Ab initio and semiempirical molecular orbital calculations are made for a heme‐containing model of the biologically active state of cytochrome P‐450 and for two chemical models of the enzyme, chromyl chloride (CrO2Cl2) and peroxytrifluoroacetic acid (CF3COOOH), known to yield similar oxidation products. The heme model for the transient activated state of P‐450 is made by analogy to a more stable active complex (compound I) formed by another heme enzyme, horse radish peroxidase with peroxides. The main aim of these studies is to characterize the active state, and in particular the electrophilic oxygen transferred to substrates by these enzymes. All models for the active state of P‐450 have negatively charged oxygen atoms and low‐energy virtual or half‐filled orbitals with substantial oxygen character. These combined results suggest that the electrophilic activity of the oxygen in all these compounds is related to overlap rather than charge‐controlled interactions with nucleophilic substrates. Calculations for a model of compound I itself yield a π cation porphyrin radical with two unpaired electrons in highly covalent nearly degenerate (dπ–Oπ) iron‐oxygen orbitals. Electric field gradients and quadrupole splittings calculated for both compound I and its reduced form (compound II) are in good agreement with experiment.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Condensed Matter Physics
- Physical and Theoretical Chemistry