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We provide a comprehensive approach to the formation and characterization of molecular monolayers of the blue copper protein Pseudomonas aeruginosa azurin on Au(111) in aqueous ammonium acetate solution. Main issues are adsorption patterns, reductive desorption, properties of the double layer, and long-range electrochemical electron transfer between the electrode and the copper center. Voltammetry, electrochemical impedance spectroscopy (EIS), in situ scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS) have been employed to disclose features of these issues. Zn-substituted azurin, cystine, and 1-butanethiol are investigated for comparison. Cyclic voltammetric and capacitance measurements show qualitatiVely that azurin is adsorbed at submicromolar concentrations over a broad potential range. The characteristics of reductive desorption suggest that azurin is adsorbed via its disulfide group to form a monolayer. The adsorption of this protein on Au(111) via a gold-sulfur binding mode is further supported by XPS measurements. In situ STM images with molecular resolution have been recorded and show a dense monolayer organization of adsorbed azurin molecules. Direct electron transfer (ET) between the copper atom of adsorbed azurin and the electrode has been revealed by differential pulse voltammetry. The rate constant is estimated from electrochemical impedance spectroscopy and shows that ET is compatible with a long-range ET mode such as that anticipated by theoretical frames. The results constitute the first case of an electrochemically functional redox protein monolayer at single-crystal metal electrodes.

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X-ray absorption spectroscopy at the sulfur K-edge (∼2470 eV) has been applied to a series of 2Fe−2S model complexes to obtain insight into their electronic structures. Since these 2Fe−2S complexes contain both terminal thiolates and bridging sulfides, contributions to covalency from both sets of ligands can be evaluated. Importantly, the pre-edge feature of sulfide can be resolved from that of thiolate due to differences in effective nuclear charge. In our previous studies, the covalency of the metal−thiolate bond in [Fe(SR)4]- was determined. In this study, sulfide covalency is quantified for the first time on the basis of an analysis of previous X-ray photoelectron and X-ray absorption spectroscopic studies of [FeCl4]- which are then applied to the bis-μ2-sulfide compound KFeS2. With references for both sulfide and thiolate covalencies thus established for open d-shell systems, comparisons are made between thiolate and sulfide bonding. Sulfide−Fe covalency in the [Fe2S2(SR)4]2- complexes is higher than thiolate−Fe covalency, indicating extensive charge donation of the bridging sulfides. Finally, this investigation of model complexes is extended to the oxidized and reduced 2Fe−2S cluster of the Rieske protein of Paracoccus denitrificans which has terminal thiolates on one Fe center, and histidines on the other Fe center. It is determined that thiolate covalency of the Fe(III) center is the same in both the oxidized and reduced Rieske clusters and similar to that of the [Fe2S2(SR)4]2- model complexes. Further, in the fully oxidized Rieske cluster, the sulfide covalency of the ferric center containing terminal histidine ligation is ∼18% higher than the Fe(III) containing terminal thiolate ligation. This is consistent with the fact that the histidine ligands are poorer donors and supports the suggestion that the terminal histidine ligation makes a significant contribution to the higher reduction potential of the Rieske protein.