Evolution of Peptidomimetics-Based Chiral Assemblies of β-Sheet, α-Helix, and Double Helix Involving Chalcogen Bonds

Developing chiral assemblies that mimic biological secondary structures, e.g., protein β-sheet, α-helix, and DNA double helix, is a captivating goal in supramolecular chemistry. Here, we create a family of biomimetic chiral assemblies from alanine-based peptidomimetics, wherein the incorporation of N-terminal 2,1,3-benzoselenadiazole groups enables the rarely utilized chalcogen bonding as the adhesive interaction. While the alanine-based acylhydrazine molecule 1L was designed as a building unit with an extended conformation, simple derivatization of 1L affords folded unilateral N-amidothiourea 2L with one β-turn and bilateral N-amidothiourea 3L with two β-turns. This derivatization leads to the evolution of molecular assemblies from β-sheet organization (1L) to single helix (α-helix mimic, 2L) and ultimately to double helix (3L), illustrating an evolutionary route relating the structures and superstructures. In the case of the double helix formed by 3L, an unexpected cis-form that brings the two β-turns into one side was observed, stabilized via the π···π interaction between two N-terminal 2,1,3-benzoselenadiazole groups. This conformation allows double-crossed N-Se···S═C chalcogen bonds to support a DNA-like P-double helix featuring intrastrand noncovalent interactions and interstrand covalent linkages, surviving in both the solid state and in dilute acetonitrile solution phase.

High-Fidelity Supramolecular Chirality Transportation Enabled Through Chalcogen Bonding

The formation of asymmetric microenvironments in proteins benefits from precise transportation of chirality across multiple levels through weak bonds in the folding and assembly process, which inspires the rational design and fabrication of artificial chiral materials. Herein, the chalcogen bonding-directed precise transportation of supramolecular chirality toward multiple levels is reported to aid the fabrication of chiroptical materials. Benzochalcogenadiazole (O, S, Se) motifs are conjugated to amino acid residues, and the solid-state assemblies afforded selective supramolecular chirality with handedness depending on the kinds of chalcogen atoms and amino acids. The chalcogen-N sequence assisted by hydrogen bonding synergistically allows for the complexation with pyrene conjugated aryl carboxylic acids to give macroscopic helical structures with active circular dichroism and circularly polarized luminescence, of which handedness is precisely determined by the pristine chiral species. Then the further application of chiral benzochalcogenadiazole motifs as seeds in directing handedness of benzamide via symmetry breaking is realized. Behaving as the dopants, embedding into the matrix of benzophenone induces the room temperature phosphorescence, whereby the thermal chiroptical switch is fabricated with color-tunable phosphorescent circularly polarized luminescence. This work utilized benzochalcogenadiazole-based chiral building units to accomplish precise transportation of supramolecular chirality in coassemblies with high-fidelity, achieving flexible manipulation of chiroptical properties and macroscopic helical sense.

Heterochiral coupling to bilateral β-turn structured azapeptides bearing two remote chiral centers

Enantioselective synthesis governed by chiral catalysts has been extensively developed, but that without any chiral auxiliaries or chiral catalysts is rare, particularly when remote stereogenic centers are involved. Here we report an enantioselectivity of heterochiral coupling in the one-pot reaction of racemic hydrazides with achiral 1,4-bis(isothiocyanine)benzene, yielding preferentially the heterochiral bilateral azapeptides over the homochiral ones. Despite bearing two hydrogen-bonded β-turn structures that allow intramolecular chiral transfer, the bilateral azapeptide products have two chiral centers separated by 14 atoms or 15 bonds, which prevent the direct intramolecular asymmetric communication between the two chiral centers. Interestingly, the heterochiral azapeptides feature intermolecular hydrogen bonding stacking between homochiral β-turns to form a superstructure of alternative M- and P-helices in the crystals. In contrast, the homochiral azapeptide counterparts adopt a β-sheet-like structure, which is less favorable compared to the helical-like superstructure from heterochiral azapeptides, accounting for the favored heterochiral coupling of the one-pot reaction. This work demonstrates enantioselective synthesis involving distant chiral centers through the formation of biomimetic superstructures, opening up new possibilities for the regulation of enantioselectivity.

