Hydrogen-bonded aromatic amide macrocycles: synthesis, properties and functions

A molecular sheaf: doubly threaded [6]rotaxane

We report that the use of a hydrogen-bonded pyrimidine-macrocycle complex can efficiently facilitate the threading of two bispyridinium ethylenes into four rings, as evidenced by X-ray crystallography of its precursor, offering a rare example of a doubly threaded [6]rotaxane in 91% yield. The unusual architecture is found to be stable with no dethreading despite the large ring size of the macrocycle with respect to the stopper.

Self-assembly of rigid amphiphilic graft cyclic-brush copolymers to nanochannels using dissipative particle dynamics simulation

The synthesis of specific artificial nanochannels remains a formidable challenge in the field of nanomaterials and synthetic chemistry. In particular, the preparation of artificial nanochannels using amphiphilic graft cyclic-brush copolymers (AGCCs) as monomers has garnered substantial attention. Nevertheless, because of the constrained time and length scales inherent in traditional molecular dynamics simulations, a comprehensive theoretical understanding of the morphological regulation mechanism governing the self-assembly of AGCCs into nanochannels remains elusive. In this study, we employed the dissipative particle dynamics (DPD) method to explore the self-assembly mechanism considering factors such as the DPD interaction parameters, concentrations, and sizes of AGCCs. By calculating the phase diagrams, we predicted the emergence of four distinct nanochannel types: short independent, long independent, parallel, and disordered channels. Importantly, the formation of these nanochannels is highly contingent on specific environmental conditions. Furthermore, we extensively discussed self-assembly processes that lead to different types of nanochannels. The self-assembly of AGCCs is revealed as a multistep process primarily influenced by the interaction parameters. However, while the monomer size and concentration do not introduce novel self-assembly morphologies, they do influence the final aggregation state. The elucidation of the self-assembly mechanism presented in this study deepens our understanding of AGCC nanochannel formation. Consequently, this is a valuable guide for the preparation of copolymer materials with specific functionalities, offering insights into targeted copolymer material design.

Proton Affinity and Conformational Integrity of a 24-Atom Triazine Macrocycle across Physiologically Relevant pH

For 24-atom triazine macrocycles, protonation of the heterocycle leads to a rigid, folded structure presenting a network of hydrogen bonds. These molecules derive from dynamic covalent chemistry wherein triazine monomers bearing a protected hydrazine group and acetal tethered by the amino acid dimerize quantitatively in an acidic solution. Here, lysine is used, and the product is a tetracation. The primary amines of the lysine side chains do not interfere with quantitative yields of the desired bis(hydrazone) at concentrations of 5-125 mg/mL. Mathematical modeling of data derived from titration experiments of the macrocycle reveals that the pKa values of the protonated triazines are 5.6 and 6.7. Changes in chemical shifts of resonances in the 1H NMR spectra corroborate these values and further support assignment of the protonation sites. The pKa values of the lysine side chains are consistent with expectation. Upon deprotonation, the macrocycle enjoys greater conformational freedom as evident from the broadening of resonances in the 1H and 13C NMR spectra indicative of dynamic motion on the NMR time scale and the appearance of additional conformations at room temperature. While well-tempered metadynamics suggests only a modest difference in accessible conformational footprints of the protonated and deprotonated macrocycles, the shift in conformation(s) supports the stabilizing role that the protons adopt in the hydrogen-bonded network.

Construction of Oligorotaxanes with Hydrogen-Bonded Aramide Macrocycles through Threaded Host-Guest Complexation

