Molecular monolayers and interfacial electron transfer of Pseudomonas aeruginosa azurin on Au(111)

We provide a comprehensive approach to the formation and characterization o f molecular monolayers of the blue copper protein Pseudomonas aeruginosa az urin on Au(111) in aqueous ammonium acetate solution. Main issues are adsor ption patterns, reductive desorption, properties of the double layer, and l ong-range electrochemical electron transfer between the electrode and the c opper center. Voltammetry, electrochemical impedance spectroscopy (EIS), in situ scanning tunneling microscopy (STM), and X-ray photoelectron spectros copy (XPS) have been employed to disclose features of these issues. Zn-subs tituted azurin, cystine, and 1-butanethiol are investigated for comparison. Cyclic voltammetric and capacitance measurements show qualitatively that a zurin is adsorbed at submicromolar concentrations over a broad potential ra nge. The characteristics of reductive desorption suggest that azurin is ads orbed via its disulfide group to form a monolayer. The adsorption of this p rotein on Au(lll) via a gold-sulfur binding mode is further supported by XP S measurements. In situ STM images with molecular resolution have been reco rded and show a dense monolayer organization of adsorbed azurin molecules. Direct electron transfer (ET) between the copper atom of adsorbed azurin an d the electrode has been revealed by differential pulse voltammetry. The ra te constant is estimated from electrochemical impedance spectroscopy and sh ows that ET is compatible with a long-range ET mode such as that anticipate d by theoretical frames. The results constitute the first case of an electr ochemically functional redox protein monolayer at single-crystal metal elec trodes.