Conjugated Cofactor Enables Efficient Temperature-Independent Electronic Transport Across ∼6 nm Long Halorhodopsin
Author(s):
Sabyasachi Mukhopadhyay, Sansa Dutta, Israel Pecht, Mordechai Sheves David Cahen
Journal:
Journal of the American Chemical Society
Year:
2015
Volume:
137
Pages
11226–11229
DOI:
10.1021/jacs.5b06501
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.