Electrochemistry of self-assembled monolayers of the blue copper protein Pseudomonas aeruginosa azurin on Au(111)

We report the self-assembly and electrochemical behaviour of the blue copper protein Pseudomonas aeruginosa azurin on Au(111) electrodes in aqueous acetate buffer (pH=4.6). The formation of monolayers of this protein is substantiated by electrochemical measurements. Capacitance results indicate qualitatively that the protein is strongly adsorbed at sub-μM concentrations in a broad potential range (about 700 mV). This is further supported by the attenuation of a characteristic cyclic voltammetric peak of Au(111) in acetate solution with increasing azurin concentration. Reductive desorption is clearly disclosed in NaOH solution (pH=13), strongly suggesting that azurin is adsorbed via its disulphide group. An anodic peak and a cathodic peak associated with the copper centre of azurin are finally observed in the differential pulse voltammograms. These peaks are, interestingly, indicative of long-range electrochemical electron transfer such as paralleled by intramolecular electron transfer between the disulphide anion radical and the copper atom in homogeneous solution, and anticipated by theoretical frames. Together with reported in situ scanning tunnelling microscopy (STM) results they constitute the first case for electrochemistry of self-assembled monolayers of azurin, even redox proteins. This integrated investigation provides a new approach to both structure and function of adsorbed redox metalloproteins at the molecular level.