Single Molecule Studies of Cyclic Peptides Using Molecular Matrix at Liquid/Solid Interface by Scanning Tunneling Microscopy

We report in this work the single molecule studies of cyclic peptide, cyclosporine A (CsA), using a molecular network formed by star-shaped oligofluorene (StOF-COOH₃) at the liquid/solid interface by scanning tunneling microscopy (STM). Individual cyclosporine A can be identified and resolved in the molecular network, and the high-resolution STM images of CsA show polygon-like characteristics with a diameter of approximately 1.7 nm. Furthermore, the complex of CsA and Mg²⁺ has also been observed to adsorb inside of the molecular matrix. The STM results reveal two adsorption characteristics for the CsA−Mg²⁺ complex, which is suggestive of asymmetrical configurations of the complex. The difference in binding energy between the two observed adsorption configurations is estimated to be 1.88 kJ·mol⁻¹. These results help set the stage for studying the fine structures and functions of various cyclic peptides at the liquid/solid interface.