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Reaction profile of the last step in cytochrome P-450 catalysis revealed by studies of model complexes (1997)

Gross, Z. ; Nimri, Shay ; Barzilay, Claudia M. ; [et al.]

Springer

JBIC 2 (1997), S. 492-506

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ISSN: 1432-1327

Keywords: Key words Cytochrome P-450 ; Compound I ; Heme enzymes ; Oxoiron ; Porphyrin cation radicals

Source: Springer Online Journal Archives 1860-2000

Topics: Biology , Chemistry and Pharmacology

Notes: 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.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s007750050161

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Elucidation of Factors Affecting the Electronic Structures of Magnesium(II) and Zinc(II) Tetraarylporphyrin Radical Cations (1995)

Barzilay, Claudia M. ; Sibilia, Sharon A. ; Spiro, Thomas G. ; [et al.]

Weinheim : Wiley-Blackwell

Chemistry - A European Journal 1 (1995), S. 222-231

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ISSN: 0947-6539

Keywords: electronic structure ; frontier orbitals ; metalloporphyrins ; radical cations ; Chemistry ; General Chemistry

Source: Wiley InterScience Backfile Collection 1832-2000

Topics: Chemistry and Pharmacology

Notes: A series of magnesium and zinc tetraarylporphyrins and their porphyrinoxidized derivatives were studied by UV/Vis, ESR, and resonance Raman spectroscopy at various temperatures. The series included tetra(meta-dichlorophenyl)porphyrinatozinc (5), tetra(ortho-dichlorophenyl)porphyrinatozinc (6), tetra(ortho-difluorophenyl)porphyrinatozinc and -magnesium (9 and 10), and tetra(pentafluorophenyl)porphyrinatozinc and -magnesium (7 and 8). The radical cations (3a-10a) were isolated by chemical one-electron oxidation of their neutral precursors (3-10). Despite the structural similarity of all these radicals, their electronic ground state varied within the series. The position of the chloro groups was found to play a key role. While the radical cation of the meta-dichloro-substituted derivative 5a exhibited A2u spectroscopic features, the ortho-dichlorophenyl derivative (6a) showed A1u spectral features. Radicals of the fluoro-substituted porphyrins, especially that of 10, were found to have state-admixed (A1u/A2u) electronic structures, and the relative contributions of the two states was found to vary with temperature and to depend on the axial ligand. The results indicate that the fluoro-substituted porphyrins are primarily A2u at low temperature, even though their room temperature spectroscopic features resemble those of A1u cations. The elucidation of factors that affect the electronic structures of the radicals in the present series is helpful in providing a greater understanding of the spin-spin interactions in the intermediates of heme-dependant enzymatic reactions and their synthetic analogues.

Additional Material: 5 Ill.