Mechanically interlocked molecules (MIMs), such as rotaxanes, catenanes and molecular knots, have attracted a lot of attention in the past years for the design of artificial molecular machines. Their use as molecular switches, machines, or motors has been intensively discussed in the literature. The mechanochemical properties of individual MIMs have been studied using single-molecule techniques such as atomic force microscopy (AFM) or optical tweezers. Recently, we reported on the AFM-based single-molecule force spectroscopy (SMFS) of oligorotaxanes presenting a folded conformation in solution1,2. We evidenced remarkable force performances and fast dynamics of such artificial molecules compared to their natural analogs. Other similar investigations presented the single-molecule characterization of rotaxanes and catenanes in solution3-5. Here we show the first example of AFM-based pulling experiments on artificial molecular overhand knots at the single-molecule level. The synthesis6 of a trefoil knot was modified to facilitate its interfacing for AFM experiments. The pre-assembly of 2,6-pyridinedicarboxyamide ligands coordinated around a Lu3+ ion was followed by the attachment of two poly(ethylene glycol) (PEG) tethers functionalized with thiolane end-groups. The molecules were attached to gold-coated substrates (by strong Au-S interactions) under highly-diluted conditions favoring the probing of individual molecules. Standard SMFS experiments on individual knots evidenced force deviations from the usual force-extension curves of flexible polymers. Characteristic force increases were observed repeatedly at pulling extensions similar to the total PEG length, thus indicating a mechanical tension applied to the knotted part of the molecule. Precise measurements of the length revealed after these deviation features helped uncovering the mechanically-induced tying pathway. Such challenging studies are essential to understand the impact of mechanical bonds on the properties of wholly synthetic architectures and are crucial for further uses of MIMs in the design of functional materials.

Probing the mechanochemical properties of a synthetic knot by AFM

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.