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Synthesis, structure, and spectroscopic properties of five-coordinate mercaptoiron(II) porphyrins. Models for the reduced state of cytochrome P450
Schappacher,Ricard,Fischer,Weiss,Montiel-Montoya,Bill,Trautwein
, p. 4639 - 4645 (2008/10/08)
The five-coordinate, high-spin (S = 2) (ethanethiolato)- and (tetrafluorobenzenethiolato)iron(II) picket-fence porphyrin derivatives [Fe(TPpivP)(SC2H5)][Na(C222)] (1), [Fe(TPpivP)(SC6HF4)][Na(C222)] (2), and [Fe(TPpivP)(SC6HF4)][Na(Cl8c6)] (3) have been synthesized and characterized. The X-ray structure of the chlorobenzene solvate of 1 has been determined. [Fe(TPpivP)(SC2H5)][Na(C222)]·C 6H5Cl (C90H110N10O10 NaSClFe) crystallizes in the monoclinic system with a = 23.394 (7) A?, b = 21.762 (7) A?, c = 17.890 (6) A?, β = 103.62 (2)°, Z = 4, and space group P21/n (Cu Kα radiation). The average Fe-Np bond distance is 2.074 (10) A?. The displacement of iron out of the 4 Np mean plane is 0.44 A?. The ethanethiolato axial ligand of iron lies in the molecular cavity formed by the four pivalamido groups of the picket-fence porphyrin. The Fe-S bond distance of 2.324 (2) A? is in complete agreement with the Fe-S bond length found in the reduced ferrous state of cytochrome P450 by EXAFS spectroscopy. Experimental Mossbauer spectra of 1-3 exhibit isomer shifts that are typical for five-coordinate ferrous high-spin porphyrins and that are practically identical with that reported for reduced P450. Additionally the observed quadrupole splitting (ΔEQ) of 1-3 is comparable to that of reduced P450, and especially the temperature dependence of ΔEQ from 3 between 4.2 and 200 K is very similar to that of the reduced enzyme. The Mo?ssbauer spectra measured under an applied field (6.4 T ⊥ γ) in the temperature range 1.8-200 K were analyzed by the usual spin Hamiltonian formalism. The resulting zero-field splitting and hyperfine structure derived for 1 and 3, respectively, are very close to what was found for reduced P450. Comparing the spectroscopic properties of five-coordinate mercaptoiron(II) porphyrins with those reported for the reduced state of cytochrome P450, we conclude that the cysteinate ligand remains bonded to iron in the ferrous state and is not protonated when reduction of the ferric to the ferrous state of P450 takes place.
