Reduction and Oxidation Processes of Blue Copper Proteins, Azurin, Pseudoazurin, Umecyanin, Stellacyanin, Plantacyanin, and Plastocyanin Approached by Cyclic and Potential Step Voltammetries

Direct electrochemistry of a series of blue copper proteins: azurin, pseudoazurin, umecyanin, stellacyanin, plantacyanin, and plastocyanin has been performed at a gold electrode modified with di-4-pyridyl disulfide and/or at a bare glassy carbon electrode. Well-resolved cyclic voltammograms with peak separation, ΔEp = 55—100 mV were obtained, allowing the facile determination of the redox potential of blue copper proteins. Protein molecules associate with the electrode surface through both electrostatic and hydrophobic interactions, of which the predominant one differs according to the combination of protein and electrode. Double-step voltammetry showed that redox processes of blue copper proteins depend profoundly on the translocation of the molecules (diffusion and/or change of orientation) at the electrode surface. The heterogeneous rate constants at pH 6.0 and 25 °C for both reduction and oxidation processes were independently determined to be the order of 10−3 to 10−4 cm s−1 by single-step voltammetry and were compared with those determined by cyclic voltammetry. Further, activation parameters for redox processes of some blue copper proteins have been determined from temperature dependence studies. The reaction pathway of blue copper proteins was discussed.