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Z4.04 - Bringing Bacteriorhodopsin Closer to Bioelectronics: Effects of Illumination and Humidity on Electron Transport 
Date/Time:
April 23, 2014   9:30am - 9:45am
 
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Dry bacteriorhodopsin, bR, (with only tightly bound H2O, needed to maintain the natural conformation, retained) is a fascinating candidate component for biomolecular electronics, considering the combination of its mechanical stability, electrical conduction and versatile optical and chemical properties. This promise and the conflicting results that have been reported, led to the present study. Electron transport across dry bR monolayers is governed by thermally activated hopping > 160K, along with tunneling via super-exchange at lower temperatures. Transport is dominated by the retinal cofactor, and protein conformation. As bR is a trans-membrane protein, in nature it is surrounded by membrane lipids from all but its top and bottom sides. For possible practical use as high a concentration of the active membrane component as possible is needed and to that end we studied monomeric lipid-deleted bR (dLbR). Monolayers of dLbR on conducting substrates are stable and maintain the photoactivity of the native membrane-bound protein. Electron transport across dLbR monolayers, both in dark- and light-adapted intermediate states, can only be probed under optical illumination at controlled humidity conditions. Such measurements were performed by conducting-probe AFM on junctions that contain only a few monomers. Such measurements need to be done at tip forces <10 nN, the limit of elastic behavior (i.e., at higher forces irreversible changes occur). We find green light-induced current enhancement and conductance modulation upon subsequent blue illumination in this configuration. The effect associated with bound water molecules on electronic transport was found by changing the external relative humidity (to affect the number of water molecules inside the protein) and probing junction conductance. Remarkably, we find significantly increased electronic conductance at higher humidities, suggesting that conformational effects are involved. Apart from their scientific value, these results emphasize the building block potential and relevance of dLbR for future bioelectronics.
 


 
 
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