Modeling Heme Peroxidase: Heme Saddling Facilitates Reactions with Hyperperoxides To Form High‐Valent Fe^IV‐Oxo Species

Abstract

Saddle-shaped hemes have been discovered in the structures of most peroxidases. How such a macrocycle deformation affects the reaction of Fe III hemes with hydrogen peroxide (H 2 O 2 ) to form high-valent Fe-oxo species remains uncertain. Through examination of the ESI-MS spectra, absorption changes and 1 H NMR chemical shifts, we investigated the reactions of two Fe III porphyrins with different degrees of saddling deformation, namely Fe III (OETPP)ClO 4 (1 OE ) and Fe III (OMTPP)ClO 4 (1 OM ), with tert-butyl hydroperoxide (tBuOOH) in CH 2 Cl 2 at À 40 °C, which quickly resulted in OÀ O bond homolysis from a highly unstable Fe III -alkylperoxo intermediate, Fe III -O(H)OR (2) into Fe IV -oxo porphyrins (3).Insight into the reaction mechanism was obtained from [tBuOOH]-dependent kinetics. At À 40 °C, the reaction of 1 OE with tBuOOH exhibited an equilibrium constant (K a = 362.3 M À 1 ) and rate constant (k = 1.87 × 10 À 2 s MÀ > 1 ) for the homolytic cleavage of the 2 OÀ O bond that were 2.1 and 1.4 times higher, respectively, than those exhibited by 1 OM (K a = 171.8 M À 1 and k = 1.36 × 10 À 2 s À 1 ). DFT calculations indicated that an Fe III porphyrin with greater saddling deformation can achieve a higher HOMO ([Fe(d z 2 ,d x 2 À y 2 )-porphyrin(a 2u )]) to strengthen the orbital interaction with the LUMO (OÀ O bond σ*) to facilitate OÀ O cleavage.