Role of metals in enzymatic reactions. Part 3.—Kinetics of complex formation in model systems involving zinc and pyridine-2-azo-p-dimethylaniline

Abstract

The temperature-jump relaxation method has been used to measure the kinetics of formation and dissociation of the 1 : 1-complex between zinc and pyridine-2-azo-p-dimethylaniline (PADA) and of the ternary complexes between PADA and the zinc-diethylenetriamine, -triethylenetetramine iminodiacetate, -ethylenediamine-N,N′-diacetate, -cysteine, and -polytriphosphate complexes. A lower limit for the rate of the reaction with zinc-nitrilotriacetate has also been determined. In most cases, the activation parameters for the formation and dissociation processes could also be obtained. The first ligand has comparatively little influence in the rate at which zinc reacts with and dissociates from the second ligand, although it causes a significant reduction in the activation enthalpy. The implication of these results with respect to the role of zinc in enzymatic reactions is discussed.