Electron transfer with azurin at Au/SAM junctions in contact with a protic ionic melt

Author: Tatyana Tretyakova
Co-authors: Dimitry Khoshtariya, Tinatin Dolidze, Rudi van Eldik
Keywords: Electron transfer, adiabatic mechanism, azurin, voltammetry
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Biological electron transfer (ET) with small redox proteins placed in contact with a liquid phase electrolyte has been studied extensively in the past. Semi-solid conditions normally providing protein stabilization may shed light on the intrinsic links between the protein’s dynamical and functional properties near the glass-forming threshold, from both fundamental and applied perspectives. A semi-solid electrolyte may provide a more realistic mimic of the environment in a cell or biological membrane compared to a simple aqueous electrolyte. Hence, kinetic studies of such assemblies are highly relevant and may improve our understanding of electron transfer in biological structures. However, systematic voltammetry studies of biological ET in semi-solid media have been limited because of the poor solution conductivity of semi-rigid media and/or the other technical difficulties. Fortunately, a number of protic ionic melts, including choline dihydrogen phosphate ([ch][dhp]), have been reported to possess remarkable bio-compatibility and significant ionic (protic) conductance, which makes them useful as additives to enhance the stability and robustness of electroactive Au/SAM/protein assemblies. This work explores interfacial biological ET in a protic electrolyte mixture that gradually extends over the liquid and semisolid electrolyte conditions with a change in composition. The Au/SAM/azurin assemblies (SAM, alkanethiol self-assambled monolayers; Az, blue cupredoxin) were placed in contact with a buffered protic ionic melt, choline dihydrogen phosphate ([ch][dhp]), containing from < 2 to 15 water molecules per [ch][dhp] and the ET was studied as a function of temperature and pressure by rapid-scan cyclic voltammetry. The extra confinement of the Az films within the highly rigidified environment affects the protein’s conformational dynamics, hence the ET rate, via the mechanism occurring in a friction controlled regime resulting in a remarkable interplay of the dynamically-controlled ET mechanism with accompanying non-ergodic and nonlinear effects for biological ET.