Simultaneous formation of helical and sheet-like assemblies from short azapeptides enables spontaneous resolution

As determined by the homochirality of amino acid building units, protein secondary structures α-helix and β-sheet are single-handed chiral superstructures extending in one and quasi-two dimensions, respectively. Synthetic molecular assemblies that mimic the structural homochirality of proteins would provide insights into the origin of biological homochirality and inform the development of chiral separation techniques. Here we fabricated a homochiral 3D assembly consisting of 1D helical and 2D sheet-like assemblies that feature molecular packings resembling α-helix and β-sheet, respectively. This was achieved by using an alanine derivative, a β-turn structured short azapeptide from p-iodobenzoylalanine-based N-amido-N'-phenylthiourea. While N-H⋯SC/OC hydrogen bonds between the β-turn scaffolds afford a 2D pleated sheet-like structure, the head-to-tail C-I⋯π halogen bonds, together with the N-H⋯OC hydrogen bonds, support a 1D helical-like assembly, serving as linkers to connect the 2D sheet-like structures into a 3D superstructure. The two biomimetic assembly modes share the N-H⋯OC hydrogen bonds and can allow 3D homochiral elongation, driving spontaneous resolution of the short azapeptides to generate conglomerate crystals.

Anions as Lewis Acids in Noncovalent Bonds

The ability of an anion to serve as electron-accepting Lewis acid in a noncovalent bond is assessed via DFT calculations. NH3 is taken as the common base, and is paired with a host of ACln - anions, with central atom A=Ca, Sr, Mg, Te, Sb, Hg, Zn, Ag, Ga, Ti, Sn, I, and B. Each anion reacts through its σ or π-hole although the electrostatic potential of this hole is quite negative in most cases. Despite the contact between this negative hole and the negative region of the approaching nucleophile, the electrostatic component of the interaction energy of each bond is highly favorable, and accounts for more than half of the total attractive energy. The double negative charge of dianions precludes a stable complex with NH3.

Orientational transitions of discotic columnar liquid crystals in cylindrical pores

Confined in a cylindrical pore with homeotropic anchoring condition, the hexagonal columnar phase of discotic liquid crystals can form a "log-pile" configuration, in which the columns are perpendicular to the long axis of the pore. However, the {100} planes of the hexagonal lattice can orient either parallel (termed (100)‖ orientation) or perpendicular ((100)⊥) to pore axis. Here we experimentally show that the (100)‖ orientation is found in narrower cylindrical pores, and the (100)‖-(100)⊥ transition can be controlled by engineering the structure of the molecules. The (100)‖ orientation is destroyed in asymmetric discotics hepta(heptenyloxy)triphenylene (SATO7); replacing the oxygen linkage in hexa(hexyloxy)triphenylene (HATO6) by sulphur (HATS6) improves the (100)‖ orientation in small pores; adding a perfluorooctyl end to each alkyl chain of HATO6 (HATO6F8) moves the (100)‖-(100)⊥ transition to larger pores. We have provided a semi-quantitative explanation of the experimental observations, and discussed them in the context of previous findings on related materials in a wider pore size range from 60 nm to 100 μm. This allows us to produce a comprehensive picture of confined columnar liquid crystals whose applications critically depend on our ability to align them.

Benzoselenadiazole-Functionalized H-Bonded Arylamide Foldamers: Solvent-Responsive Properties and Helix Self-Assembly Directed by Chalcogen Bonding in Solid State

In this study, a series of H-bonded arylamide foldamers bearing benzoselenadiazole ends with solvent-responsive properties have been synthesized. In dichloromethane or dimethyl sulfoxide solvents, the molecules exhibit meniscus or linear structures, respectively, which can be attributed to the unique intramolecular hydrogen bonding behavior evidenced by 1D 1H NMR and 2D NOESY spectra. UV-vis spectroscopy experiments show that the absorption wavelength of H-bonded arylamide foldamers are significantly red-shifted due to the presence of benzoselenadiazole group. In addition, the crystal structures reveal that effective intermolecular dual Se ⋅ ⋅ ⋅ N interactions between benzoselenadiazole groups induce further assembly of the monomers. Remarkably, supramolecular linear and double helices structures are constructed under the synergistic induction of intramolecular hydrogen bonding and intermolecular chalcogen bonding. Additionally, 2D DOSY diffusion spectra and theoretical modelling based on density functional theory (DFT) are performed to explore the persistence of intermolecular Se ⋅ ⋅ ⋅ N interactions beyond the crystalline state.

