented.