Hemichrome

Hemochromes are the symmetrical bis(ligand) adducts of the Fe11 porphyrins, e.g. the bis(imidazole) moiety [11] presumably occurring in reduced cytochrome a or bs (1, 47). The bis(pyridine) hemochromes Fe(P)Py2 are all rather labile in solution the same is true for the corresponding hemichrome derivatives, [Fe(P)Py2] , which are the cationic oxidation products of the hemochromes. Reliable spectral and other physicochemical data on these species can only be obtained in solution when an excess of L is present to suppress the dissociation equilibrium (5) (M = Fe) (20, 24), which is the origin of subsequent oxidation (n = 0) or solvolysis reactions (n = 0 or 1). 

La Mar (150) and Walker (156) have found a thermodynamic cis effect in the formation of hemichrome salts [Fe(TRP)L2]Cl (->-[27]) according to equilibrium (77) which was studied by 1H-NMR and optical spectroscopy for L = l-Melm (Table 22). As the electron-donating power of the para-phenyl-substituent of the porphyrin increases, the total formation constant, /J2, increases. This is because the product of the reaction contains a positively charged center which is stabilized by electron-donating groups. As a Hammet relation exists, the mesomeric part of the electronic transmission is also operative, and hence dative porphyrin-to-metal tr-bonding seems to be involved. 

Table 22. Of effects of the poiphinato ligand on the equilibrium constants for the formation of hemichrome salts, [Fe(TRP)(l-MeIm)2)Cl. A Figures taken from spectrophotometric titrations (156), B from H-NMR data (150)...

A recent H-NMR study of imidazole bonding in synthetic hemichromes led La Mar to the conclusion that 7r-bonding is important for these systems, and that imidazole acts primarily as a 7r-acceptor (158). However, our evidence on the osmochrome/ osmichrome system lends support to the idea that the jr-donor function of Him should not be overlooked, and more experiments are needed to understand the hemichrome systems. The difficulty encountered in their theoretical treatment is due to the fact that their optical spectra are of the mixed hyper/hypso type (50). Thus there will be a variety of transmission paths for electronic effects which remain to be illuminated. The present article may serve as an intuitive guide and a challenge to further research. 

Andreyuk GM, Kisel MA. Conversion of hemoglobin to hemichrome by lysophospholipids. Biochemistry (Mosc) 64, 867, 1999. 

A novel cytochrome b model compound containing two heme binding sites was tested by EPR and Mossbauer spectroscopy. Both sites were found to be occupied in a 1 1 stoichiometry. For one of the sites a normal b-hemichrome EPR signal with rhombic -tensor symmetry was found as expected for a bis-histidine ligation with parallel histidine planes. For the other site unusual EPR features (e.g. axial y-tensor) were obtained which were ascribed to a configuration with perpendicular histidine planes.267 A new type of cytochrome b was described for a preparation from the cytoplasmic fraction of an archaeon, Acidianus ambivalens. This is thus the first soluble cytochrome found in this... 

Denatured globin hemochrome Heat denaturation of myoglobin reduction of globin hemichrome Intact Fe(II) Denatured Pink or red... 

Denatured globin hemichrome Heat denaturation of metmyoglobin oxidation of globin hemochrome Intact Fe(III) Denatured Gray or brown... 

Some investigators have soaked cooked meat slices in freshly prepared 1% sodium dithionite solution to maintain the pigments in the reduced state. However, this is not truly representative of the fresh meat slice, since dithionite will also reduce the brown globin hemichromes, intensifying the pink color to a level greater than that actually observed after slicing. 

Iron porphyrins, the prosthetic group of the heme proteins are of course of great interest. Tetraphenylporphine, (TPPH2) (Fig. 2) is not a naturally occurring porphyrin. But because of its ease of preparation and close similarity to other porphyrins, tetraphenylporphinatoiron (TFPFe) derivatives have been extensively studied. Hemes are Fe(II) compounds such as TPPFe(II), and hemins are rive coordinate Fe(III) derivatives such as TFPFe(III) X (X = halide, pseudohalide, carboxylate, alkoxide, etc.)16). Six coordinate Fe(III) prophyrins are known as hemichromes, such as [TPPFe(III) L2] X (L = pyridine, piperidine, imidazole, etc.)16). Hemichromes are usually low spin, S = 1/2, systems 0 eff 1-9 B.M.) and hemins are of the high spin,... 

The splitting of (TPPFe)20 by imidazole (Him) in organic solvents to produce the hemichrome [TPPFe(III) (HIm)2]+ has been examined kinetically34). Although the rate of the reaction is strongly dependent on acid concentration, the mechanism is not simple. Some type of (TPPFe)20 Him adduct is formed in a fast equilibrium prior to bridge cleavage. The adduct is not spectrally identifiable and may be due to interaction of the porphyrin with Him rather than between the Fe(III) and Him. 

Heating of meat results in the denaturation of the globin linked to iron, as well as in the oxidation of iron to the ferric state and formation of different brown heme pigments named hemichrome. 

Hemichromes precipitate and form Heinz bodies, which attach hydrophobically to the erythrocyte membrane, increasing membrane permeability and the rate of lysis. Loss of heme and oxidation of sulfhydryl groups do not appear to be necessary for precipitation. 

Kannan, R., Labotka, R., and Low, P. S. (1988). Isolation and characterization of the hemichrome-stabilized membrane protein aggregates from sickle erythrocytes. Major site of autologous antibody binding. J. Biol. Chem. 263,13766-13773. 

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