Model Fe(III) Porphyrin Systems: Axial Ligation in Oxidized Cytochrome P-450.S

G. C. Papaefthymiou, Massachusetts Institute of Technology
Richard B. Frankel, Massachusetts Institute of Technology
S. Foner, Massachusetts Institute of Technology
S. C. Tang, Massachusetts Institute of Technology
S. Koch, Massachusetts Institute of Technology
R. H. Holm, Stanford University

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Copyright © 1976 EDP Sciences. The original publication is available at http://dx.doi.org/10.1051/jphyscol:1976644.

NOTE: At the time of publication, the author Richard B. Frankel was not yet affiliated with Cal Poly.

Abstract

In an effort to develop useful experimental criteria to distinguish among the viable axial ligation possibilities of O, N, and S coordination in oxidized cytochrome P-450. S (substrate-bound, high-spin), syntheticmodel compounds with O-(Fe(PPIXDME) (OEt) and Fe(PPIXDME) (OC₆H₄NO₂)) and S-(Fe(PPIXDME) (S₆H₄NO₂) and Fe(OEP) (SPh)) donor axial ligands were prepared and studied by Mössbauer spectroscopy. Ferric hemoglobin and myoglobin were used as model compounds with N-axial coordination. The isomer shifts (δ) and quadrupole splittings (ΔE_Q) observed are typical of high-spin Fe(III) porphyrins and are comparable with results for ox-P-450.S. However, these parameters do not exhibit a cleSarcut dependence on the nature of the axial ligand and cannot be used for identification of enzyme ligation. Mössbauer spectra in external magnetic fields (H₀ ≤ 120 kOe) together with magnetization measurements (H₀ ≤ 60kOe) were, therefore, obtained and analyzed assuming a spin-Hamiltonian, He = D[S²_z - 1/3 S(S + 1] + E(S²_x - S²_y) + 2 β H.S, in order to obtain estimates of the crystal field parameters, D and E, and the saturation magnetic hyperfine field, H⁰_hf, in the PPIXDME series. For ferric myoglobin and the oxygen-ligated porphyrins - H⁰_hf ≥ 500 kOe, Considerably larger than the value of - 450 kOe observed for ox-P-450. S. Only in the thiolate complex Fe(PPIXDME) (SC₆H₄NO₂) for which - H⁰_hf = 476 ± 10 kOe is the enzyme value reasonably closely approached. In addition, magnetization and EPR measurements show that complexes with O- and N- donor ligands have axial or near axial symmetry while Fe(PPIXDME) (SC₆H₄NO₂) and Fe(OEP) (SPh) show deviations from axial symmetry with E/D ≈ 0.05, which compares more favorably with the unusually large degree of rhombicity, E/D ≈ 0.087, observed for ox-P-450.S. These observations support the designation of cysteinate sulfur as the axial ligand in ox-P-450. S.