Chirality Supramolecular Systems: Helical Assemblies, Structure Designs, and Functions.

Chirality, as one of the most striking characteristics, exists at various scales in nature. Originating from the interactions of host and guest molecules, supramolecular chirality possesses huge potential in the design of functional materials. Here, an overview of the recent progress in structure designs and functions of chiral supramolecular materials is present. First, three design routes of the chiral supramolecular structure are summarized. Compared with the template-induced and chemical synthesis strategies that depend on accurate molecular identification, the twisted-assembly technique creates chiral materials through the ordered stacking of the nanowire or films. Next, chirality inversion and amplification are reviewed to explain the chirality transfer from the molecular level to the macroscopic scale, where the available external stimuli on the chirality inversion are also given. Lastly, owing to the optical activity and the characteristics of the layer-by-layer stacking structure, the supramolecular chirality materials display various excellent performances, including smart response, shape-memorization, superior mechanical performance, and applications in biomedical fields. To sum up, this work provides a systematic review of the helical assemblies, structure design, and applications of supramolecular chirality systems.

Short Oligourea Foldamers as N- or C-Caps for Promoting α-Helix Formation in Water.

While foldamers have been extensively studied as protein mimics and especially as α-helix mimics, their use as capping motif to enhance α-helix propensity remains comparatively much limited. In this study, we leverage the structural similarities between urea-based helical foldamers and α-helix to investigate the efficacy of oligoureas as N- or C-caps for reinforcing α-helical structures in water. Short oligoureas, comprising 3 to 4 residues, were strategically introduced at the N- or C-terminus of two peptide sequences (S-peptide and an Ala-rich model sequence). The impact of these foldamer insertions on peptide conformation was examined using electronic circular dichroism (ECD) and solution NMR. This research identifies specific foldamer sequences capable of promoting α-helicity when incorporated at either terminus of the peptides. Not only does this work broaden the application scope of foldamers, but it also provides valuable insights into novel strategies for modulating peptide conformation in aqueous environments. The findings presented in this study may have implications for peptide design and the development of bioactive foldamer-based peptide mimics.

Bimodal Use of Chiral α-Trifluoromethylalanine in Aib Foldamers: Study of the Position Impact Towards the Helical Screw-Sense Preference.

Oligomers of the achiral α-aminoisobutyric acid (Aib) adopt a 310 helical conformation in which the screw-sense preference can be controlled by a single chiral residue. The use of the fluorinated residue α-Trifluoromethylalanine (α-TfmAla) revealed a unique way to both induce and measure the screw-sense preference of such oligomers acting as 19F NMR probe. This work proposes a systematic study of the effect of this fluorinated chiral inducer on the helical screw-sense preference of poly-Aib oligomers. The impact of the position of the fluorinated residue into pentamers (N-terminal, central or C-terminal) as well as the nature of the C-terminal capping of the peptides was thoroughly studied in light of complete structural analysis. A deeper understanding of the fluorine effect was achieved confirming the unique ability of α-TfmAla as a helical screw-sense controller.

Luminescence changeable CO2-storage cylinder: Triple-stranded helical Eu(III)/Tb(III) fluorinated MOFs with amide linkers.

Triple-stranded helical lanthanide MOFs with CO2 adsorption properties were investigated. Lanthanide MOFs ([Eu0.1Tb0.9(hfa)3(dpa)]n) are composed of lanthanide luminophores (Eu(III) and/or Tb(III) ions), fluorinated antenna ligands (hfa: hexafluoroacetylacetonate), and polyamide-type linker ligands (dpa: 4-(diphenylphosphoryl)-N-(4-(diphenylphosphoryl)phenyl)benzamide). The cylindrical structure was characterized by single-crystal X-ray analysis, thermogravimetric analysis, and gas adsorption measurements. The inner surfaces of the cylindrical channels were covered with the fluorine atoms of the hfa ligands. The emission intensity ratio (IEu / ITb) in [Eu0.1Tb0.9(hfa)3(dpa)]n is affected by the CO2 gas adsorption behavior. The change in IEu / ITb value was caused by the intermolecular interactions between the CO2 gas molecules and the fluorinated ligands, resulting in an electronic structural change of the lowest triplet excited state in the photosensitized hfa ligands.

