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.
Proteins in hyperthermophiles exhibit extremely high thermal stability unlike general proteins. These thermostable proteins are stabilized by weak molecular interactions such as hydrogen bonding, charge interactions and van der Waals (vdW) interactions, along with the hydrophobic effect. An in-depth understanding of the stabilization mechanisms will enable us to rationally design artificial molecules with very high thermal stability. Here we show thermally stable supramolecular assemblies composed of six identical amphiphilic molecules having an indented hydrophobic surface, held together by weak intermolecular interactions (vdW and cation-π interactions) and the hydrophobic effect in water. The disassembly temperature of one of the assemblies is over 150°C, which is higher than that of the most hyperthermophilic protein reported to date (PhCutA1). Study of the relationship between the structure of the components and the stability of the assemblies indicates that the hyperthermostability is achieved only if all the weak interactions and the hydrophobic effect work cooperatively.
Spontaneous formation of the heteroleptic cadmium(II) bis(terpyridine) complex under ambient conditions can be achieved by a combination of 6,6''-di(2,6-dimethoxylphenyl)-substituted and unsubstituted terpyridine-based ligands. Building on this dynamic heteroleptic complexation, diverse metallo-supramolecular macrocycles and cages were readily assembled in quantitative yields from the predesigned multicomponent systems. The complementary ligation reinforced self-recognition to facilitate the shape-dependent self-sorting of a four-component dynamic library into two well-defined parallelograms. In addition, the subtle lability difference between homoleptic and heteroleptic complexes led to the site-selective Cd -Zn transmetalation in the Sierpiński triangle. Facile construction of a dodecanuclear tetrahedral metallocage was also realized by using two self-recognizable tritopic building blocks. The photophysical study of the metallo-supramolecules assembled from the d metal ions revealed intense ligand-based photoluminescence in solution. The self-assembly strategy described here provides an efficient methodology for building pre-programmable, sophisticated supramolecular architectures furnished with photoactivity.
Separation of a homogeneous mixture of different components to reach an ordered out-of-equilibrium state in solution has attracted continuous attention. While this can be achieved using external chemical fuels or photo energy, an alternative energy source is heat. Here we realize a temperature-controlled cycle of transitions between ordered and disordered states based on a mixture of two kinds of building blocks that self-assemble into cubic structures (nanocubes). An almost statistical mixture of nanocubes (disordered state) is thermodynamically most stable at lower temperature (25 °C), while homoleptic assemblies composed of single components are preferentially produced at higher temperature (100 °C) followed by rapid cooling. The scrambling of the building blocks between the nanocubes takes place through the exchange of free building blocks dissociated from the nanocubes. Based on this mechanism, it is possible to accelerate, retard, and perfectly block the scrambling by the guest molecules encapsulated in the nanocubes.
A novel method for the semi-quantitative evaluation of molecular meshing in molecular complexes and assemblies (SAVPR: surface analysis with varying probe radii) is proposed. SAVPR revealed that the extremely high stability of hexameric assemblies (nanocubes) is due to tight molecular meshing between the components in the assemblies, indicating the importance of van der Waals interactions in hydrophobic molecular assemblies.
True understanding of dispersion interaction in solution remains elusive because of difficulty in the precise evaluation of its interaction energy. Here, the effect of substituents with different polarizability on dispersion interactions in water is discussed based on the thermodynamic parameters determined by isothermal titration calorimetry for the formation of discrete aggregates from gear-shaped amphiphiles (GSAs). The substituents with higher polarizability enthalpically more stabilize the nanocube, which is due to stronger dispersion interactions and to the hydrophobic effect. The differences in the thermodynamic parameters for the nanocubes from the GSAs with CH 3 and CD 3 groups are also discussed to lead to the conclusion that the H/D isotope effect on dispersion interactions is negligibly small, which is due to almost perfect entropy-enthalpy compensation between the two isotopomers.
Sensing systems of nonpolar gas molecules without functional groups such as natural gas and liquefied petroleum gas (LPG) remain difficult to develop because of lacking selective detection of such molecules over other gas molecules. Here we report a supramolecular fluorescence sensor for LPG using a 2-nm-sized cube-shaped molecular container i.e. a nanocube self-assembled from six molecules of gear-shaped amphiphiles (GSA) in water. The nanocube selectively encapsulates LPG, while it does not bind other gas molecules. Upon encapsulation of LPG in the nanocube, the fluorescence from the nanocube is enhanced by 3.9 times, which is caused by the restricted motion of the aromatic rings of GSA in the nanocube based on aggregation-induced emission. Besides the high selectivity, high sensitivity, quick response, high stability of the nanocube for LPG, and easy preparation of GSA satisfy the requirement for its practical use for an LPG sensor.
The effect of the methyl groups in neutral gear-shaped amphiphiles (GSAs) on the stability of nanocubes was investigated using a novel C-symmetric GSA, which was synthesized using selective alternate trilithiation of a pentabrominated hexaphenylbenzene derivative. The lack of only one methyl group in the GSA decreased the association constant for the assembly of the nanocube by 3 orders of magnitude. A surface analysis recently developed by the authors (SAVPR: surface analysis with varying probe radii) was carried out for characteristic isomers of the nanocube consisting of C-symmetric GSAs. It was found that the methyl groups near the equator of the nanocube play a significant role in the stabilization of the nanocubes.
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