Modeling Heme Peroxidase: Heme Saddling Facilitates Reactions with Hyperperoxides To Form High-Valent FeIV -Oxo Species.

Saddle-shaped hemes have been discovered in the structures of most peroxidases. How such a macrocycle deformation affects the reaction of FeIII hemes with hydrogen peroxide (H2 O2 ) 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 FeIII porphyrins with different degrees of saddling deformation, namely FeIII (OETPP)ClO4 (1OE ) and FeIII (OMTPP)ClO4 (1OM ), with tert-butyl hydroperoxide (tBuOOH) in CH2 Cl2 at -40 °C, which quickly resulted in O-O bond homolysis from a highly unstable FeIII -alkylperoxo intermediate, FeIII -O(H)OR (2) into FeIV -oxo porphyrins (3). Insight into the reaction mechanism was obtained from [tBuOOH]-dependent kinetics. At -40 °C, the reaction of 1OE with tBuOOH exhibited an equilibrium constant (Ka =362.3 M-1 ) and rate constant (k=1.87×10-2  sM->1 ) for the homolytic cleavage of the 2 O-O bond that were 2.1 and 1.4 times higher, respectively, than those exhibited by 1OM (Ka =171.8 M-1 and k=1.36×10-2   s-1 ). DFT calculations indicated that an FeIII porphyrin with greater saddling deformation can achieve a higher HOMO ([Fe(d z 2 ,d x 2 - y 2 )-porphyrin(a2u )]) to strengthen the orbital interaction with the LUMO (O-O bond σ*) to facilitate O-O cleavage.