Real-Time Fluctuations in Single-Molecule Rotaxane Experiments Reveal an Intermediate Weak Binding State during Shuttling

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Real-Time Fluctuations in Single-Molecule Rotaxane Experiments Reveal an Intermediate Weak Binding State during Shuttling

Damien Sluysmans, Perrine Lussis, Charles-André Fustin, Andrea Bertocco, David A. Leigh, and Anne-Sophie Duwez

UR MolSys, NanoChem group, University of Liège (BE)

Institute of Condensed Matter and Nanosciences (IMCN),

Université Catholique de Louvain (BE)

Department of Chemistry, University of Manchester (UK)

We report on the use of atomic force microscopy
(AFM) to identify and characterize an intermediate state in
macrocycle shuttling in a hydrogen bonded amide-based molecular
shuttle. The [2]rotaxane consists of a benzylic amide macrocycle
mechanically locked onto a thread that bears both fumaramide and
succinic amide-ester sites, each of which can bind to the
macrocycle through up to four intercomponent hydrogen bonds.
Using AFM-based single-molecule force spectroscopy, we mechanically triggered the translocation of the ring between the two principal binding sites (“stations”) on the axle. Equilibrium
fluctuations reveal another interacting site involving the two
oxygen atoms in the middle of the thread. We characterized the
ring occupancy distribution over time, which confirms the intermediate in both shuttling directions. The study provides
evidence of weak hydrogen bonds that are difficult to detect using other methods and shows how the composition of the thread can significantly influence the shuttling dynamics by slowing down the ring motion between the principal binding sites. More generally, the study illustrates the utility that single-molecule experiments, such as force spectroscopy, can offer for elucidating the structure and dynamics of synthetic molecular machines.

2352. Sluysmans et al., J. Am. Chem. Soc. 2021, 143, 2348-2352.