Peptides nickel complexes

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

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. 

Nickel(III) peptide complexes have a tetragonally-distorted octahedral geometry as shown by electron spin resonance studies (19) and by reaction entropies for the Ni(III,II) redox couple (17). Axial substitutions for Ni(III)-peptide complexes are very fast with formation rate constants for imidazole greater... 

Cu (H G ) in Figure 1. The two protonated nickel(III) complexes then undergo substitution reactions for the terminal peptide nitrogen with rate constants of 0.94 s 1 and 17 s 1, respectively (21). It is interesting that the corresponding nickel(II) complexes have similar but somewhat larger rate constants. Thus, Ni (H gG )H dissociates with a rate constant of... 

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- ... 

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

Nickel(lll) deprotonated peptide complexes can be easily obtained in solution by chemical or electrochemical oxidation of the corresponding nickel(U) complexes. They are moderately stable in aqueous solutions and have been widely characterized by EPR, electron spectroscopy and cyclic voltammetry.3047,3058,3087,3088 Some selected examples of nickel(III) peptide complexes are reported in Table 115. 

Table 115 Selected Examples of Nickel(IU) Deprotonated Peptide Complexes... 

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-... 

In nickel(III) peptide complexes, there is a strong in-plane field provided by the deprotonated peptide linkages (117, 118). Two axially coordinated water molecules are present in the tetragonally distorted complexes which exchange much more rapidly than for the [14]aneN4 species with a substitution rate of constant >106 M l sec-1 for the formation of the imidazole complex (141). However, except for the terminal peptide group, equatorial substitution is very slow. Substitution and rearrangement (125) reactions of these species reveal acid-... 

A complication arises from a consideration of potentials. The midpoint reduction potentials of model peptide complexes of Ni111, which appear to involve deprotonated peptide donor sites, lie between +0.79 and +0.96 V. These values are much greater than the values found for the Niin/Niu couple of the hydrogenase from D. gigas (—0.145888 or —0.220 V890). An obvious conclusion is that nickel in hydrogenase is bound by different amino acid groups from those involved in the... 

Dinuclear nickel(II) complexes are formed with the dipeptide Gly-L-Cys. The nickel atoms are coordinated by the amino and deprotonated peptide nitrogens and the sulfur groups, with the sulfur atoms bridging the two nickel atoms... 

Figure 7-1 Activation of T cells by metal ions. Nickel and other metal ions appear to activate specific T cells by several different molecular mechanisms. (1) T cells with their T cell receptor respond to complexes of nickel with MHC-peptide similar to other hapten-peptide complexes. (2) Nickel forms a direct linker between MHC and the T cell receptor independent of the peptide with some similarities to superantigen-mediated T cell stimulation. (3) The processing of self peptides is disturbed by nickel resulting in cryptic self peptides presented to a T cell receptor.

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