Metal ions and chelates

The ability of a metal ion to increase the rate of hydrolysis of a peptide has enormous implications in biology, and many studies have centred upon the interactions and reactions of metal complexes with proteins. However, hydrolysis is not the only reaction of this type which may be activated by chelation to a metal ion, and chelated esters are prone to attack by any reasonably strong nucleophile. For example, amides are readily prepared upon reaction of a co-ordinated amino acid ester with a nucleophilic amine (Fig. 3-11). In this case, the product is usually, but not always, the neutral chelated amide rather than a depro-tonated species. 

Tomaszewski, M. and Thimann, K. V. 1966. Interactions of phenolic acids, metallic ions and chelating agents on auxin-induced growth. Plant Physiol. 41, 1443-1454... 

P6. Plooke, D. J., and Vallee, B. L., Interaction of alkaline phosphatase of E. coli with metal ions and chelating agents. Biochemistry 1, 1039-1043 (1962). 

Space does not permit a complete survey of all inorganic species the reader is referred to an extensive account by Lytle [16] of luminescence over the entire periodic table, and to a recent monograph [2]. To be discussed here will be certain simple luminescent ions, chelates of non-transition-metal ions, and chelates of transition-metal ions. 

Metal Ions and Chelating Agents in Antiviral Chemotherapy... 

Stability As high as possible Tolerance against in vivo condition, cellular compartment high metallic ions and chelating compounds... 

Metal Deactivators. The abiUty of metal ions to catalyse oxidation can be inhibited by metal deactivators (19). These additives chelate metal ions and increase the potential difference between the oxidised and reduced states of the metal ions. This decreases the abiUty of the metal to produce radicals from hydroperoxides by oxidation and reduction (eqs. 15 and 16). Complexation of the metal by the metal deactivator also blocks its abiUty to associate with a hydroperoxide, a requirement for catalysis (20). 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

Citric acid is utilized in a large variety of food and industrial appHcations because of its unique combination of properties. It is used as an acid to adjust pH, a buffer to control or maintain pH, a chelator to form stable complexes with multivalent metal ions, and a dispersing agent to stabilize emulsions and other multiphase systems (see Dispersants). In addition, it has a pleasant, clean, tart taste making it useful in food and beverage products. 

However, EDTA has the widest general application in analysis because of its powerful complexing action and commercial availability. The spatial structure of its anion, which has six donor atoms, enables it to satisfy the coordination number of six frequently encountered among the metal ions and to form strainless five-membered rings on chelation. The resulting complexes have similar structures but differ from one another in the charge they carry. 

For ketones and aldehydes in which adjacent substituents permit the possibility of chelation with a metal ion, the stereochemistry can often be interpreted in terms of the steric requirements of the chelated TS. In the case of a-alkoxyketones, for example, an assumption that both the alkoxy and carbonyl oxygens are coordinated with the metal ion and that addition occurs from the less hindered face of this chelate correctly predicts the stereochemistry of addition. The predicted product dominates by as much as 100 1 for several Grignard reagents.120 Further supporting the importance of chelation is the correlation between rate and stereoselectivity. Groups that facilitate chelation cause an increase in both rate and stereoselectivity.121 This indicates that chelation not only favors a specific TS geometry, but also lowers the reaction barrier by favoring metal ion complexation. 

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