Kanazawa University and Kyoto University visualize how cyclic molecules assembled on a substrate capture guest molecules

A joint research group including Professor Hitoshi Asakawa of the Faculty of Chemistry, Institute of Science and Engineering and Professor Mikihiro Shibata of the Nano Life Science Institute at Kanazawa University and Professor Tomoki Ogoshi of the Graduate School of Engineering and Program-Specific Senior Lecturer Takashi Sumikama of the Graduate School of Biostudies at Kyoto University, has successfully visualized, at the single-molecule level, how cyclic molecules assembled on a substrate cooperate to capture external guest molecules. The findings were published in the online edition of the Journal of the American Chemical Society.

The research group focused on host-guest complexes formed when linear molecules are incorporated into pillar[5]arenes, which are cyclic molecules with microscopic internal pores, arranged at high density on a substrate. By combining two types of AFM technologies, frequency modulation AFM (FM-AFM) and high-speed AFM (HS-AFM), they succeeded in directly observing both the spatial distribution and temporal changes at the single-molecule level. Through FM-AFM observation, they visualized extremely small structural changes of less than 100 picometers accompanying the formation of the host-guest complexes, clarifying the spatial distribution at the molecular level.

As a result, it was discovered that the host-guest complexes were not uniformly distributed but formed heterogeneously, exhibiting a "cooperative behavior" where the capture of a guest molecule by one host molecule influences surrounding molecules, making subsequent binding more likely to occur. In particular, a characteristic distribution was observed where binding occurred preferentially at locations approximately 3 nanometers apart.

In collaboration with molecular dynamics simulations, this phenomenon is thought to stem from interactions and steric constraints among the densely accumulated host molecules. Furthermore, statistical analysis quantitatively demonstrated the cooperative effect, revealing that the binding behavior changes significantly when molecules are densely packed. Additionally, through HS-AFM observation, they directly visualized the dynamics in which the formation and dissociation of host-guest complexes are repeated.

The lifetime of individual complexes ranged on a time scale from milliseconds to seconds, successfully capturing single-molecule-level dynamics that could not be obtained by the conventional analysis method based on averaged measurements.

This demonstrated that the reversible binding and dissociation processes in host-guest interactions can be directly tracked at the single-molecule level. The phenomenon of reversible binding and dissociation of functional molecules is a fundamental mechanism used in various molecular systems, including in living organisms, and this research visualized the basic principle at the single molecule level.

These findings are expected to lead to future applications of cyclic molecules in chemical sensors with higher sensitivity and storage materials with higher functionality and performance, as well as to the realization of a safe and secure society and solutions to energy and environmental problems.

Journal Information
Publication: Journal of the American Chemical Society
Title: Cooperative Host-Guest Complexation in Densely Assembled Host Structures on Surfaces Revealed at the Single-Molecular Level
DOI: 10.1021/jacs.6c00891