Tunneling explains efficient electron transport via protein junctions
Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved April 9, 2018 (received for review November 15, 2017)
Significance
Investigation of the charge transport mechanism across a monolayer of a redox active protein is important for the fundamental understanding of the naturally occurring electron transfer processes, such as those in photosynthesis or respiration. Inelastic electron tunneling spectroscopy measurements of a redox active protein may provide direct experimental evidence that the tunneling charges are, in fact, passing through the protein molecules. Results of our study of conductance via well-controlled azurin monolayer solid-state junctions show the direct involvement of the Cu(II) site in assisting electron transport, underscoring this site’s vibronic characteristics associated with the charge transport mechanism. Our study widens the scope of currently available methodologies and also adds to the potential of using proteins in bioelectronics.
Abstract
Metalloproteins, proteins containing a transition metal ion cofactor, are electron transfer agents that perform key functions in cells. Inspired by this fact, electron transport across these proteins has been widely studied in solid-state settings, triggering the interest in examining potential use of proteins as building blocks in bioelectronic devices. Here, we report results of low-temperature (10 K) electron transport measurements via monolayer junctions based on the blue copper protein azurin (Az), which strongly suggest quantum tunneling of electrons as the dominant charge transport mechanism. Specifically, we show that, weakening the protein–electrode coupling by introducing a spacer, one can switch the electron transport from off-resonant to resonant tunneling. This is a consequence of reducing the electrode’s perturbation of the Cu(II)-localized electronic state, a pattern that has not been observed before in protein-based junctions. Moreover, we identify vibronic features of the Cu(II) coordination sphere in transport characteristics that show directly the active role of the metal ion in resonance tunneling. Our results illustrate how quantum mechanical effects may dominate electron transport via protein-based junctions.
Acknowledgments
We thank Prof. Spiros Skourtis (University of Cyprus), Dr. Cunlan Guo, and Mr. Ben Kayser (Weizmann Institute of Science) for fruitful discussions. J.A.F. thanks the Azrieli Foundation for the award of an Azrieli Fellowship. M.S. and D.C. thank the Israel Science Foundation, the Minerva Foundation, the Nancy and Stephen Grand Center for Sensors and Security, the Benoziyo Endowment Fund for the Advancement of Science, and J & R Center for Scientific Research for partial support. M.S. holds the Katzir–Makineni Chair in Chemistry; D.C. held the Schaefer Professorial Chair in Energy Research. J.C.C. acknowledges funding from the Spanish Ministry of Economy, Industry, and Competitiveness (Projects FIS2014-53488-P and FIS2017-84057-P) and thanks the German Research Foundation (DFG) and Collaborative Research Center (SFB) 767 for sponsoring his stay at the University of Konstanz as a Mercator Fellow.
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Cover image: Pictured is the reef-building coral Acropora millepora, releasing egg–sperm bundles into seawater. Phillip A. Cleves et al. used the CRISPR/Cas9 genome-editing system to introduce mutations into specific genes in fertilized A. millepora eggs. The authors detected mutations in about half of the resulting larvae. The results demonstrate the feasibility of using genome editing to understand the biological roles of individual genes in corals. See the article by Cleves et al. on pages 5235–5240. Image courtesy of Patrick Buerger (Commonwealth Scientific and Industrial Research Organization, Canberra, Australia).
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© 2018. Published under the PNAS license.
Submission history
Published online: April 30, 2018
Published in issue: May 15, 2018
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Acknowledgments
We thank Prof. Spiros Skourtis (University of Cyprus), Dr. Cunlan Guo, and Mr. Ben Kayser (Weizmann Institute of Science) for fruitful discussions. J.A.F. thanks the Azrieli Foundation for the award of an Azrieli Fellowship. M.S. and D.C. thank the Israel Science Foundation, the Minerva Foundation, the Nancy and Stephen Grand Center for Sensors and Security, the Benoziyo Endowment Fund for the Advancement of Science, and J & R Center for Scientific Research for partial support. M.S. holds the Katzir–Makineni Chair in Chemistry; D.C. held the Schaefer Professorial Chair in Energy Research. J.C.C. acknowledges funding from the Spanish Ministry of Economy, Industry, and Competitiveness (Projects FIS2014-53488-P and FIS2017-84057-P) and thanks the German Research Foundation (DFG) and Collaborative Research Center (SFB) 767 for sponsoring his stay at the University of Konstanz as a Mercator Fellow.
Notes
This article is a PNAS Direct Submission.
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Competing Interests
The authors declare no conflict of interest.
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