The interaction of cysteine methyl ester with metal ions. Part II. The metal-ion catalysed base hydrolysis of cysteine methyl ester

Abstract

The base hydrolysis of cysteine methyl ester has been studied in aqueous solution at 25° and I= 0·1 M, and specific rate constants for the hydrolysis of the various pH-dependent forms of the ester have been determined. Rate constants for the base hydrolysis of the ester function in the bis-complexes of nickel(II), zinc(II), lead(II), cadmium(II), and mercury(II) with cysteine methyl ester have also been obtained at 25° and I= 0·1 M. The hydrolytic behaviour of the bis-complexes is consistent with the kinetic scheme ME₂+ OH^–→ MEA^–+ MeOH; MEA^–+ OH^–→ MA₂^2–+ MeOH. The metal complexes undergo hydrolysis slightly faster than the free ligand. Thermodynamic parameters for the hydrolysis of the ester and the nickel(II) complex are compared. The effect of adding a number of chelating species to the 1 : 1 nickel(II)–ester system has also been studied, and there is kinetic evidence that mixed-ligand complexes are formed with 2-aminoethanethiol.