Reaction profile of the last step in cytochrome P-450 catalysis revealed by studies of model complexes |
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Authors: | Z Gross Shay Nimri Claudia M Barzilay Liliya Simkhovich |
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Institution: | (1) Department of Chemistry, Technion – Israel Institute of Technology, Haifa 32000, Israel Tel.: +972-4-8293954; Fax: +972-4-8233735; e-mail: chr10zg@tx.technion.ac.il, IL |
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Abstract: | A series of oxoiron(IV) porphyrin cation radical complexes was investigated as compound I analogs of cytochrome P-450. Both
the spectroscopic features and the reactivities of the complexes in oxygen atom transfer to olefins were examined as a function
of only one variable, the axial ligand trans to the oxoiron(IV) bond. The results disclosed two important kinetic steps – electron transfer from olefin to oxoiron(IV)
and intramolecular electron transfer from metal to porphyrin radical – which are affected differently by the axial ligands.
The large kinetic barrier of the latter step in the reaction of olefins with the perchlorato-bound oxoiron(IV) porphyrin cation
radical complex enabled the trapping of a reaction intermediate in which the metal, but not the porphyrin radical, is reduced.
The first electron transfer step is probably followed by σ-bond formation, which readily accounts for formation of isomerized
organic products at low temperatures. It is finally postulated that part of the enhanced oxygenation activities of cytochrome
P-450 monooxygenases and chloroperoxidases is due to a lowering of the energy barrier for the second electron transfer step
via participation of their redox-active cysteinate ligand.
Received: 16 January 1997 / Accepted: 24 May 1997 |
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Keywords: | Cytochrome P-450 Compound I Heme enzymes Oxoiron Porphyrin cation radicals |
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