The catalysis of ascorbate oxidation by ionic copper and its complexes

Abstract

The kinetics of the oxidation of ascorbate, catalyzed by ionic copper, with special reference to the effects of a range of chelating and complexing agents, have been investigated by determining the rate of ascorbate disappearance. The effects of ascorbate and copper concentrations, oxygen tension, and pH on the rate of reaction confirmed or extended earlier observations. Though the chelating agents each reduced the rate by about 50% at a molar ratio to copper of 1:1, further increase in concentration led to various effects. Diethyldithiocarbamate (Dieca) and ethylenediamine tetraacetate (EDTA) inhibited the reaction completely at ratios of 2:1, but 8-hydroxyquinoline (oxine) and 7-iodo-8-hydroxyquinoline-5-sulfonic acid (ferron) required rather more reagent. Quinaldinic acid, 2,2′-dipyridyl and 1,10-phenanthroline were even less effective, and higher chelates of the last two also modified the form of the reaction. Though thiourea inhibited the oxidation, its effect was reversed since it was destroyed during the reaction. Carbon monoxide (95%)-oxygen (5%) inhibited the reaction completely; this could not be reversed by light. Cyanide and azide each stimulated the reaction at low molar ratios to copper, but inhibited at higher ratios, though the effective ratio depended also on the ascorbate concentration employed. The observations are discussed in terms of a simple mechanism, in which the various reagents interfere either by displacing ascorbate from an initial complex with cupric ion or at some subsequent stage.