Peptides ligand transfer reactions

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-... 

Some small peptide-heme complexes have been prepared, including an undecapeptide (residues 11-21)668"669 and an octapeptide (residues 14-21). TTiese are useful models as they include the two cysteine residues that covalently link the heme to the peptide, and one of the axial ligands. The axial Met-80 residue is absent, but the position can be filled by methionine or by other ligands as required.670 Work with several octapeptide complexes shows that the rates of outer-sphere electron transfer appear to be independent of the axial ligand, and faster than the reaction for cytochrome c. Other comparisons show that the orientation of the axial methionine in cytochrome c and the contacts between heme and protein are important controlling factors in the electronic structure of the heme. Aqua and hydroxo complexes of iron(III) octapeptide complexes are also useful models for studying spin equilibria in iron(III) hemoproteins.671... 

Formation of the nickel(III) complexes is generally monitored by the changes in the UV-visible spectra which show up intense charge transfer bands (27 (KK)-31000 and 38 000-40 000 cm see Table 115) in place of the less intense d-d bands observed in the parent nickel(II) complexes. The reduction potentials of the Ni /Ni° peptide couples are in the range 0.79-0.96 V (m. NHE) and are not very sensitive to the nature of the equatorial ligands. Nickel(III) peptide complexes are thus moderately strong oxidants in aqueous solutions. The electron transfer kinetics of the reduction reaction have been studied as well as the electron transfer kinetics of the reactions between copper(III,II) and nickel(in,II) peptide complexes. ... 

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