The combination of low-temperature scanning tunnelling microscopy with a mass-selective electro-spray ion-beam deposition established the investigation of large biomolecules at nanometer and sub-nanometer scale. Due to complex architecture and conformational freedom, however, the chemical identification of building blocks of these biopolymers often relies on the presence of markers, extensive simulations, or is not possible at all. Here, we present a molecular probe-sensitisation approach addressing the identification of a specific amino acid within different peptides. A selective intermolecular interaction between the sensitiser attached at the tip-apex and the target amino acid on the surface induces an enhanced tunnelling conductance of one specific spectral feature, which can be mapped in spectroscopic imaging. Density functional theory calculations suggest a mechanism that relies on conformational changes of the sensitiser that are accompanied by local charge redistributions in the tunnelling junction, which, in turn, lower the tunnelling barrier at that specific part of the peptide.

Department of Chemistry University of Oxford Oxford UK

Department of Condensed Matter Physics Faculty of Mathematics and Physics Charles University Prague Czech Republic

Institut de Physique École Polytechnique Fédérale de Lausanne Lausanne Switzerland

Institute of Applied Physics and Laboratory for Emerging Nanometrology Technische Universität Braunschweig 38104 Braunschweig Germany

Institute of Physics of the Czech Academy of Science Prague Czech Republic

Max Planck Institute for Solid State Research Stuttgart Germany

National Institute of Materials Physics 077125 Magurele Romania

School of Integrated Circuits and Electronics Beijing Institute of Technology Beijing 100081 China

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