Metal ion-amino acid complexation

The rate constants for the formation of divalent metal ion-amino acid complex formation have been determined by T-jump measurements representative data are given in Table 7. As is generally found for bidentate complex formation, the mechanism involves a two-step process (equations 12... 

Table 7 Rate Constants for the Formation Reactions of Metal Ion-Amino Acid Complexes ...

METAL ION-AMINO ACID COMPLEXATION IN SUPRAMOLECULAR CHEMISTRY... 

Metal-bound amino acid complexes, 206-209 Metal ions, in biological systems, 153—154 Methylamine dehydrogenase, 69—70 reaction diagram, 54/... 

Metal ions, amino acids Acidic, pH = 3 DEHPA Acidic, pH = 0 Complexes Counter transport of H+ [57]... 

The versatile binding modes of the Cu2+ ion with coordination number from four to six due to Jahn-Teller distortion is one of the important reasons for the diverse structures of the Cu-Ln amino acid complexes. In contrast, other transition metal ions prefer the octahedral mode. For the divalent ions Co2+, Ni2+, and Zn2+, only two distinct structures were observed one is a heptanuclear octahedral [LnM6] cluster compound, and the other is also heptanuclear but with a trigonal-prismatic structure. 

There are several such toxic agents that cause considerable medical, public and political concern. Two examples are discussed here the heavy metal ions (e.g. lead, mercury, copper, cadmium) and the fluorophosphonates. Heavy metal ions readily form complexes with organic compounds which are lipid soluble so that they readily enter cells, where the ions bind to amino acid groups in the active site of enzymes. These two types of inhibitors are discussed in Boxes 3.5 and 3.6. There is also concern that some chemicals in the environment, (e.g. those found in industrial effluents, rubbish tips and agricultural sprays), although present at very low levels, can react with enhanced reactivity groups in enzymes. Consequently, only minute amounts concentrations are effective inhibitors and therefore can be toxic. It is suggested that they are responsible for some non-specific or even specific diseases (e.g. breast tumours). 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

In a series of reactions similar to those described for amino acid complexes, pyruvic acid undergoes metal-catalyzed condensation with aldehydes and also halogenation (Scheme 14). Again, the metal ion presumably functions by accelerating the formation of the methylene carbanion. 

In more recent years attention has turned from studying the equilibria of binary metal-amino acid complexes to that of ternary complex formation in aqueous media, particularly to complexes of the type (aa)—M11—L, where L is some other ligand or a different amino acid to (aa), and M11 is a kinetically labile metal ion. Ternary complexes involving kinetically inert metal ions, e.g. Co,w and Pt", are more well known since they can be separated from mixtures and studied in isolation. Such is not the case with the labile systems. Because of the facile nature of their equilibria they must be studied in situ (claims regarding the separation of labile species by chromatographic procedures... 

Most of the attention regarding amino acid complexation has been centred on animal fluids. There is considerable evidence, for example, that amino acid complexes are involved in the (active) transport of metal ions across various biological membranes.39 Complexation of the trace elements is also considered essential in reducing concentrations of the free or hydrated metal ions and hence preventing the formation of unwanted hydroxy species and limiting the toxicity of the metal ions. 

Very little is known about the metabolism of metal complexes, though a number of ammine complexes of cobalt(III) were used as nitrogen sources for aspergillus niger nearly forty years ago (150), and much more recently, both tris(ethylenediamine)cobalt(III) ions (757) and amino acid complexes (152) of cobalt(III) have served as nitrogen sources for species of pseudomonas. Correlations through enzymic selectivity are therefore as yet not... 

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