IMIDAZOLE-LIGATED 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 cha racterized for the first time with magnetic circular dichroism (MCD) s pectroscopy. The chlorins (dihydroporphyrins) employed are octaethylch lorin, mesochlorin, and ''methyl'' chlorin (2,2,4-trimethyl-deuterochl orin, featuring a gem-dimethyl-substituted peripheral carbon). The spe cies studied include the bis(1-methylimidazole)ferric, bis(1-methylimi dazole)ferrous, (1,2-dimethylimidazole)ferrous, and 1-methylimidazole/ CO or NO complexes together with analogous bis(pyridine)- and bis(I-bu tylamine)ferrous adducts and their CO-ligated derivatives. The results presented establish MCD spectral signatures for use in determining wh ether an iron chlorin-containing protein bears either a histidine or a lysine axial ligand. Analysis of data obtained with different ring-re duced mesochlorin isomers shows that the method is insensitive to the site of pyrrole ring reduction. This latter observation will facilitat e the use of mesochlorins in reconstitution experiments with structura lly-defined heme proteins in order to investigate mixed-ligand complex es 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 st ate, and are relatively insensitive to changes in the equatorial plane such as the site of pyrrole ring reduction. Therefore, it appears tha t MCD spectroscopy will be of particular use in the identification of proximal and distal axial ligands in chlorin-containing proteins, as h as repeatedly been shown to be the case with protoheme-based iron porp hyrins.