Induced-fit expansion and contraction of a self-assembled nanocube finely responding to neutral and anionic guests

Abstract

Induced-fit or conformational selection is of profound significance in biological regulation. Biological receptors alter their conformation to respond to the shape and electrostatic surfaces of guest molecules. Here we report a water-soluble artificial molecular host that can sensitively respond to the size, shape, and charged state of guest molecules. The molecular host, i.e. nanocube, is an assembled structure consisting of six gear-shaped amphiphiles (GSAs). This nanocube can expand or contract its size upon the encapsulation of neutral and anionic guest molecules with a volume ranging from 74 to 535 Å3 by induced-fit. The responding property of this nanocube, reminiscent of a feature of biological molecules, arises from the fact that the GSAs in the nanocubes are connected to each other only through the hydrophobic effect and very weak intermolecular interactions such as van der Waals and cation-π interactions.

Supramolecular fluorescence sensor for liquefied petroleum gas

“…The effect of the encapsulation of guest molecules on the fluorescence property of the nanocube was investigated. The encapsulation of n-alkanes from n-propane to n-decane in the nanocube 13 brought about the increase in the fluorescence intensity without changing the fluorescence wavelength ( Fig. 2d).…”

“…12). The reason why smaller alkanes than propane (methane and ethane) cannot be trapped in the nanocube 13 would be due to entropic disadvantage arising from that more methane or ethane molecules should be needed to properly fill the inner space of the nanocube.…”

“…A main difference between the nanocube and other molecular hosts is that the nanocube finely responds to the guest molecules and outputs a strong fluorescence signal. This high responding property of the nanocube arises from its precise induced-fit ability 13 , which causes slight change in the molecular meshing between the GSAs in the nanocube to modulate AIE and ACQ. As the GSA (1·Cl 2 ) can easily be synthesized in a large scale 38 , the supramolecular LPG sensor can be fabricated from inexpensive materials (1·Cl 2 and water) and several commercial devices (UV lamp, detector, and monitor) shown in Fig.…”

“…After addition of Na 2 [ 10 B 12 H 12 ] to an aqueous solution of [1 6 ] 12+ , the solution was heated at 60°C for 30 min to complete the encapsulation. Complexes between [1 6 ] 12+ and liquid alkane molecules were prepared according to the literature 13 .…”

“…nanocube consisting of six molecules of gearshaped amphiphiles (GSAs), which tightly mesh with each other 12 . The nanocube has an~1-nm-sized hydrophobic inner cavity, where several hydrophobic and anionic species are encapsulated to alter the size of the nanocube in an induced-fit manner 13 . It was found that the self-assembly of the nanocube caused a blue fluorescence from hexaphenylbenzene moieties in GSAs at~450 nm through aggregation-induced emission (AIE) 14 and that the fluorescence intensity was enhanced upon encapsulation of hydrophobic guest molecules by a balance between AIE and aggregation-caused quenching (ACQ) 15 of the GSAs.…”