Quantum Effects Explain the Twist Angle in the Helical Structure of DNA.

Why are DNA bases stacked in a double helix structure? We combined three theoretical approaches to demonstrate how one core concept derived from quantum mechanics (Pauli repulsion) annihilates the contribution of dispersion to the π-π stacking. The helical architecture is governed by a combination of exchange and electrostatic forces, a result that is interpreted from both a computational and a biological perspective.

Separation of enantiomers of chiral fullerene derivatives through enantioselective encapsulation within an adaptable helical cavity of syndiotactic poly(methyl methacrylate) with helicity memory.

Optically active left (M)- and right (P)-handed helical syndiotactic poly(methyl methacrylate)s (M- and P-st-PMMAs) with a helicity memory enantioselectively encapsulated the racemic C60 derivatives, such as 3,4-fulleroproline tert-butyl ester (rac-1) and tetraallylated C60 (rac-2), as well as the C60-bound racemic 310-helical peptides (rac-3) within their helical cavities to form peapod-like inclusion complexes and a unique "helix-in-helix" superstructure, respectively. The enantiomeric excess (ee) and separation factor (enantioselectivity) (α) of the analyte 1 (ee = 23%-25% and α = 2.35-2.50) encapsulated within the helical cavities of the M- and P-st-PMMAs were higher than those of the analytes 2 and 3 (ee = 4.3%-6.0% and α = 1.28-1.50). The optically pure (S)- and (R)-1 were found to more efficiently induce an excess one-handed helical conformation in the st-PMMA backbone than the optically pure (S)- and (R)-1-phenylethylamine, resulting in intense mirror-image vibrational circular dichroism (VCD) spectra in the PMMA IR regions. The excess one-handed helices induced in the st-PMMAs complexed with (S)- and (R)-1 were memorized after replacement with the achiral C60, and the complexes exhibited induced electric CDs in the achiral C60 chromophore regions.

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.

Can Magnetic Dipole Transition Moment Be Engineered?

The development of chiral compounds with enhanced chiroptical properties is an important challenge to improve device applications. To that end, an optimization of the electric and magnetic dipole transition moments of the molecule is necessary. Nevertheless, the relationship between chemical structure and such quantum mechanical properties is not always clear. That is the case of magnetic dipole transition moment (m) for which no general trends for its optimization have been suggested. In this work we propose a general rationalization for improving the magnitude of m in different families of chiral compounds. Performing a clustering analysis of hundreds of transitions, we have been able to identify a single group in which |m| value is maximized along the helix axis. More interestingly, we have found an accurate linear relationship (up to R2 =0.994) between the maximum value of this parameter and the area of the inner cavity of the helix, thus resembling classical behavior of solenoids. This research provides a tool for the rationalized synthesis of compounds with improved chiroptical responses.

Enantiopure [6]-Azairidahelicene by Dynamic Kinetic Resolution of a Configurationally Labile [4]-Helicene.

A pair of enantiopure [6]-azairidahelicenes incorporating chirality at the metal center and on the helicenic ligand were synthesized by dynamic kinetic resolution (dkr) of a configurationally labile [4]-helicenic ligand (4-(2-pyridyl)-benzo[g]phenanthrene, L1H) using bis-cyclometalated chiral-at-metal only iridium(III) precursors as chiral inductors. The origin of the observed dkr is attributed to the different conformation and stability of diastereomeric reaction intermediates formed during the cyclometalation process. The isolated enantiomers exhibited circularly polarized phosphorescence (CPP), with |gphos| values of 1.8×10-3.

Helicoselective Synthesis of Indolohelicenoids through Organocatalytic Central-to-Helical Chirality Conversion.

We report the helicoselective and convergent construction of indolohelicenoids with excellent efficiency and stereocontrol. This reaction proceeds through a chiral-phosphoric-acid-catalyzed enantioselective cycloaddition and eliminative aromatization sequence, which can be finely controlled by adjusting the reaction temperature. Mechanistic studies reveal that the chiral phosphoric acid cooperatively serves as both a bifunctional and Brønsted acid catalyst, enabling one-pot central-to-helical chirality conversion. Additionally, the optical properties of the synthesized indolohelicenoids were characterized to explore their potential applications in organic photoelectric materials.