Molecular Binding Behavior of Pyridine-2,6-dicarboxamide-Bridged Bis(β-cyclodextrin) with Oligopeptides: Switchable Molecular Binding Mode

Durable organic nonlinear optical membranes for thermotolerant lightings and in vivo bioimaging

Organic nonlinear optical materials have potential in applications such as lightings and bioimaging, but tend to have low photoluminescent quantum yields and are prone to lose the nonlinear optical activity. Herein, we demonstrate to weave large-area, flexible organic nonlinear optical membranes composed of 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate@cyclodextrin host-guest supramolecular complex. These membranes exhibited a record high photoluminescence quantum yield of 73.5%, and could continuously emit orange luminescence even being heated at 300 °C, thus enabling the fabrication of thermotolerant light-emitting diodes. The nonlinear optical property of these membranes can be well-preserved even in polar environment. The supramolecular assemblies with multiphoton absorption characteristics were used for in vivo real-time imaging of Escherichia coli at 1000 nm excitation. These findings demonstrate to achieve scalable fabrication of organic nonlinear optical materials with high photoluminescence quantum yields, and good stability against thermal stress and polar environment for high-performance, durable optoelectronic devices and humanized multiphoton bio-probes.

Chiroptical study of cryptophanes subjected to self-encapsulation

In 1,1,2,2-tetrachloroethane-d₂ , the ¹²⁹ Xe NMR spectrum of the Xe@cryptophane-223 complex bearing seven acetate groups (Xe@1 complex) shows an unusually broad signal compared with that of its congeners (Chapellet, LL. et al. J. Org. Chem. 2015;80:6143-6151). To interpret this unexpected behaviour, a ¹ H NMR analysis and a thorough study of the chiroptical properties of 1 as a function of the nature of the solvent have been performed. The ¹ H NMR spectra of 1 reveal that a self-encapsulation phenomenon takes place in DMSO-d₆ and 1,1,2,2-tetrachloroethane-d₂ solvents. Thanks to the separation of the two enantiomers of 1 by HPLC on chiral stationary phase, the two enantiomers of 1 have been studied in detail by polarimetry, electronic (ECD), and vibrational (VCD) circular dichroism spectroscopies. Except for ECD spectroscopy, these chiroptical techniques reveal spectroscopic changes as a function of the nature of the solvent. For instance, in DMSO and 1,1,2,2-tetrachloroethane, in which the self-encapsulation phenomenon takes place, the sign of the specific optical rotation of [CD(-)₂₅₄ ]-1 and [CD(+)₂₅₄ ]-1 is changed. These results have then been compared with those obtained with cryptophane-223 bearing only one acetate group on the propylenedioxy linker (compound 2) and with cryptophane-223 bearing six acetate groups (compound 3). A self-encapsulation phenomenon is also observed with compound 2. Finally, compounds 2 and 3 show different chiroptical properties compared with those obtained with the two enantiomers of compound 1.

Spectrophotometric study of the selective binding behavior of aliphatic oligopeptides by bridged bis(β-cyclodextrin) linked by a 4,4′-diaminodiphenyl disulfide tether

The conformation and binding behavior of 4,4′-diaminodiphenyl disulfide bridged bis(β-cyclodextrin) (1) towards representative aliphatic oligopeptides, i.e., Leu-Gly, Gly-Leu, Glu-Glu, Met-Met, Gly-Gly, Gly-Gly-Gly, and Gly-Pro, were investigated by circular dichroism, fluorescence, and 1H and 2D NMR spectroscopy at 25 °C in phosphate buffer (pH 7.20). The results indicated that 1 acts as an efficient fluorescent sensor and displays remarkable fluorescence enhancement upon addition of optically inert oligopeptides. Owing to the cooperative host–linker–guest binding mode in which the linker and guest are coincluded in the two cyclodextrin cavities, the bis(β-cyclodextrin) 1 gives high binding constants of up to 103–104 (mol/L)–1 for oligopeptides. The bis(β-cyclodextrin) 1 can recognize not only the size and shape of oligopeptides but also the hydrophobicity, giving an exciting residue selectivity of up to 61.3 for the Gly-Leu/Glu-Glu pair. These phenomena are discussed from the viewpoints of multiple recognition and induce-fit interactions between host and guest.

Peptide Recognition: Encapsulation and α-Helical Folding of a Nine-Residue Peptide within a Hydrophobic Dimeric Capsule of a Bowl-Shaped Host

A dimeric capsule of coordination bowl 1 encapsulated a nine-residue peptide (Trp-Ala-Glu-Ala-Ala-Ala-Glu-Ala-Trp; 2) within the large hydrophobic cavity in water, and stabilized the alpha-helical conformation of bound 2. An NMR titration experiment revealed that monomeric bowl 1 recognized two Trp residues at the both terminals of 2 through 1/2 = 1:1 to 2:1 complexation. The 1:1 and 2:1 species exist in equilibrium even in the presence of excess 1. It was found that the formation of the 2:1 complex, in which two bowls of 1 wrapped the whole of 2, became dominant by the addition of NaNO3 due to the fact that the enhanced ion strength increased the hydrophobic interaction between Trp residues and the cavity of 1. The alpha-helical conformation of 2 within the dimeric capsule of 1 was elucidated from detailed NOESY analysis.

Efficient fluorescent sensors of oligopeptides by dithiobis(2-benzoylamide)-bridged bis(β-cyclodextrin)s: structure in solution, binding behavior, and thermodynamic origin

Two 6,6'-bis(beta-cyclodextrin)s linked by 2,2'-dithiobis[2-(benzoylamino)ethyleneamino] and 2,2'-dithiobis[2-(benzoylamino)diethylenetriamino] bridges (1 and 2) have been synthesized as cooperative multipoint recognition receptor models for non-aromatic oligopeptides. Their structures in solution and inclusion complexation mechanism are comprehensively investigated by means of circular dichroism, 2D NMR spectra and temperature-dependent fluorescence titrations. The results show that the cooperative 'host-linker-guest' binding mode and the extensive desolvation effect jointly contribute to the guest-induced fluorescence enhancement of bis(beta-cyclodextrin)s. Further examinations on the binding behavior of hosts 1-2 with a series of di- and tri-peptides demonstrate that bis(beta-cyclodextrin) 1 can recognize not only the size/shape of oligopeptides but also the dipeptide sequence, giving an exciting residue selectivity up to 37.5 for Gly-Gly-Gly/Glu-Glu pair and a high sequence selectivity up to 5.0 for Gly-Leu/Leu-Gly pair. These fairly good selectivities are discussed from the viewpoint of cooperative binding, multiple recognition and induced-fit interactions between host and guest.

Binding Behavior of Aliphatic Oligopeptides by Bridged and Metallobridged Bis(β-cyclodextrin)s Bearing an Oxamido Bis(2-benzoic) Carboxyl Linker

beta-Cyclodextrin dimers bearing an oxamido bis(2-benzoic) carboxyl linker (1) or its metal complexes (2 and 3) were newly synthesized, and their inclusion complexation behavior with a series of representative aliphatic oligopeptides, i.e., Leu-Gly, Gly-Leu, Gly-Pro, Glu-Glu, Gly-Gly, Gly-Gly-Gly, and Glu(Cys-Gly), was elucidated by means of UV/vis, circular dichroism, fluorescence, and 2D NMR spectroscopy in Tris-HCl buffer solution (pH 7.4) at 25 degrees C. The results obtained indicated that metallobridged bis(beta-cyclodextrin)s 2 or 3 could significantly enhance the original molecular binding abilities of parent bis(beta-cyclodextrin) 1 toward model substrates through the cooperative binding of two cyclodextrin moieties and the additional chelation effect supplied by the coordinated metal centers. It is interesting that hosts 2 and 3 displayed an entirely different fluorescence behavior upon complexation with guest oligopeptides. Among the guest peptides examined, 3 showed the highest complex formation constant of 68 200 M(-)(1) for Glu-Glu, up to 510-fold as compared with 1 (135 M(-)(1)), while 1 gave excellent molecular selectivity for Glu(Cys-Gly)/Glu-Glu pair, up to 51-fold. The molecular binding ability and selectivity were discussed from the viewpoints of the induced-fit and multiple recognition mechanism between host and guest.