Multiple non-covalent-interaction-directed supramolecular double helices: the orthogonality of hydrogen, halogen and chalcogen bonding

In this study, a benzoselenadiazole- and pyridine-bifunctionalized hydrogen-bonded arylamide foldamer was synthesized. A co-crystallization experiment with 1,4-diiodotetrafluorobenzene showed that a new type of supramolecular double helices, which were induced by three orthogonal interactions, namely, three-center hydrogen bonding (O⋯H⋯O), I⋯N halogen bonding and Se⋯N chalcogen bonding, have been constructed in the solid state. This work presents a novel instance of multiple non-covalent interactions that work together to construct supramolecular architectures.

Pnictogen bonding in imide derivatives for chiral folding and self-assembly

Pnictogen bonding (PnB) is an attraction interaction that originates from the anisotropic distribution of electron density of pnictogen elements, which however has been rarely found in nitrogen atoms. In this work, for the first time, we unveil the general presence of N-involved PnB in aromatic or aliphatic imide groups and reveal its implications in chiral self-assembly of folding. This long-neglected interaction was consolidated by Cambridge structural database (CSD) searching as well as subsequent computational studies. Though the presence of PnB has limited effects on spectroscopic properties in the solution phase, conformation locking effects are sufficiently expressed in the chiral folding and self-assembly behavior. PnB anchors the chiral conformation to control the emergence and inversion of chiroptical signals, while intramolecular PnB induces the formation of supramolecular tilt chirality. It also enables the chiral folding of imide-containing amino acid or peptide derivatives, which induces the formation of unique secondary structural sequences such as β-sheets. Finally, the effects of PnB in directing folded helical structures were revealed. Examples of cysteine and cystine derivatives containing multiple N⋯O and N⋯S PnBs constitute an α-helix like secondary structure with characteristic circular dichroism. This work discloses the comprehensive existence of imide-involved PnB, illustrates its important role in folding and self-assembly, and sheds light on the rational fabrication of conformation-locked compounds and polymers with controllable chiroptical activities.

Noncovalent interaction network of chalcogen, halogen and hydrogen bonds for supramolecular β-sheet organization

An alanine-based bilateral building block, linked by 2,5-thiophenediamide motifs and equipped with C-terminal 4-iodoaniline groups, was designed, allowing a noncovalent interaction network consisting of intramolecular chalcogen bonds and intermolecular halogen/hydrogen bonds, which cooperatively maintain a supramolecular β-sheet organization in the solid state, as well as in dilute CH3CN solution with a high g factor of -0.017.

Competition Between the Two σ-Holes in the Formation of a Chalcogen Bond

A chalcogen atom Y contains two separate σ-holes when in a R₁ YR₂ molecular bonding pattern. Quantum chemical calculations consider competition between these two σ-holes to engage in a chalcogen bond (ChB) with a NH₃ base. R groups considered include F, Br, I, and tert-butyl (tBu). Also examined is the situation where the Y lies within a chalcogenazole ring, where its neighbors are C and N. Both electron-withdrawing substituents R₁ and R₂ act cooperatively to deepen the two σ-holes, but the deeper of the two holes consistently lies opposite to the more electron-withdrawing group, and is also favored to form a stronger ChB. The formation of two simultaneous ChBs in a triad requires the Y atom to act as double electron acceptor, and so anti-cooperativity weakens each bond relative to the simple dyad. This effect is such that some of the shallower σ-holes are unable to form a ChB at all when a base occupies the other site.