Bioinspired Binding and Conversion of Linear Monoterpenes by Polyaromatic Coordination Capsules.

Linear monoterpenes, versatile reaction biosubstrates, are bound and subsequently converted to various cyclic monomers and oligomers with excellent selectivity and efficiency, only in natural enzymes. We herein report bioinspired functions of synthetic polyaromatic cavities toward linear monoterpenes in the solution and solid states. The cavities are provided by polyaromatic coordination capsules, formed by the assembly of Pt(II) ions and bent bispyridine ligands with two anthracene panels. By using the capsule cavities, the selective binding of citronellal from mixtures with other monoterpenes and its preferential vapor binding over its derivatives are demonstrated in water and in the solid state, respectively. The capsule furthermore extracts p-menthane-3,8-diol, with high product- and stereoselectivity, from a reaction mixture obtained by the acid-catalyzed cyclization of citronellal in water. Thanks to the inner and outer polyaromatic cavities, the catalytic cyclization-dimerization of vaporized citronellal efficiently proceeds in the acid-loaded capsule solid and product/stereoselectively affords p-menthane-3,8-diol citronellal acetal (∼330% yield based on the capsule) under ambient conditions. The solid capsule reactor can be reused at least 5 times with enhanced conversion. The present study opens up a new approach toward mimicking terpene biosynthesis via synthetic polyaromatic cavities.

A Closed Cavity Strategy for Selective Dipeptide Binding by a Polyaromatic Receptor in Water.

Precise recognition of peptides is a daunting task owing to the substantial number of available amino acids and their combination into various oligo/polymeric structures in addition to the high hydration of their flexible frameworks. Here, we report the selective recognition of a dipeptide through a closed cavity strategy, in contrast to previous synthetic receptors with open cavities. A polyaromatic receptor with a virtually isolated, hydrophobic cavity exclusively binds one molecule of phenylalanine dipeptide from a mixture with its amino acid and tripeptide in water via multiple CH-π and hydrogen-bonding interactions in the complementary cavity. The binding selectivity persists even in the presence of other dipeptides, such as leucine-leucine, leucine-phenylalanine, tyrosine-phenylalanine, tryptophan-tryptophan, and aspartame, revealed by NMR/MS-based competitive binding experiments. ITC studies reveal that the selective binding of the phenylalanine dipeptide is relatively strong (Ka = 1.1 × 105 M-1) and an enthalpically and entropically favorable process (ΔH = -11.7 kJ mol-1 and TΔS = 17.0 kJ mol-1). In addition, the present receptor can be used for the emission detection of the dipeptide through a combination with a fluorescent dye in water.

A Porous Polyaromatic Solid for Vapor Adsorption of Xylene with High Efficiency, Selectivity, and Reusability.

Development of porous materials capable of capturing volatile organic compounds (VOCs), such as benzene and its derivatives, with high efficiency, selectivity, and reusability is highly demanded. Here we report unusual vapor adsorption behavior toward VOCs by a new porous solid, composed of a polyaromatic capsule bearing a spherical nanocavity with subnano-sized windows. Without prior crystallization and high-temperature vacuum drying, the porous polyaromatic solid exhibits the following five features: vapor adsorption of benzene over cyclohexane with 90 % selectivity, high affinity toward o-xylene over benzene and toluene with >80 % selectivity, ortho-selective adsorption ability (>50 %) from mixed xylene isomers, tight VOCs storage even under high temperature and vacuum conditions, and at least 5 times reusability for xylene adsorption. The observed adsorption abilities are accomplished at ambient temperature and pressure within 1 h, which has not been demonstrated by organic/inorganic porous materials reported previously.

Selective Encapsulation and Unusual Stabilization of cis-Isomers by a Spherical Polyaromatic Cavity.

To explore new cavity functions, we herein employed cis-trans stereoisomers with a N=N, C=C, or C=N unit as guest indicators for a polyaromatic capsule. Thanks to the rigid, spherical cavity with a diameter of ∼1 nm, azobenzene and stilbene derivatives are quantitatively encapsulated by the capsule with 100 % cis-selectivity in water. The isomerization of the cis-azo compound is suppressed against heat and light in the cavity, due to the confinement effect. Furthermore, C,N-diphenyl imine derivatives are quantitatively encapsulated by the capsule in water and adopt an otherwise unstable cis-form. The polyaromatic cavity suppresses the hydrolysis of the imines in water, even at elevated temperature, due to the shielding effect. Accordingly, the properties of the cis-trans isomers could be largely altered through supramolecular manipulation.

Cyclic monoterpenes trapped in a polyaromatic capsule: unusual selectivity, isomerization, and volatility suppression.

Cyclic monoterpenes (CMTs) are intractable natural products with high volatility and strong odors so that there has been no molecular receptor capable of selectively and tightly trapping CMTs in both solution and the solid state. We herein report that a polyaromatic capsule acts as a functional nanoflask for CMTs with the following five features: (i) the capsule can selectively bind menthone from mixtures with other saturated CMTs in water. In contrast, (ii) treatment of the capsule with mixtures of menthone and π-conjugated CMTs gives rise to ternary host-guest complexes with high pair-selectivity. Notably, (iii) the encapsulated menthone displays unusual isomerization from a typical chair conformer to otherwise unstable conformers upon heating. (iv) The selective binding of volatilized CMTs is demonstrated by the capsule even in the solid state at atmospheric pressure. Furthermore, (v) the volatilities of CMTs are significantly suppressed at elevated temperatures by the capsule upon encapsulation in solution as well as in the solid state.