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1. Anion-binding catalyst designs for enantioselective synthesis

   Visco, M. D.; Attard, J. W.; Guan, Y.; Mattson, A. E. (January 2017, Tetrahedron letters)

   Select hydrogen bond donors can catalyze reactions of ion pairs through the recognition of anions. This mode of action can be exploited in enantioselective catalysis if a suitable chiral hydrogen bond donor is applied. Beyond just anionic recognition, an enantioselective anion-binding catalyst often must host numerous non-covalent interactions, including hydrogen bonding, general base, π-π, and π-cation, to achieve high levels of enantiocontrol. Anion-binding catalysts can be strategically designed to support those non-covalent interactions required to render a process highly stereoselective. Tactics applied in anion-binding catalyst development include enhancing arene substituents for improved π-stacking, linking two anion-binding units together on a single scaffold, expanding types of functional groups for anion recognition, and building frameworks with bifunctional modes of action. The intent of this digest is to highlight observations that suggest as anion-binding catalyst designs advance, their associated synthetic methodologies for complex molecule construction become increasingly impressive. 

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2. Folding of prestin’s anion-binding site and the mechanism of outer hair cell electromotility

   https://doi.org/10.7554/eLife.89635.3  

   Lin, Xiaoxuan; Haller, Patrick R; Bavi, Navid; Faruk, Nabil; Perozo, Eduardo; Sosnick, Tobin R (December 2023, eLife)

   Plsess, Stephan A (Ed.)

   Prestin responds to transmembrane voltage fluctuations by changing its cross-sectional area, a process underlying the electromotility of outer hair cells and cochlear amplification. Prestin belongs to the SLC26 family of anion transporters yet is the only member capable of displaying electromotility. Prestin’s voltage-dependent conformational changes are driven by the putative displacement of residue R399 and a set of sparse charged residues within the transmembrane domain, following the binding of a Cl⁻anion at a conserved binding site formed by the amino termini of the TM3 and TM10 helices. However, a major conundrum arises as to how an anion that binds in proximity to a positive charge (R399), can promote the voltage sensitivity of prestin. Using hydrogen–deuterium exchange mass spectrometry, we find that prestin displays an unstable anion-binding site, where folding of the amino termini of TM3 and TM10 is coupled to Cl⁻binding. This event shortens the TM3–TM10 electrostatic gap, thereby connecting the two helices, resulting in reduced cross-sectional area. These folding events upon anion binding are absent in SLC26A9, a non-electromotile transporter closely related to prestin. Dynamics of prestin embedded in a lipid bilayer closely match that in detergent micelle, except for a destabilized lipid-facing helix TM6 that is critical to prestin’s mechanical expansion. We observe helix fraying at prestin’s anion-binding site but cooperative unfolding of multiple lipid-facing helices, features that may promote prestin’s fast electromechanical rearrangements. These results highlight a novel role of the folding equilibrium of the anion-binding site, and help define prestin’s unique voltage-sensing mechanism and electromotility. 

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3. Supramolecular Incarceration and Extraction of Tetrafluoroberyllate from Water by Nanojars

   https://doi.org/10.1021/acs.inorgchem.2c01198  

   Al Isawi, Wisam A.; Zeller, Matthias; Mezei, Gellert (May 2022, Inorganic Chemistry)