The development of efficient and selective organic synthetic approaches for complex molecules has garnered significant attention due to the need for precise control over molecular structures and functions. Rotaxanes, a type of mechanically interlocked molecules (MIMs), have shown promising applications in various fields including sensing, catalysis, and material science. However, the highly selective synthesis of oligo[n]rotaxanes (mostly n≥3) through controlling host-guest complexation and supramolecular threading assembly process still remains an ongoing challenge. In particular, the utilization of two-dimensional (2D) macrocycles with structural shape-persistency for the synthesis of oligo[n]rotaxanes is rare. In this concept, research on cooperatively threaded host-guest complexation with hydrogen-bonded (H-bonded) aramide macrocycles and selective synthetic protocols of oligo[n]rotaxanes has been summarized. The high efficiency and selectivity in synthesis are ascribed to the synergistic interplay of multiple non-covalent bonding interactions such as hydrogen bonding and intermolecular π-π stacking of macrocycles within the unique supramolecular structure of threaded host-guest complexes. This review focuses on the latest progress in the concepts, synthesis, and properties of H-bonded aramide macrocycle-based oligorotaxanes, and presents an in-depth outlook on challenges in this emerging field.

Micro-simulation insights into the functional and mechanistic understanding of glycyrrhizin against asthma

Asthma is a common chronic respiratory disease, which causes inflammation and airway stenosis, leading to dyspnea, wheezing and chest tightness. Using transgelin-2 as a target, we virtually screened the lead compound glycyrrhizin from the self-built database of anti-asthma compounds by molecular docking technology, and found that it had anti-inflammatory, anti-oxidative and anti-asthma pharmacological effects. Then, molecular dynamics simulations were used to confirm the stability of the glycyrrhizin-transgelin-2 complex from a dynamic perspective, and the hydrophilic domains of glycyrrhizin was found to have the effect of targeting transgelin-2. Due to the self-assembly properties of glycyrrhizin, we explored the formation process and mechanism of the self-assembly system using self-assembly simulations, and found that hydrogen bonding and hydrophobic interactions were the main driving forces. Because of the synergistic effect of glycyrrhizin and salbutamol in improving asthma, we revealed the mechanism through simulation, and believed that salbutamol adhered to the surface of the glycyrrhizin nano-drug delivery system through hydrogen bonding and hydrophobic interactions, using the targeting effect of the hydrophilic domains of glycyrrhizin to reach the pathological parts and play a synergistic anti-asthmatic role. Finally, we used network pharmacology to predict the molecular mechanisms of glycyrrhizin against asthma, which indicated the direction for its clinical transformation.

Bracelet-like Complexes of Lithium Fluoride with Aromatic Tetraamides, and Their Potential for LiF-Mediated Self-Assembly: A DFT Study

Geometries and binding energies of complexes between a LiF molecule and a model aromatic tetraamide are obtained using various DFT methods. The tetraamide consists of a benzene ring and four amides positioned so that the LiF molecule can bind via Li⋯O=C or N-H⋯F interactions. The complex with both interactions is the most stable one, followed by the complex with only N-H⋯F interactions. Doubling the size of the former resulted in a complex with a LiF dimer sandwiched between the model tetraamides. In turn, doubling the size of the latter resulted in a more stable tetramer with bracelet-like geometry having the two LiF molecules also sandwiched but far apart from each other. Additionally, all methods show that the energy barrier to transition to the more stable tetramer is small. The self-assembly of the bracelet-like complex mediated by the interactions of adjacent LiF molecules is demonstrated by all computational methods employed.

Multi-Responsive Molecular Encapsulation and Release Based on Hydrogen-Bonded Azo-Macrocycle

Research on stimuli-responsive host-guest systems is at the cutting edge of supramolecular chemistry, owing to their numerous potential applications such as catalysis, molecular machines, and drug delivery. Herein, we present a multi-responsive host-guest system comprising azo-macrocycle 1 and 4,4'-bipyridinium salt G1 for pH-, photo-, and cation- responsiveness. Previously, we reported a novel hydrogen-bonded azo-macrocycle 1. The size of this host can be controlled through light-induced E↔Z photo-isomerization of the constituent azo-benzenes. The host is found in this work to be capable of forming stable complexes with bipyridinium/pyridinium salts, and implementing guest capture and release with G1 under light in a controlled manner. The binding and release of the guest in the complexes can also be easily controlled reversibly by using acid and base. Moreover, the cation competition-induced dissociation of the complex 1a₂⊃G1 is achieved. These findings are expected to be useful in regulating encapsulation for sophisticated supramolecular systems.