Polypseudorotaxanes, by which α-cyclodextrin (α-CD) rings shuttle alongside a poly(ethylene glycol) (PEG) chain, are promising candidates for molecular machines. Nevertheless, their molecular dynamics have remained unclear. Researchers have now used fast-scanning atomic power microscopy (FS-AFM) to visualise α-CD rings shifting alongside a PEG chain. This breakthrough establishes FS-AFM as a strong device for analyzing supramolecular polymers and paves the best way for designing environment friendly molecular motors.
Think about a microscopic locomotive shifting forwards and backwards alongside a monitor, propelling itself with none exterior power. On the molecular stage, this idea kinds the muse of molecular motors — intricate techniques that might allow superior supplies, focused drug supply, and the event of nanoscale robotics.
Impressed by nature’s molecular machines, scientists have been growing synthetic counterparts for the reason that first artificial molecular machine was created in 1994. This analysis has progressed quickly, culminating within the 2016 Nobel Prize in Chemistry for breakthroughs in molecular machine design. One promising candidate is polypseudorotaxane, a construction the place a poly(ethylene glycol) (PEG) polymer chain is threaded by a number of α-cyclodextrin (α-CD) rings. In aqueous options, these rings self-assemble onto the PEG chain and transfer alongside its size. Nevertheless, the precise structural adjustments behind this motion have remained unclear — till now.
Lately, scientists from the Japan Superior Institute of Science and Know-how (JAIST) have visualized the dynamic shuttling of α-CD rings alongside the PEG chain in actual time, revealing localized structural adjustments that had been beforehand unclear. Utilizing a specialised microscope referred to as fast-scanning atomic power microscopy (FS-AFM), the staff, led by Affiliate Professor Ken-ichi Shinohara, captured real-time photographs of α-CD rings shifting alongside the PEG chain. Their examine, revealed in Macromolecules on March 4, 2025, introduces a brand new methodology for analyzing the construction of supramolecular polymers — an strategy that was beforehand unattainable and will pave the best way for extra superior molecular machines.
“Though PEG@α-CD polypseudorotaxane is extensively used, the structural adjustments that happen as α-CD rings shuttle alongside the polymer chain stay poorly understood. By revealing its construction on the solid-liquid interface, our examine will contribute to the event of artificial polymer motors pushed by thermal fluctuations,” explains Dr. Shinohara.
To organize the polypseudorotaxane, the researchers combined PEG100k with α-CD in an aqueous answer and allowed the pattern to relaxation for greater than six hours. This course of led to the formation of a white stable, which they then analyzed utilizing FS-AFM in a 15 millimolar potassium chloride aqueous answer. In contrast to common optical microscopes, AFM makes use of an ultra-sharp tip on a tiny lever to scan surfaces, capturing nanoscale options and producing high-resolution photographs.
Imaging of the PEG100k chain alone revealed a extremely versatile, dumbbell-shaped construction with globules at each ends. This flexibility gave it spring-like properties, permitting it to broaden and contract freely. In consequence, when relaxed, the chain appeared a lot shorter (averaging 48.1 nm) than its precise stretched-out size of 790 nm. When α-CD rings had been added, they decreased the chain’s flexibility. Imaging the PEG100k@α-CD polypseudorotaxane confirmed a considerably longer (499.6 nm on common) and a extra inflexible construction, with the end-cap formations stopping the α-CD rings from slipping off. Curiously, regardless of being much less versatile, the chain nonetheless exhibited a spring-like movement, as α-CD rings continued to shuttle alongside its size.
“We noticed that the polypseudorotaxane exhibited shrinking and lengthening motions pushed by the shuttling of α-CD rings alongside the polymer chain. These actions primarily occurred within the uncovered, self-shrinking PEG segments, the place repeated growth and contraction had been noticed because the α-CD rings moved,” explains Dr. Shinohara. Molecular dynamics simulations additional confirmed these findings, reproducing the shrinking and lengthening motions noticed within the FS-AFM experiments.
Though absolutely practical molecular machines stay a long-term purpose, this examine lays the groundwork for understanding molecular movement in supramolecular techniques. “FS-AFM is a promising method for analyzing supramolecular supplies, particularly when typical spectroscopic strategies are unsuitable for structural evaluation,” remarks Dr. Shinohara. These insights might result in energy-efficient molecular motors that harness thermal vitality at room temperature for managed motion.
