Nickel complexation with peptide

IrCli- has been used to prepare several nickel(III) complexes with peptides.3047,3048... 

Zinc(n), unlike copper(II) and rtickel(n), does not form very stable complexes with peptides. A study of 12 dipeptides containing the Gly, Ala, Leu, and Pro residues with zinc(II) showed that the only two species formed in detectable quantities were [ZnLH]+ and Z11L2H2. This is in contrast to copper(II) and nickel(II), in which the major species are MH iL and MH 3L. The main reason for the difference in... 

It is believed that nickel penetrates the skin and acts as a hapten, complexing with selected peptide and/or amino-acid ligands to distort intercellular or cellular proteins, stimulating a type IV delayed (cell-mediated) hypersensitivity reaction [398]. Nickel water-soluble salts, like nickel chloride and nickel sulphate, are strong sensitizers [213, 215], The chloride induced in sweat is apparently an important factor in dissolving the metallic nickel, permitting the soluble nickel salts to act. 

The sluggish substitution properties of copper(III) and nickel(III) peptide complexes have permitted the isolation of complexes with these oxidation states (14, 15). Thus, the tri-valent peptide complexes pass through a cation exchange resin which readily strips copper(II) or nickel(II) from the corresponding complexes. We now have a little more information about the substitution characteristics of the trivalent metal complexes. 

Ni" forms square-planar bis-complexes with the amidate anions of L-Val, L-Phe, and L-Pro. The structure of bis(Gly)-bis(imidazole)nickel(ii) has been reported and the configuration around the metal atom is cis-O(carboxyl), cis-N(amine), cis-N(imidazole). Tetra- and penta-peptide complexes of nickel(ii) consume oxygen in neutral solutions as the metal ion catalyses peptide oxidation to give a number of products, including amides of amino-acids and peptides, oxo-acids, and C02- ... 

The observation (112-115) that neutral aqueous solutions of [Ni11H 3G4]2 consume molecular oxygen with the appearance of a strongly absorbing transient at 350 nm lead to detailed investigations and discovery of nickel(III)-peptide complexes (113). The oxidized nickel complexes have absorption maxima around 325 and 240 nm (e = 5240 and 11,000 M I cm-1, respectively for [NiM1H 3G4]-). Reduction potentials (116) (Table II), measured by cyclic voltammetry, show a small dependence on ligand structure which can be correlated... 

Electrochemical studies (62, 64,106,137) have shown that nickel(III) complexes with macrocycles, peptides, and diimine ligands are rel-... 

Relevant to the conflicting reports of copper versns nickel reqnirements for enzyme activity,biochemical stndies demonstrated the existence of a labile nickel associated with the a snbnnit of ACS/CODH. Very recently, model stndies on a metal-ion captnre of a peptide-backbone, nonlabile [MN2S2] (26) nnit have established the capability of snch a nickel dithiolate to bind exogeneons metals. A qualitative ranking of the binding ability of complex (26) with Zn +, Cn+, and Ni + was established by a metal-ion displacement experiment (Zn + < < Cn+), as shown in Scheme 9. ... 

A study done with four optically pure dipeptides, L-Phe-L-Met, L-Met-L-Phe, L-Phe-L-Leu, and L-Leu-L-Phe, showed that the major species formed at physiological pH with nickel(II) is NiL. This is different from the complex with copper(II), in which the major product was CuH-iL. This difference is due to the fact that the first complexation with nickel(II) occurs at pH 4.5, while the copper(II) complexes begin to form between pH 2 and 3. It is this lesser ability of nickel(II) to deprotonate the peptide nitrogen that causes the difference. In the case of the NiH iL complex the formation of L-Phe-L-Leu is more difficult than L-Leu-L-Phe. This suggests that the nickel(II)-aromatic ring interaction influences the stability, as was seen with the copper(II) complexes. 

Unlike both copper(II) and nickel (II), zinc (II) does notform very strong complexes with LHRH. Studies have shown that, at pH 7.0, only 20% of LHRH is complexed with zinc(II). In addition to binding to the imidazole of the histidine residue, the zinc(II) ion is also weakly coordinated by a carbonyl of a peptide group. NMR studies have indicated that it is the carbonyl oxygen of the His-Trp peptide bond that coordinates the metal ion. 

Amino-acids and Peptides.— The co-monoprotonated forms of lysine and ornithine are reactive in the formation of complexes with nickel(ii) and cobalt(ii). 

Tervalent copper and nickel are involved in the autoxidation reactions of [Cu(H 3G4)] and [Ni(H 3G4)] respectively. In the case of nickel, decomposition of [Ni(H 3G4)] proceeds by decarboxylation of the terminal carboxy-group adjacent to the peptide nitrogen. - With copper, decomposition of [Cu-(H sG4)] proceeds through a carbon-centred free radical produced by abstraction of a hydrogen atom from the peptide backbone. Bulky carbon substituents assist the stabilization of the higher-oxidation state ions, and a study of the stabilities of leucyl tripeptide complexes with copper(ii) and nickel(u) has been reported. Copper(iii) and nickel(iii) tripeptide complexes of a-aminoisobutyric acid are thermally stable but are readily decomposed by photochemical pathways. Resonance Raman and other studies with copper(iii) peptide complexes have also been reported. ... 

Relatively stable complexes of nickel(iu) with deprotonated peptide ligands can be produced chemically or electrochemically from the corresponding nickel(ii) species. The complex with glycylglycylclycinamide, [NiCH-aGga)], has a reduction potential,... 

Kros et al. reported a polymer-peptide conjugate prepared via nickel-mediated NCA polymerization and a subsequent polymerization of an isocyanide, again using the nickel complex as initiator [111]. The active catalyst is attacked by the more electrophilic isocyanide and the coordinated amine reacts with the isocyanide to yield a carbene-like initiator for the isocyanide polymerization (see Fig. 14). The product can be purified from free residual homopolymers by selective solvent... 

The results reported in the literature and summarized in the previous sections reveal that cadmium(II) can form complexes with all natural amino acids and peptides. The thermodynamic stabilities of the cadmium(II) complexes are, however, relatively low as compared to those of the corresponding copper(II) and nickel(II) complexes. On the other hand, these data are comparable to those of the essential zinc(II) and the small differences are dependent on the effects of various side chain residues. These effects together with the structural characteristics of the cadmium(II) complexes can be evaluated separately for amino acids and peptides. 

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