Electron transfer with azurin at Au–SAM junctions in contact with a protic ionic melt: impact of glassy dynamics

Abstract

Gold electrodes were coated with alkanethiol SAM-azurin (Az, blue cupredoxin) assemblies and placed in contact with a water-doped and buffered protic ionic melt as the electrolyte, choline dihydrogen phosphate ([ch][dhp]). Fast-scan protein-film voltammetry was applied to explore interfacial biological electron transfer (ET) under conditions approaching the glass-transition border. The ET rate was studied as a function of the water amount, temperature (273-353 K), and pressure (0.1-150 MPa). Exposure of the Az films to the semi-solid electrolyte greatly affected the protein's conformational dynamics, hence the ET rate, via the mechanism occurring in the extra complicated dynamically-controlled regime, is compared to the earlier studies on the reference system with a conventional electrolyte (D. E. Khoshtariya et al., Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 2757-2762), allowing for the disclosure of even more uncommon mechanistic motifs. For samples with low water content (ca. 3 or less waters per [ch][dhp]), at moderately low temperatures (below ca. 298 K) and/or high pressure (150 MPa), the voltammetric profiles systematically deviated from the standard Marcus current-overvoltage pattern, deemed as attributable to a breakdown of the linear response approximation through the essential steepening of the Gibbs energy wells near the glass-forming threshold. Electrolytes with a higher water content (6 to 15 waters per [ch][dhp]) display anomalous temperature and pressure performances, suggesting that the system crosses a broad nonergodic zone which arises from the interplay of ET-coupled large-scale conformational (highly cooperative) modes of the Az protein, inherently linked to the electrolyte's (water-doped [ch][dhp]) slowest collective relaxation(s).