Electron transfer with self-assembled copper ions at Au-deposited biomimetic films : mechanistic "anomalies" disclosed by temperature- and pressure-assisted fast-scan voltammetry

It has been suggested that electron transfer (ET) processes occurring in complex environments capable of glass transitions, specifically in biomolecules, under certain conditions may experience the medium ’ s nonlinear response and nonergodic kinetic patterns. The interiors of self-assembled organic films (SAMs) deposited on solid conducting platforms (electrodes) are known to undergo glassy dynamics as well, hence they may also exhibit the abovementioned ‘ irregularities ’ . We took advantage of Cu 2+ ions as redox-active probes trapped in the Au-deposited − COOH-terminated SAMs, either L-cysteine, or 3-mercaptopropionic acid diluted by the inert 2-mercaptoethanol, to systematically study the impact of glassy dynamics on ET using the fast-scan voltammetry technique and its temperature and high-pressure extensions. We found that respective kinetic data can be rationalized within the extended Marcus theory, taking into account the frictionally controlled (adiabatic) mechanism for short-range ET, and complications due to the medium ’ s nonlinear response and broken ergodicity. This combination shows up in essential deviations from the conventional energy gap (overpotential) dependence and in essentially nonlinear temperature (Arrhenius) and high-pressure patterns, respectively. Biomimetic aspects for these systems are also discussed in the context of recently published results for interfacial ET involving self-assembled blue copper protein (azurin) placed in contact with a glassy environment

Electron transfer with myoglobin in free and strongly confined regimes: disclosing diverse mechanistic role of the Fe-coordinated water by temperature- and pressure-assisted voltammetric studies

<div><p>The naturally occurring electron-transfer (ET) event for myoglobin (Mb) can be mimicked through its functionalization at diversely modified metal platforms to allow for the electron exchange either in freely diffusing or immobilized regimes. In this work, horse muscle Mb was involved in the electron exchange with Au electrodes modified by dissimilar, thin or thick alkanethiol SAMs, terminated either by unicomponent (–OH) or 1 : 1 mixed (–OH/–COOH) functional (externally exposed) entities, respectively. The systematic, temperature- and pressure-supported cyclic voltammetry studies perfectly confirmed certainty of two kinds of ET patterns for Mb, embodying: (a) different operational kinetic regimes (including protein’s freely diffusing and strongly confined motifs) and (b) different intrinsic physical mechanisms (including dynamically controlled and non-adiabatic modes). Our analysis of obtained and published data for Mb and the reference redox-active protein, cytochrome <i>c</i>, specified further the central mechanistic role of the Fe-(heme-)coordinated water whose displacement is directly coupled to ET, and can be, in turn, controlled by the conformational organization and intrinsic fluctuational mobility of the Mb macromolecule.</p></div