An Interesting Reaction of Cupric Ions with Ferricyanide and Ferrocyanide

Abstract

The possibility that Cu(III), or cupryl ion [Cu(I)0°], participates in enzymic reactions of copper with dioxygen and other oxidizing substrates, as Dr. Hamilton suggests in the preceeding paper, is an extremely interesting one which opens new vistas of mechanism. However, absolutely unambiguous criteria for the 3+ oxidation level of copper in these processes are required. That is the principal point of this note. Here we show that in the presence of excess ferricyanide ion, the EPR spectrum of Cu²⁺ ions disappears, and reappears when excess ferrocyanide ion is added. As Dr. Hamilton suggests for galactose oxidase, this may be explicable by the reversible oxidation-reduction of Cu²⁺ to low spin Cu³⁺. On the other hand, Cu may form ion pairs with ferricyanide, the magnetic interaction in the pairs broadening the EPR spectrum to indetectability. The addition of (low spin) ferrocyanide would then so dilute the ferricyanide that EPR-detectable Cu²⁺ -ferrocyanide ion pairs are observed.