S. Chaberek

Chelating agents as metal buffers in biological systems. I. Principles and use of ethylenediaminetetraacetic acid

Abstract The interactions which influence the concentration of free metal ions in aqueous solutions of chelating agents are described. Graphical representation of metal chelate equilibria in the form of pM-pH functions is proposed as the most convenient method of taking into account the effects of interfering substances. This method is applied to metal-ion buffer systems containing ethylenediaminetetraacetic acid, and graphs are presented to illustrate the influence of hydrogen-ion concentration, metal-hydroxide precipitation, and oxalate precipitation.

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.