Single-molecule electrometry


Mass and electrical charge are fundamental properties of biological macromolecules. Although molecular mass has long been determined with atomic precision, a direct and precise determination of molecular charge remains an outstanding challenge. Here we report high-precision (<1e) measurements of the electrical charge of molecules such as nucleic acids, and globular and disordered proteins in solution. The measurement is based on parallel external field-free trapping of single macromolecules, permits the estimation of a dielectric coefficient of the molecular interior and can be performed in real time. Further, we demonstrate the direct detection of single amino acid substitution and chemical modifications in proteins. As the electrical charge of a macromolecule strongly depends on its three-dimensional conformation, this kind of high-precision electrometry offers an approach to probe the structure, fluctuations and interactions of a single molecule in solution.

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Figure 1: Long-term trapping of single nucleic acid and protein molecules in solution.
Figure 2: ETe to determine the electrical charge of a trapped species.
Figure 3: The effective electrical charge of nucleic acids and proteins measured using ETe.
Figure 4: Sequential ETe to detect chemical modifications and amino acid exchanges in a disordered protein.
Figure 5: Real-time measurement of the electrical charge of a single molecule.


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We acknowledge M. Borgia for the generous gift of unlabelled ProTαC protein, D. Nettels for help with fluorescence correlation spectroscopy (FCS) instrumentation and providing FCS data-analysis software, S. Chesnov from the Functional Genomics Center Zurich for mass spectrometry analysis, A. Caflisch, A. Soranno, G. Manning and R. Netz for discussions, and the Swiss National Science Foundation and University of Zurich for financial support. Nanofabrication was carried out at the FIRST Center for Micro- and Nanoscience, ETH Zurich.

Author information




F.R. performed the experiments and analysed the data. F.Z. and F.R. purified and labelled the proteins and nucleic acids and performed and analysed the FCS experiments. N.M., M.R., M.W. and A.O. provided the unlabelled Stm-l protein. F.Z. and B.S. contributed biochemical expertise, including protein selection. M.K. conceived the project, performed the theoretical analysis, interpreted the data and wrote the manuscript. F.R., F.Z. and B.S. provided feedback on the manuscript.

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Correspondence to Madhavi Krishnan.

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The authors declare no competing financial interests.

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Ruggeri, F., Zosel, F., Mutter, N. et al. Single-molecule electrometry. Nature Nanotech 12, 488–495 (2017).

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