Reaction of Chlorocruorin with Heme Iron Ligands and Carbonyl Reagents1

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Abstract
Chlorocruorin was purified from Potamilla leptochaeta and the spectral properties of its derivatives were investigated. Ferri- or ferrochlorocruorin did not exhibit a ferrihemochrome or ferrohemochrome spectrum, respectively. Oxy- and carbonmonoxy-ferrochlorocruorin did show ferrohemochrome-type spectra. Ferrihemochromes were formed, however, when oxyor ferrichlorocruorin was treated with 0.02–0.05% SDS, and they were transformed to ferrohemochromes by reduction with sodium dithionite. Ferrihemochrome formation was also brought about by increasing the pH of a ferrichlororin solution to 9, or by liganding of extrinsic imidazole or cyanide to the ferric pigment. Therefore, it is apparent that at least one of the coordination positions on the heme iron in ferri- and ferrochlorocruorin is vacant or occupied by a weak-field ligand. Titration studies of ferrichlorocruorin with imidazole indicated that this supposedly vacant coordination position was occupied first by the imidazole, and that the intrinsic ligand of protein origin was replaced finally at higher concentrations. The extrinsic ligands in the cyanide and imidazole complexes of ferrichlorocruorin were excluded from their coordination positions as the protein moiety assumed conformations inherent to the reduced pigment. Spectral analyses indicated that the intrinsic ligand is an imidazole moiety of a histidyl residue. When chlorocruorin was intact, carbonyl reagents such as cyanide and sodium bisulfite did not add to the formyl group of chlorocruoroheme. When the protein conformation was perturbed by SDS, addition to ferrichlorocruorin occurred appreciably. This addition was accelerated if the heme iron coordination position had been occupied by strong field ligands, and was reversed to some extent as the chlorocruorin complexes were reduced.

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