Molecular machines play an important role in a variety of key biological and cellular activities such as muscle contraction and intracellular vesicle transport. Many reports have been published on designing fully synthetic molecular machines and some successful examples show that they are able to use biased Brownian motion to perform work or collectively induce movement of much larger objects. For more than a decade, much of the information about how biological machines operate has been gleaned from force measurements made on single molecules. In contrast, few single-molecule force spectroscopy investigations have been successively realized on synthetic molecular machines. The rarity of such studies comes from the difficulty of developing proper tools and preparing appropriate molecules that can be interfaced with these techniques, especially when one wants to probe sub-molecular motions. Here, we report on the study of macrorotaxanes, i.e. [2]rotaxanes made of a macrocycle threaded onto a polymer chain, by AFM-based single-molecule force spectroscopy. Specific monomers and functions have been included on the polymer thread, such as 2,6-pyridine dicarboxamide, to assess the ability of macrorotaxanes to act as single molecule chemical sequencers. The macrocycle is initially hold into place on the thread by a 3+1 Pd(II) complex between a tridendate pyridine bis-amide ligand (macrocycle) and a monodendate pyridine ligand (thread). The bulky stopper near the macrocycle is equipped with a functional group allowing its chemisorption on a surface. A poly(ethylene oxide) chain is attached to the macrocycle to act as tether to link the macrocycle with the AFM tip. Force measurements indicate that the macrocycle does travel along the thread after being caught by the AFM tip and detaches from the binding station. Characteristic force profiles show the signature of the interaction between the macrocycle and functions present on the polymer thread, demonstrating the sequencing ability of the system.

Probing the ring motion along the polymer track of a macrorotaxane at the single molecule level

ACS Spring Meeting 2021

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