Imidazole- and alkylamine-ligated iron(II,III) chlorin complexes as models for histidine and lysine coordination to iron in dihydroporphyrin-containing proteins: characterization with magnetic circular dichroism spectroscopy

An extensive series of five- and six-coordinate (1-methylimidazole)-, (1-butylamine)-, and (pyridine)iron(II, III) chlorin complexes are characterized for the first time with magnetic circular dichroism (MCD) spectroscopy. The chlorins (dihydroporphyrins) employed are octaethylchlorin, mesochlorin, and “methyl” chlorin (2,2,4-trimethyldeuterochlorin, featuring a gem-dimethyl-substituted peripheral carbon). The species studied include the bis(1-methylimidazole)ferric, bis(1-methylimidazole)ferrous, (1,2-dimethylimidazole)ferrous, and 1-methylimidazole/CO or NO complexes together with analogous bis(pyridine)- and bis(1-butylamine)ferrous adducts and their CO-ligated derivatives. The results presented establish MCD spectral signatures for use in determining whether an iron chlorin-containing protein bears either a histidine or a lysine axial ligand. Analysis of data obtained with different ring-reduced mesochlorin isomers shows that the method is insensitive to the site of pyrrole ring reduction. This latter observation will facilitate the use of mesochlorins in reconstitution experiments with structurally-defined heme proteins in order to investigate mixed-ligand complexes which are difficult to generate synthetically. In general, the MCD spectra of iron chlorin complexes are most sensitive to the identity, number, and type of axial ligand, along with the oxidation and spin state, and are relatively insensitive to changes in the equatorial plane such as the site of pyrrole ring reduction. Therefore, it appears that MCD spectroscopy will be of particular use in the identification of proximal and distal axial ligands in chlorin-containing proteins, as has repeatedly been shown to be the case with protoheme-based iron porphyrins.