Histidine, cobalt complex

Cobalt(ll) forms many complexes which can exhibit oxygen-carrying properties (2,19). Reversible oxygen uptake in solutions of cobalt (ll)-histidine (33-36), and cobalt (II) in the presence of a-amino acids and peptides (37—39) has been known for some time. The reaction of cobalt (II) with dipeptide was first observed in enzymic studies involving glycyl-glycine (40). 

Molecular structures of cobalt II histidine complexes, reproduced with permission from Candlin and Harding (1970). Left-hand Mdl, with molecule I in the d form and molecule II in the l. Right-hand Mll. Nitrogen atoms are shaded oxygen atoms are shown heavier... 

It is interesting to note the authors claim that the amount of ester incorporated onto the hydrogel was equal to the amount of template used during the formulation step. They also assumed that the ester occupied all of the imprinted cavities. The authors have used UV spectroscopy and ESR to characterize the structure of the complex prior to the removal of the metal center. Although the spectra were not shown, they reported studies which showed that cobalt formed a complex with 20, methacrylic acid (MAA), methacryloyl histidine (MA-His), and the template in the molar ratio 1 1 1 1 1. The authors interpretation of the spectra and explanation of the cobalt complexes with the polymer are confusing at best. The schematic diagrams that they report incorporate a hydroxyl group in the active site, which is not consistent with their spectral interpretations. 

In contrast with these findings for cobalt(u) complexes of octahedral configuration, the interaction of Oa with a cobalt(n) histidine complex has been investigated in strongly basic media. In solutions of IM-NaOH, the spectrum of the cobalt(n) species is consistent with that of a tetrahedral complex with four nitrogen donors. The rate law for the reaction is... 

The structures of the ligand-cobalt complexes used are shown in Figure 4. The voltammetry of these complexes show considerable sensitivity to the electrode material. Figure 5 illustrates this point for cobalt bis-histidine. Under certain conditions adsorption processes can be clearly seen. In addition the pH of the solution clearly has a major effect on the composition of the complex in solution as shown by the voltammetry illustrated in Figure 6. Behavior like this has been observed before for Vitamin Bi2. Even the choice of counterion can significantly alter the shapes of the voltammetric responses and the associated rate constants. 

A calorimetric study has been made (97) of the oxygenation of the cobalt (II) complexes formed with histidine, histamine and ethylenediamine in aqueous media at 25 °C. 

Thus, the mechanism of MT antioxidant activity might be connected with the possible antioxidant effect of zinc. Zinc is a nontransition metal and therefore, its participation in redox processes is not really expected. The simplest mechanism of zinc antioxidant activity is the competition with transition metal ions capable of initiating free radical-mediated processes. For example, it has recently been shown [342] that zinc inhibited copper- and iron-initiated liposomal peroxidation but had no effect on peroxidative processes initiated by free radicals and peroxynitrite. These findings contradict the earlier results obtained by Coassin et al. [343] who found no inhibitory effects of zinc on microsomal lipid peroxidation in contrast to the inhibitory effects of manganese and cobalt. Yeomans et al. [344] showed that the zinc-histidine complex is able to inhibit copper-induced LDL oxidation, but the antioxidant effect of this complex obviously depended on histidine and not zinc because zinc sulfate was ineffective. We proposed another mode of possible antioxidant effect of zinc [345], It has been found that Zn and Mg aspartates inhibited oxygen radical production by xanthine oxidase, NADPH oxidase, and human blood leukocytes. The antioxidant effect of these salts supposedly was a consequence of the acceleration of spontaneous superoxide dismutation due to increasing medium acidity. 

Among protein aromatic groups, histidyl residues are the most metal reactive, followed by tryptophan, tyrosine, and phenylalanine.1 Copper is the most reactive metal, followed in order by nickel, cobalt, and zinc. These interactions are typically strongest in the pH range of 7.5 to 8.5, coincident with the titration of histidine. Because histidine is essentially uncharged at alkaline pH, complex-ation makes affected proteins more electropositive. Because of the alkaline optima for these interactions, their effects are most often observed on anion exchangers, where complexed forms tend to be retained more weakly than native protein. The effect may be substantial or it may be small, but even small differences may erode resolution enough to limit the usefulness of an assay. 

For tridentate amino acids with three non-equivalent donor atoms such as L-aspartic acid or L-cysteine, the isomers possible are illustrated below (252-254). There have been a number of reports of the preparation of L-aspartic acid complexes.1180,1181,1182. In the earlier work the isomers were not identified, however in the later study, the complexes were tentatively identified by comparison of their spectroscopic properties with those of the corresponding cobalt(III) complexes.1183 The order of elution of the complexes on HPLC was also similar to that observed for the corresponding cobalt(III) complexes. Mixed complexes containing l- or D-aspartate and L-histidine were also prepared.1182 A crystal structure of one salt obtained from this kind of system, bis(L-histidinato-0,Ar,Ar )chromium(III) nitrate, has been determined.1184... 

As cobalt(III) complexes (d6), cobalamins strictly adhere to an octahedral geometry, with an axial benzimidazole nitrogen ligand. It appears that in protein-bound B12 this axial ligand is replaced by a histidine provided by the peptide chain, with consequent configurational changes that influence the reactivity of the... 

While the majority ofmetal-mediatedproteinmodifications covered in this survey involve activation of small molecules (e g. O2, H2O) by metal complexes, there are some instances in which complexation of the metal ion itself serves as the mechanism of modification. Complexes with low-valent cobalt and chromium undergo relatively rapid hgand exchange, permitting solution complexes to coordinate with exposed amino acid side chains (e g. histidine) on the surface of proteins. Oxidation of this adduct converts the complex to a substitution-inert species. The extreme inertness of Cr(III) and Co(III) complexes toward hgand-substitution... 

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