Comparative study of the optical and magnetic circular dichroism spectra of bis-thioether and -imidazole complexes of iron(III) tetraphenyl- and octaethyl-porphyrin. Models of haem co-ordination in bacterioferritins

Abstract

The absorption and low-temperature (4.2 K) magnetic circular dichroism (MCD) spectra of the low-spin iron(III) complexes [Fe(oep)(tht)₂]⁺, [Fe(tpp)(tht)₂]⁺, [Fe(oep)(Him)₂]⁺ and [Fe(tpp)(Him)₂]⁺, where oep is 2,3,7,8,12,13,17,18-octaethylporphyrinate, tpp is 5,10,15,20-tetraphenylporphyrinate, tht is tetrahydrothiophene and Him is imidazole, have been compared to provide spectra of well defined bis-co-ordinated iron(III) porphyrin models for the diagnosis of axial haem ligation in proteins. Comparison of the shapes of the porphyrin-to-Fe^III charge-transfer (c.t.) bands and the magnitudes of the MCD C terms in the near-infrared region show that the electronic spectra of low-spin iron(III) complexes of oep, and hence of protoporphyrin IX (3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropanoic acid), have only one transition, namely, a_1u(π)→ e_g(d) whereas the spectra of complexes of tpp contain two one-electron transitions a_1u, a_2u(π)→ e_g(d). Substitution of a phenyl group for a hydrogen atom at the porphyrin methine bridge causes a large energy shift of the a_2u→ e_g(d) transition but leaves the other one unaffected. On changing the axial ligation of [Fe(oep)]⁺ from bis(imidazole) to bis(tht) the energy of the a_1u(π)→ e_g(d) c.t. band shifts from 6330 (1580) to 4700 cm^–1(2130 nm). The energy shift is twice that observed when bis(imidazole)ligation is changed to imidazole–thioether ligation showing that the shifts of the c.t. bands obey an approximate additivity rule. The EPR and MCD spectra of the complex [Fe(oep)(tht)₂]⁺ match well the spectra of the haem group in the bacterial iron storage protein, bacterioferritin, and hence provide strong support for the assignment of bis(methionine) co-ordination of the haem in this protein, a ligation scheme not previously reported for haem in any protein.