Thioether Ligation in Iron-Porphyrin Complexes: Models for Cytochrome c

Article abstract of DOI:10.1021/ja00409a024

Iron-meso-tetraphenylporphyrin complexes having thioether ligation have been synthesized and investigated as models for cytochrome c.Using a 5-(N-imidazoyl)valeramido-derivatized tail porphyrin of iron(II) and tetrahydrothiophene, it was possible to isolate a mixed-ligand complex having thioether imidazole coordination, triphenylporphinato>(tetrahydrothiophene)iron(II) (1).The crystal structure of 1 was determined.An analogous iron(III) model for ferricytochrome c was characterized by EPR is solution (g = 2.90, 2.37, 1.48).A structural comparison between the iron(II)/iron(III) redox pair of complexes bis(tetrahydrothiophene)(meso-tetraphenylporphinato)iron(II) (5) and bis(pentamethylene sulfide)(meso-tetraphenylporphinato)iron(III) perchlorate (7) was made by X-ray analysis and reveals Fe-S bond lengths which are notably insensitive to oxidation state change.The structural analysis for bis(tetrahydrothiophene)(meso-tetraphenylporphinato)iron(III) perchlorate (6) is also briefly reported.All complexes have low-spin ground states.The main implications for cytochrome c are as follows: (i) Fe-S bond lengths in methionine-ligated hemoproteins are expected to be about 2.33 Angstroem and rather insensitive to oxidation state change, (ii) coordinate bond length changes are unlikely to contribute to Franck-Condon barriers of electron transfer, and (iii) the intrinsic stability of the Fe(III)-S(thioether) bond is sufficiently high that a protein conformation-enforced methionine-iron contact need not be invoked.Crystal data: 1, a = 13.170(4) Angstroem, b = 15.037(11) Angstroem, c = 25.422(8) Angstroem, β = 90.29(2) deg, monoclinic, space group P2₁/c, Z = 4; 5, a = 13.225(3) Angstroem, b = 17.967(5) Angstroem, c = 10.283(2) Angstroem, α = 91.07(2) deg, β = 99.22(2) deg, γ = 76.59(2) deg, triclinic, space group P<*>, Z = 2; 7, a = 17.830(3) Angstroem, b = 18.781(3) Angstroem, c = 18.187(3) Angstroem, orthorhombic, space group P2₁2₁2₁, Z = 4; 6, a = 13.007(7) Angstroem, b = 19.188(9) Angstroem, c = 11.256(5) Angstroem, α = 93.99(3) deg, β = 107.17(4) deg, γ = 95.01(4) deg, triclinic, space group P1, Z = 2.