   The previously unexplored noncovalent binding of the highly toxic tetrafluoroberyllate anion (BeF42–) and its extraction from water into organic solvents are presented. Nanojars resemble anion-binding proteins in that they also possess an inner anion binding pocket lined by a multitude of H-bond donors (OH groups), which wrap around the incarcerated anion and completely isolate it from the surrounding medium. The BeF4-binding propensity of [BeF4⊂{CuII(OH)(pz)}n]2– (pz = pyrazolate; n = 27–32) nanojars of different sizes is investigated using an array of techniques including mass spectrometry, paramagnetic 1H, 9Be, and 19F NMR spectroscopy, and X-ray crystallography, along with thermal stability studies in solution and chemical stability studies toward acidity and Ba2+ ions. The latter is found to be unable to precipitate the insoluble BaBeF4 from nanojar solutions, indicating a very strong binding of the BeF42– anion by nanojars. 9Be and 19F NMR spectroscopy allows for the unprecedented direct probing of the incarcerated anion in a nanojar and, along with 1H NMR studies, reveals the fluxional structure of nanojars and their inner anion-binding pockets. Single-crystal X-ray diffraction provides the crystal and molecular structures of (Bu4N)2[BeF4⊂{Cu(OH)(pz)}32], which contains a novel Cux-ring combination (x = 9 + 14 + 9), (Bu4N)2[BeF4⊂{Cu(OH)(pz)}8+14+9], and (Bu4N)2[BeF4⊂{Cu(OH)(pz)}6+12+10] and offers detailed structural parameters related to the supramolecular binding of BeF42– in these nanojars. The extraction of BeF42– from water into organic solvents, including the highly hydrophobic solvent n-heptane, demonstrates that nanojars are efficient binding and extracting agents not only for oxoanions but also for fluoroanions. 

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4. Orthogonal, modular anion–cation and cation–anion self-assembly using pre-programmed anion binding sites

   https://doi.org/10.1039/D2SC05121D  

   Dhara, Ayan; Fadler, Rachel E.; Chen, Yusheng; Köttner, Laura A.; Van Craen, David; Carta, Veronica; Flood, Amar H. (March 2023, Chemical Science)

   Subcomponent self-assembly relies on cation coordination whereas the roles of anions often only emerge during the assembly process. When sites for anions are instead pre-programmed, they have the potential to be used as orthogonal elements to build up structure in a predictable and modular way. We explore this idea by combining cation (M + ) and anion (X − ) binding sites together and show the orthogonal and modular build up of structure in a multi-ion assembly. Cation binding is based on a ligand (L) made by subcomponent metal-imine chemistry (M + = Cu + , Au + ) while the site for anion binding (X − = BF 4 − , ClO 4 − ) derives from the inner cavity of cyanostar (CS) macrocycles. The two sites are connected by imine condensation between a pyridyl-aldehyde and an aniline-modified cyanostar. The target assembly [LM-CS-X-CS-ML], + generates two terminal metal complexation sites (LM and ML) with one central anion-bridging site (X) defined by cyanostar dimerization. We showcase modular assembly by isolating intermediates when the primary structure-directing ions are paired with weakly coordinating counter ions. Cation-directed (Cu + ) or anion-bridged (BF 4 − ) intermediates can be isolated along either cation–anion or anion–cation pathways. Different products can also be prepared in a modular way using Au + and ClO 4 − . This is also the first use of gold( i ) in subcomponent self-assembly. Pre-programmed cation and anion binding sites combine with judicious selection of spectator ions to provide modular noncovalent syntheses of multi-component architectures. 

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5. Solution Studies of a Water-Stable, Trivalent Antimony Pnictogen Bonding Anion Receptor with High Binding Affinities for CN ^– , OCN ^– , and OAc ^–

   https://doi.org/10.1021/acs.inorgchem.3c01887  

   Qiu, Jinchun; Bateman, Curt N.; Lu, Shuai; George, Gary C.; Li, Xiaopeng; Gorden, John D.; Vasylevskyi, Serhii; Cozzolino, Anthony F. (August 2023, Inorganic Chemistry)

   The solution phase anion binding behavior of a water-stable bidentate pnictogen bond donor was studied. A modest change in the visible absorption spectrum allowed for the determination of the binding constants. High binding constants were observed with cyanide, cyanate, and acetate, and these were corroborated with density functional theory (DFT) calculations. The receptor could be recovered free from the anion following treatment with methyl triflate, confirming that it remains intact. The tight binding of cyanide and water stability were exploited to use this system as a supramolecular catalyst in a phase-transfer Strecker reaction, further demonstrating the utility of pnictogen bonding as a tool in noncovalent catalysis. 

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