Redox activity distinguishes solid-state electron transport from solution-based electron transfer in a natural and artificial protein: cytochrome C and hemin-doped human serum albumin

Abstract

Integrating proteins in molecular electronic devices requires control over their solid-state electronic transport behavior. Unlike "traditional" electron transfer (ET) measurements of proteins that involve liquid environments and a redox cycle, no redox cofactor is needed for solid-state electron transport (ETp) across the protein. Here we show the fundamental difference between these two approaches by macroscopic area measurements, which allow measuring ETp temperature dependence down to cryogenic temperatures, via cytochrome C (Cyt C), an ET protein with a heme (Fe-porphyrin) prosthetic group as a redox centre. We compare the ETp to electrochemical ET measurements, and do so also for the protein without the Fe (with metal-free porphyrin) and without porphyrin. As removing the porphyrin irreversibly alters the protein's conformation, we repeat these measurements with human serum albumin (HSA), 'doped' (by non-covalent binding) with a single hemin equivalent, i.e., these natural and artificial proteins share a common prosthetic group. ETp via Cyt C and HSA-hemin are very similar in terms of current magnitude and temperature dependence, which suggests similar ETp mechanisms via these two systems, thermally activated hopping (with ~0.1 eV activation energy) >190 K and tunneling by superexchange <190 K. Also, ET rates to and from the Fe redox centres (Fe(2+) <=> Fe(3+) + e(-)), measured by electrochemistry of HSA-hemin are only 4 times lower than those for Cyt C. However, while removing the Fe redox centre from the porphyrin ring markedly affects the ET rate, it hardly changes the ETp currents through these proteins, while removing the macrocycle (from HSA, which retains its conformation) significantly reduces ETp efficiency. These results show that solid-state ETp across proteins does not require the presence of a redox cofactor, and that while for ET the Fe ion is the main electron mediator, for ETp the porphyrin ring has this function.

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

“…This lack of correlation between ETp and protein monolayer width (electrode separation) underscores that different orientations of the same protein can present distinct charge transport media between the two electrodes. This can also be related to the cofactor position with respect to the electrodes because, as we have shown previously, cofactors are crucial mediators in ETp, even though, in contrast to ET, they do not need to be redox-active to achieve efficient ETp (28,33).…”

“…It is essential to note that the barrier for ETp across WT CytC is influenced and possibly dominated by the protein's contact to the electrode, and as such it is distinct from the E a of ET reactions between a donor and acceptor (usually determined by the driving force and reorganization energy), as treated by Marcus theory. In this context, it is important to note that we have previously shown that in contrast to an electrochemical process, ETp across a protein does not require a redox process (33).…”

Sensing of molecules using quantum dynamics

“…Eq. 10 can be used directly in the modeling of electron transport through solid-state protein junctions (27,28) or redox junctions (29)(30)(31)(32). The semiconductor's band structure should be used for quantitative descriptions of conduction through typical FET bridges.…”

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.…”