Conjugated Cofactor Enables Efficient Temperature-Independent Electronic Transport Across ∼6 nm Long Halorhodopsin

Abstract

We observe temperature-independent electron transport, characteristic of tunneling across a ∼6 nm thick Halorhodopsin (phR) monolayer. phR contains both retinal and a carotenoid, bacterioruberin, as cofactors, in a trimeric protein-chromophore complex. This finding is unusual because for conjugated oligo-imine molecular wires a transition from temperature-independent to -dependent electron transport, ETp, was reported at ∼4 nm wire length. In the ∼6 nm long phR, the ∼4 nm 50-carbon conjugated bacterioruberin is bound parallel to the α-helices of the peptide backbone. This places bacterioruberin's ends proximal to the two electrodes that contact the protein; thus, coupling to these electrodes may facilitate the activation-less current across the contacts. Oxidation of bacterioruberin eliminates its conjugation, causing the ETp to become temperature dependent (>180 K). Remarkably, even elimination of the retinal-protein covalent bond, with the fully conjugated bacterioruberin still present, leads to temperature-dependent ETp (>180 K). These results suggest that ETp via phR is cooperatively affected by both retinal and bacterioruberin cofactors.

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

“…We have,t hus far,o bserved ETp that is completely temperature-independent via holo-Az and halorhodopsin. [4,31,32] This type of ETp suggests (coherent) tunneling up to room temperature.Azisoriented in relation to the surface via the covalent bonds between its cysteine thiolate residues and the surface.W eh ypothesize that its temperature-independent ETp is due to the existence,i na ddition to the covalent bond between the protein and one electrode,o f strong electronic coupling between the other end of azurin and the second electrode.That latter end contains the Cu ion with its first coordination shell being less than 5away from its respective electrode.Insuch aconfiguration, the electrode contact may well be more efficient than otherwise.…”

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

“…[4,31,32] This type of ETp suggests (coherent) tunneling up to room temperature.Azisoriented in relation to the surface via the covalent bonds between its cysteine thiolate residues and the surface.W eh ypothesize that its temperature-independent ETp is due to the existence,i na ddition to the covalent bond between the protein and one electrode,o f strong electronic coupling between the other end of azurin and the second electrode.That latter end contains the Cu ion with its first coordination shell being less than 5away from its respective electrode.Insuch aconfiguration, the electrode contact may well be more efficient than otherwise. [4,31,32] This type of ETp suggests (coherent) tunneling up to room temperature.Azisoriented in relation to the surface via the covalent bonds between its cysteine thiolate residues and the surface.W eh ypothesize that its temperature-independent ETp is due to the existence,i na ddition to the covalent bond between the protein and one electrode,o f strong electronic coupling between the other end of azurin and the second electrode.That latter end contains the Cu ion with its first coordination shell being less than 5away from its respective electrode.Insuch aconfiguration, the electrode contact may well be more efficient than otherwise.…”

Protein bioelectronics: a review of what we do and do not know

“…For macroscopic contacts, a dense protein monolayer is prepared on conducting substrates, such as a highly doped Si wafer, or template-stripped gold/silver (rms roughness  1 nm), which can serve as substrates and bottom electrode of protein-based junctions. 89,90,151,219 Low roughness is important for macroscopic junctions, because otherwise the probability for electrical shorts is too high, leading to questionable results and/or low junction yields. Common top contacts are Hg or In-Ga eutectic (E-GaIn) electrodes.…”