Solid-state electron transport via cytochrome c depends on electronic coupling to electrodes and across the protein

Abstract

Electronic coupling to electrodes, Γ, as well as that across the examined molecules, H, is critical for solid-state electron transport (ETp) across proteins. Assessing the importance of each of these couplings helps to understand the mechanism of electron flow across molecules. We provide here experimental evidence for the importance of both couplings for solid-state ETp across the electron-mediating protein cytochrome c (CytC), measured in a monolayer configuration. Currents via CytC are temperature-independent between 30 and ∼130 K, consistent with tunneling by superexchange, and thermally activated at higher temperatures, ascribed to steady-state hopping. Covalent protein-electrode binding significantly increases Γ, as currents across CytC mutants, bound covalently to the electrode via a cysteine thiolate, are higher than those through electrostatically adsorbed CytC. Covalent binding also reduces the thermal activation energy, E a , of the ETp by more than a factor of two. The importance of H was examined by using a series of seven CytC mutants with cysteine residues at different surface positions, yielding distinct electrode-protein(-heme) orientations and separation distances. We find that, in general, mutants with electrode-proximal heme have lower E a values (from high-temperature data) and higher conductance at low temperatures (in the temperatureindependent regime) than those with a distal heme. We conclude that ETp across these mutants depends on the distance between the heme group and the top or bottom electrode, rather than on the total separation distance between electrodes (protein width).bioelectronics | temperature dependence | protein conduction

A Solid‐State Protein Junction Serves as a Bias‐Induced Current Switch

“…[10] TheA u-bound CytC mutant (E104C) yields oriented monolayers, sufficientlyrobust to carry out solid-state electron-transport measurements [19] at both room and cryogenic temperatures( approximately 10-15 K). Briefly, mm-sized Au electrodepairs were fabricated on aSiwafer by photolithography.M onolayers of CytC(E104C) were prepared by using the surface-exposed cysteine 104 residue that binds covalently to the micro-fabricated Au electrodei ns uch aw ay that its hemin is proximal to the top Au nanowire (AuNW) electrode( 0.8-0.9nm between the Fe III ion and the physical top contact).…”

“…[10] Thei mportance of this geometry is that the cofactor is not exposed on the surface.Asaresult, the coupling between the C-heme/ hemin cofactor and nearest Au electrode is weak, preserving the integrity of the C-heme/hemin cofactor electronic states when the protein becomes part of the junction. Here,f or the CytC(E104C), the C-heme/hemin cofactor lies approximately 1nma way from the top AuNW electrode,e mbedded within its peptide matrix.…”

Protein Electronic Conductors: Hemin–Substrate Bonding Dictates Transport Mechanism and Efficiency across Myoglobin

“…[14][15][16] Ad efined orientation can be achieved by binding the protein by as pecific group on its surface to as uitable substrate,a sw as found for Az. [14][15][16] Ad efined orientation can be achieved by binding the protein by as pecific group on its surface to as uitable substrate,a sw as found for Az.…”

Effects of Film Morphology and Surface Chemistry on the Direct Electrochemistry of Cytochrome c at Boron-Doped Diamond Electrodes

“…Finally, it should be noted that solid-state electron transport studies using mutant forms of cytochrome c integrated into a molecular electronic device (contacted to two electrodes) have produced useful insights on the importance of the protein’s electronic coupling to the electrode surface and orientation on the current that flows through the device [75,77,78]. Fundamentally, the current flow through the solid-state device is an electron flux across the protein without the need for a redox process, while the current measured in the electrochemical process is due to electronsshuttling to and from the Fe(III) and Fe(II) oxidation states of the heme.…”

“…In contrast removing the porphyrin ring irreversibly alters the protein conformation and reduces current flow in the solid-state measurement. Importantly, the solid-state measurements revealed a general trend of greater current flow when the heme is closest to the electrode (covalently or electrostatically bound) [77,78]. The results, which have relevance for solution-based electron transfer with the redox protein, suggest that the Fe heme cofactor-electrode proximity and likely the cofactor-electrode coupling are important for the most efficient current flow.…”