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