Redox-Driven Chiral Inversion of Water-Soluble Pillar[5]arene with l-Cystine Derivative in the Aqueous Medium

Chiroptical Recognition of Carboxylates with Charge-Neutral Double-Stranded Zinc(II) Helicates

Chirality analysis of small molecules for the determination of their enantiopurity is nowadays ruled by streamlined chromatographic methods which utilize chiral stationary phases. Chiroptical probes which rely on host-guest interactions are so far overshadowed by the latter but have the benefit of depending only on common spectroscopic techniques such as CD spectroscopy to distinguish enantiomers and to quantify their ratio. Interest into this receptor-based approach is constantly rising because non-invasive high-throughput screenings with a minimal waste production can be performed. In this study we investigate the possibility to utilize metal-based containers in form of charge-neutral helicates able to recognize anions for this purpose. Key building block of the helicates are triazole units which show rotational freedom and give rise to either a meso-structure or a racemic mixture of the right- and left-handed complex. A chiroptical response of the probe is observed upon recognition of chiral mono- or dicarboxylates and chirality analysis of tartrate is conducted by CD spectroscopy.

Planar Chirality for Acid/Base Responsive Macrocyclic Pillararenes Induced by Amino Acid Derivatives: Molecular Dynamics Simulations and Machine Learning

Chirality is ubiquitous in nature, ranging from a DNA helix to a biological macromolecule, snail's shell, and even a galaxy. However, the precise control of chirality at the nanoscale is a challenge due to the structure complexity of supramolecular assemblies, the small energy differences between different enantiomers, and the difficulty in obtaining polymorphic crystals. The planar chirality of water-soluble pillar[5]arenes (called WP5-Na with Na ions in the side chain) host triggered by the addition of chiral L-amino acid hydrochloride (L-AA-OEt) guests and acid/base is rationalized by the relative stability of different chiral isomers, being estimated by molecular dynamics (MD) simulations and quantum chemical calculations. As an increase in the pH value, the change from a positive to a negative value of the free energy difference (ΔG) between two conformations, pR-WP5-Na⊃L-AA-OEt and pS-WP5-Na⊃L-AA-OEt, suggests an inversed preference of the pS-WP5-Na conformer induced by the deprotonated L-arginine ethyl ester (L-Arg-OEt) at pH = 14, which is supported by the circular dichroism (CD) experiments. On the basis of 2256 WP5-Na⊃L-Ala-OEt and 3299 WP5-Na⊃L-Arg-OEt conformers sampled from MD, the gradient boosting regression (GBR) model exhibits a satisfactory performance (R² = 0.91) in predicting the chirality of WP5-Na complexations using host-guest binding descriptors, including the geometry matching and binding sites and modes (electrostatics and hydrogen bonding). The machine learning model also performs well on external tests of different hosts (using different side chains and cavity sizes) with the addition of 22 other different guests, with the average chirality prediction accuracy of ML versus experimental CD determinations of 92.8%. The easily accessible host-guest features, binding position coordination and size matching between the cavity and guest, exhibit a close correlation to the chirality of different macrocyclic molecules, water-soluble pillar[6]arenes (WP6) versus WP5, in complexation with different amino acid guests. The exploration of efficient host-guest features in ML displays the great potential of building a large space of various assembled systems and accelerating the on-demand design of chiral supramolecular systems at the nanoscale.

Circular Dichroism Based Chirality Sensing with Supramolecular Host-Guest Chemistry

Optical methods are promising to address the ever-increasing demands for chirality analysis in drug discovery and related fields because they are amenable to high-throughput screening. Circular dichroism-based chiroptical sensing using host-guest chemistry is especially appealing due to the fast equilibrium kinetics, wide substrate scope, and potential for sustainable development. In this Minireview, we give an overview on this emerging field. General aspects of molecular recognition and chirality transfer are analyzed. Chirality sensors are discussed by dividing them into three classes according to their structural features. Applications of these chirality sensors for chirality analysis of the products of asymmetric reactions and for the real-time monitoring of reaction kinetics are demonstrated with selected examples. Moreover, challenges and research directions in this field are also highlighted.