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).