Copper glycine complexation constants

Several features of the above studies were reinvestigated in a detailed kinetic study of the copper(II) complexes of glycine methyl ester and phenylalanine ethyl ester in glycine buffer at pH 7.3 (26). Glycine was selected as a buffer in this study in order that a small increase in the glycine concentration caused by the hydrolysis reaction would not increase the concentration of copper(II) complexes to a significant extent. It was found that the rate constant for the hydrolysis of the copper(II) complex of DL-phenylalanine ethyl ester was 106 times greater than the rate constant obtained for the alkaline hydrolysis of the free ester (25). 

Because of the ability of ion-selective electrodes to measure the activity of free ions, it has been possible to obtain thermodynamic formation constants and to have information on the number of binding sites and the stoichiometries of complexes. Thus, formation constants have been calculated for copper(II) complexes with glycine, glutamic acid and tris(hydroxymethyl)aminomethane [420,421]. 

Solvolysis of Organic Ligands.— The catalysis of hydrolysis of organic esters by metal ions or complexes has been much studied for many years. Recent examples of kinetic studies include hydrolysis of oxalate esters catalysed by a variety of ions, and of the bis[-L-(-J-)-histidine methyl ester] complexes of copper(n) and of nickel(n). The relative catalytic effects of several copper(n) complexes, including Cu(imda), Cu(nta), and Cu(dien) +, on the hydrolysis of methyl glycinate have been determined. Rate constants for base hydrolysis of this ester correlate with stability constants for mixed complexes of this ester with the above-named copper(n) complexes. ... 

The glycine complex of cobalt(n)-bipy, like that of copper(n>-bipy, has a comparatively high stability constant relative to the bis-glydnate of the metal. It has been shown that this can be attributed to a lower dissociation rate constant for the former (55 s at 25 °C) than for the latter (330 s ), the formation rate constants being rather similar (1.6 x 10 and 2.0 x 10 1 mol s respectively). 

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

Because /3 is the symbol for the product of individual stability constants, we write j8 = KxK for the combination of copper with glycine and for all other cases where a metal becomes saturated after combining with two portions of a ligand. For zinc and ethylenediamine, where the zinc becomes saturated after fixing three portions of the ligand, we write = KxK2K and, for the complex of a metal that is saturated when it has combined with one unit of ligand (for instance, zinc and EDTA), = Kx. 

For methyl glycinate at 25°C, ko - is 7.6 x 10 M s and k for water attack is 4.3 x 10 s. The observed first order rate constant for water attack can be converted to a second order rate constant by dividing by the molar concentration of water OChk) = k/55.5 = 7.7 X 10 M s ). The ratio Icoh- hjo = 10" and is a measure of the relative nucleophilicities of hydroxide ion and water towards the copper complex. The ratio kou-/kg = 7.6 X lOVl.28 10 and this is within the normal range of rate enhancements (10 -10 fold) observed for copper(II) promoted hydrolysis of carboxylic esters where copper(II) interacts directly with the alkoxycarbonyl group of the ester. 

The rate constants for the formation and dissociation of the mixed complex [Cu(bipy)(glycinate)]+ have been measured, and it was shown that, within experimental error, the rate-constant for attack by the glycine zwitterion is zero. For the reaction of Cu(bipy) + with the glycinate ion (gly), the forward rate constant is 1.6 x 10 lmol- s at 25 C. This compares with values of 4 x 10 lmol s for Cu + + gly and 4 X 10 lmol- s for Cu(gly)+ + gly and it is suggested that the differences can be accounted for in terms of statistical factors, char effects, and an enhancement term associated with the presence of bipy in the inner co-ordination sphere of the copper ion. A preliminary report shows that the rate of complex formation between the histaminium ion (6) and Cu"... 

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