The Copper-Poly-L-histidine Complex. I. The Environmental Effect of the Polyelectrolyte on the Oxidase Activity of Copper Ions

The copper-catalyzed oxidation by molecular oxygen of ascorbate and homogentisate ions, hydroquinone as well as tetramethyl-p-phenylenediammonium and 1-(2,5-dihydroxyphenyl)isopropylammonium cations, was studied in the presence of polyhistidine (PLH). It was found that the addition of PLH enhances the catalytic efficiency of Cu(II) toward negatively charged and neutral substrates, but inhibits it toward positively charged substrates. The PLH-Cu(II) unlike the Cu(II)-catalyzed reaction becomes zero order in substrate concentration at relatively low concentration of substrate and exhibits Michaelis-Menten kinetics. The specific effect of PLH on the Cu(II)-catalyzed reactions, which is abolished in the presence of moderate concentration of inert electrolytes, is interpreted by the electrostatic effect of the polyelectrolyte on the concentration of the reactants involved in the close vicinity of the bound copper. The analogy between the PLH-Cu(II)-catalyzed reactions and copper-containing oxidases is discussed.