Richard L. Gustafson

Stabilities of metal chelates of pyridoxamine

Abstract The acid dissociation constants of pyridoxamine, and the formation constants of its 1:1 and 2:1 chelates with Cu(II), Ni(II), Co(II), Fe(III), Mn(II), Zn(II), and Cd(II) ions have been determined by a potentiometric method. The structures of the metal chelate compounds formed are deduced with the aid of the titration curves, and the relative stabilities are interpreted in the light of the tentative mechanism of the transamination reaction.

33 Metal Chelate Compounds in Homogeneous Aqueous Catalysis

The catalytic reactions of metal ions may be divided into two general classifications: reactions in which the metal chelate compound is permanently altered, as a result of the reaction which takes place, and reactions in which the metal chelate compound remains unchanged. The first class includes both redox reactions, in which the metal ion changes valence, and reactions in which no change of oxidation state takes place. Examples of metal-catalyzed redox reactions are the oxidation of oxalate through the formation of the Mn(III) chelate and the oxidation of ascorbic acid by the Cu(II) ion. Examples of reactions in which the chelating compound is changed without the involvement of the metal ion in a redox step are catalysis by various metal ions of β-keto acid decarboxylation, transamination reactions of Schiff bases derived from pyridoxal, and the hydrolytic cleavage of various Schiff bases through chelate formation. Reactions of the second type, which take place without effecting a permanent change in the structure or composition of the metal chelate compound, may be considered to represent a true metal chelate catalysis. The peptidase action of metalactivated enzymes is one of a large number of examples of this type which have been proposed for biological systems. The action of Cu(II) chelates of various diamines is described as an example of metal chelate catalysis in the hydrolysis of diisopropylfluorophosphate. The probable nature of these catalytic reactions is outlined, and the factors which seem to render a metal chelate compound an effective catalyst are described.

FORMATION OF POLYNUCLEAR COMPLEXES IN AQUEOUS SOLUTION

It is well known that highly charged metal ions undergo polymerization in aqueous solution by the formation of bridges through 0x0 or hydroxo groups. The tendency to form complex aggregates may be reduced considerably in many cases by chelating the metal ions with suitable polydentate ligands. The degree of formation of polynuclear species is thus reduced because (1) the positive charge of the metal is lowered, and (2) the number of coordination sites available for olation is reduced. The initial steps in the hydrolysis and olation of a wide variety of chelated and unchelated metal ions may be represented by the following equations: