Complexation-driven assembly of imine-linked helical receptors showing adaptive folding and temperature-dependent guest selection

Carbazole Framework as Functional Scaffold for the Design of Synthetic Receptors

Carbazole serves as a prominent framework in the design of synthetic receptors, being a valuable scaffold for supramolecular chemistry, thanks to its planarity, fluorescence and versatility. This review provides a comprehensive analysis of notable examples of carbazole-based receptors, highlighting the impact of structural modifications on binding affinity and selectivity toward different guests.

Foldameric receptors with domain-swapping cavities capable of selectively binding and transporting monosaccharides

The development of synthetic receptors capable of selectively binding and transporting saccharides is crucial but highly challenging. In this study, two foldameric receptors 1 and 2, consisting of two repeating monomers, indolocarbazole and naphthyridine units, with different aromatic spacers in the middle of their sequences, have been synthesised. These receptors fold into helical conformations, and the two strands of each receptor are assembled to create domain-swapping cavities for binding monosaccharides by multiple hydrogen bonds. According to 1H NMR, CD spectroscopy, mass spectrometry, and ITC experiments, receptor 1 forms two distinct 2 : 2 complexes with methyl β-D-galactopyranoside and methyl β-D-glucopyranoside: (1-MM)2⊃(methyl β-D-galactopyranoside·2H2O)2 and (1-MP)2⊃(methyl β-D-glucopyranoside)2. Despite being composed of identical foldamer strands, these two complexes exhibit notably different folding and assembly modes to achieve optimal stability. The binding affinities of 1 for methyl β-D-galactopyranoside and methyl β-D-glucopyranoside are estimated to be log K = 12.7 and 13.3, respectively, in 5% (v/v) DMSO/CH2Cl2. On the other hand, receptor 2 forms a stable 2 : 2 receptor/guest complex with methyl β-D-glucopyranoside, (2-MP)2⊃(methyl β-D-glucopyranoside)2, with an association constant of log K = 13.9, which is significantly higher than that of methyl β-D-galactopyranoside (log K = 11.1) and methyl α-D-glucopyranoside (log K = 10.6). Furthermore, receptor 2 facilitates the selective transport of methyl β-D-glucopyranoside over other glycosides across an organic phase (CH2Cl2) in U-tube experiments.

Helical aromatic foldamers with tubular cavities capable of accommodating multiple halide ions

A series of pyridinium-indolocarbazole oligomers with different lengths, capable of adopting helical conformations with tubular cavities, has been synthesized. These oligomers can accommodate two, three, or four iodide ions in a linear arrangement within their cavities, which can be exchanged with other halides.

Stimuli-Responsive Molecular Duplexes Displaying Duplex-to-Duplex Switching

Synthetic duplexes with high stabilities have promising potential for mimicking biomolecular functions and developing supramolecular smart materials. Herein, we describe the synthesis and stimuli-responsive properties of molecular duplexes derived from indolocarbazole-pyridine (I-P) oligomers. These duplexes adopt nonclassical helical structures, stabilized by I-P hydrogen-bonding pairs in anhydrous chlorinated solvents. Notably, the longest duplex 62 (11-mer)2 displays remarkable stability, forming twenty hydrogen bonds; its exchange energy barrier was determined to be ΔG≠=22.0 kcal ⋅ mol-1 at 75 °C in anhydrous (CDCl2)2. Upon the addition of water, a hydrated duplex 62 (11-mer)2⊃10H2O was formed, with one water molecule inserted between each I-P hydrogen-bonding pair. The Hill coefficient (n) for this process is 6.1, demonstrating extremely positive cooperativity. Conversely, the hydrated duplex 62 (11-mer)2⊃10H2O was completely converted into the original anhydrous duplex 62 (11-mer)2 when the temperature was increased. Interconversion between these two distinct duplexes can be repeatedly carried out by varying the temperature. Furthermore, reversible switching between hetero-duplexes and homo-duplexes was also demonstrated by controlling the temperature, with concomitant changes in the characteristic emission signals.

Domain Swapping in Abiotic Foldamers

Foldamer sequences that adopt tertiary helix-turn-helix folds mediated by helix-helix hydrogen bonding in organic solvents have been previously reported. In an attempt to create genuine abiotic quaternary structures, i.e. assemblies of tertiary structures, new sequences were prepared that possess additional hydrogen bond donors at positions that may promote an association between the tertiary folds. However, a solid state structure and extensive solution state investigations by Nuclear Magnetic Resonance (NMR) and Circular Dichroism (CD) show that, instead of forming a quaternary structure, the tertiary folds assemble into stable domain-swapped dimer motifs. Domain swapping entails a complete reorganization of the arrays of hydrogen bonds and changes in relative helix orientation and handedness that can all be rationalized.

Redox-Regulated and Guest-Driven Transformations of Aromatic Oligoamide Foldamers in Advanced Structures

In this study, we demonstrate that an aromatic oligoamide sequence assembles into a trimeric helix-turn-helix architecture with a disulfide linkage, and upon cleavage of this linkage, it reconstructs into an antiparallel double helix. The antiparallel double helix is accessible to encapsulate a diacid guest within its cavity, forming a 2:1 host-guest complex. In contrast, hydrogen-bonding interactions between the trimeric-assembled structure and guests induce a conformational shift in the trimeric helix, resulting in a cross-shaped double-helix complex at a 2:2 host-guest ratio. Interconversions between the trimeric helix and the antiparallel double helix, along with their respective host-guest complexes, can be initiated through thiol/disulfide redox-mediated regulation.