The nanoscale imaging of charge flow in proteins is crucial to understanding several life processes, including respiration, metabolism and photosynthesis1,2,3. However, existing imaging methods are only effective under non-physiological conditions or are limited to photosynthetic proteins1. Here, we show that electrostatic force microscopy can be used to directly visualize charge propagation along pili of Geobacter sulfurreducens with nanometre resolution and under ambient conditions. Charges injected at a single point into individual, untreated pili, which are still attached to cells, propagated over the entire filament. The mobile charge density in the pili, as well as the temperature and pH dependence of the charge density, were similar to those of carbon nanotubes4 and other organic conductors5,6,7. These findings, coupled with a lack of charge propagation in mutated pili that were missing key aromatic amino acids8, suggest that the pili of G. sulfurreducens function as molecular wires with transport via delocalized charges, rather than the hopping mechanism that is typical of biological electron transport2,3,9.
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This research was supported by the Office of Naval Research (grant nos N00014-13-1-0550 and N00014-12-1-0229), the Office of Science (BER), US Department of Energy (award no. DE-SC0006790) and the National Science Foundation Centre for Hierarchical Manufacturing (grant no. CMMI-1025020). The authors thank A. Parsegian for discussions, Asylum Research for technical support, M. Vargas and M. Sharma for help with cell culturing, D. Callaham for help with TEM and S. Thirunavukkarasu and T. Emrick for use of the W.M. Keck Nanostructures Facility (University of Massachusetts Amherst) for EFM studies. The Asylum Research MFP-3D equipment used for EFM was purchased via the Polymer-Based Materials for Harvesting Solar Energy and Energy Frontier Research Centre funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (grant no. DE-SC0001087). N.S.M. holds a Career Award from the Scientific Interface from the Burroughs Wellcome Fund.
The authors declare no competing financial interests.
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Malvankar, N., Yalcin, S., Tuominen, M. et al. Visualization of charge propagation along individual pili proteins using ambient electrostatic force microscopy. Nature Nanotech 9, 1012–1017 (2014). https://doi.org/10.1038/nnano.2014.236
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