RESUMEN

This article focuses on developing selective receptors for chloride. A series of novel receptors based on a rigid naphthalene spacer bearing tetrafluoropyridines (TFPs) in various substitution patterns has been designed and synthsized. The topology of the receptors was confirmed by X-ray cristallography. Their complexation efficiency was essentially studied in the gas phase through mass spectrometry experiments. Density Functional Theory (DFT) calculations and NMR titration were also performed.

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Steric manipulation is a known concept in molecular recognition but there is currently no linear free energy relationship correlating sterics to the stability of receptor-anion complexes nor to the reactivity of the bound anion. By analogy to Tolman cone angles in cation coordination chemistry, we explore how to define and correlate cone angles of organo-trifluoroborates (R-BF3 -) to the affinities observed for cyanostar-anion binding. We extend the analogy to a rare investigation of the anion's reactivity and how it changes upon binding. The substituent on the anion is used to define the cone angle, θ. A series of 10 anions were studied including versions with ethynyl, ethylene, and ethyl substituents to tune steric bulk across the sp, sp2 and sp3 hybridized α-carbons bearing 0, 1 and 2 hydrogen atoms. A linear relationship between affinity and cone angle is observed for anions bearing substituents larger than the -BF3 - headgroup. This correlation predicted affinities of two new anions to within ±5 %. We explored how complexation affects the reactivity of fluoride exchange. The yield of fluoride transfer from R-BF3 - to Lewis acid triphenylborane is correlated with cone angle. We predict that other rigid macrocycles, like commercially available bambusuril, could follow these trends.

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Arylethynyl-substituted dipyrrolyldiketone BF2 complexes as anion-responsive π-electronic molecules exhibited characteristic electronic properties derived from conformation changes upon anion binding, which caused an increase in UV/vis absorption and associated two-photon absorption. The anion complexes showed expanded planar regions assisted by intramolecular interactions, resulting in charge-by-charge ion-pairing assemblies in the solid state.

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The in situ generation of active photoredox organic catalysts upon anion-binding co-catalysis by making use of the ionic nature of common photosensitizers is reported. Hence, the merge of anion-binding and photocatalysis permitted the modulation of the photocatalytic activity of simple acridinium halide salts, building an effective anion-binding - photoredox ion pair complex able to promote a variety of visible light driven transformations, such as anti-Markovnikov addition to olefins, Diels-Alder and the desilylative C-C bond forming reactions. Anion-binding studies, together with steady-state and time-resolved spectroscopy analysis, supported the postulated ion pair formation between the thiourea hydrogen-bond donor organocatalyst and the acridinium salt, which proved essential for unlocking the photocatalytic activity of the photosensitizer.

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For a comparison of the interaction modes of various chalcogen-bond donors, 2-chalcogeno-imidazolium salts have been designed, synthesized, and studied by single crystal X-ray diffraction, solution NMR and DFT as well as for their ability to act as activators in an SN1-type substitution reaction. Their interaction modes in solution were elucidated based on NMR diffusion and chemical shift perturbation experiments, which were supported by DFT-calculations. Our finding is that going from lighter to the heavier chalcogens, hydrogen bonding plays a less, while chalcogen bonding an increasingly important role for the coordination of anions. Anion-π interactions also show importance, especially for the sulfur and selenium derivatives.

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Inspired by nature, artificial hydrogen bond-based anion receptors have been developed to achieve high anion selectivity; however, their binding affinity is usually low. The potency of these receptors is usually increased by the introduction of aryl substituents, which withdraw electrons from their binding site through the resonance effect. Here, we show that the polarization of the C(sp3 )-H binding site of bambusuril receptors, and thus their potency to bind anions, can be modulated by the inductive effect. The presence of electron-withdrawing groups on benzyl substituents of bambusurils significantly increases their binding affinities to halides, resulting in the strongest iodide receptor reported to date with an association constant greater than 1013  M-1 in acetonitrile. A Hammett plot showed that while the bambusuril affinity toward halides linearly increases with the electron-withdrawing power of their substituents, their binding selectivity remains essentially unchanged.

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The first examples of [2]catenanes capable of selective anion transport across a lipid bilayer are reported. The neutral halogen bonding (XB) [2]catenanes were prepared via a chloride template-directed strategy in an unprecedented demonstration of using XB⋅⋅⋅anion interactions to direct catenane assembly from all-neutral components. Anion binding experiments in aqueous-organic solvent media revealed strong halide over oxoanion selectivity, and a marked enhancement in the chloride and bromide affinities of the catenanes relative to their constituent macrocycles. The catenanes additionally displayed an anti-Hofmeister binding preference for bromide over the larger iodide anion, illustrating the efficacy of employing sigma-hole interactions in conjunction with the mechanical bond effect to tune receptor selectivity. Transmembrane anion transport studies conducted in POPC LUVs revealed that the catenanes were more effective anion transporters than the constituent macrocycles, with high chloride over hydroxide selectivity, which is critical to potential therapeutic applications of anionophores. Remarkably these outperform existing acyclic halogen bonding anionophores with regards to this selectivity. Record chloride over nitrate anion transport selectivity was also observed. This represents a rare example of the direct translation of intrinsic anion binding affinities to anion transport behaviour, and demonstrates the key role of the catenane mechanical bond effect for enhanced anion transport selectivity.

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Enantioselective oxa-Pictet-Spengler reactions of tryptophol with aldehydes proceed under weakly acidic conditions utilizing a combination of two catalysts, an indoline HCl salt and a bisthiourea compound. Mechanistic investigations revealed the roles of both catalysts and confirmed the involvement of oxocarbenium ion intermediates, ruling out alternative scenarios. A stereochemical model was derived from density functional theory calculations, which provided the basis for the development of a highly enantioselective stereodivergent variant with racemic tryptophol derivatives.

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Much effort has been devoted to elucidate mechanisms of amyloid fibril formation using various kinds of additives, such as salts, metals, detergents, and biopolymers. Here, we review the effects of additives with a focus on polyphosphate (polyP) on amyloid fibril formation of ß2-microglobulin (ß2m) and α-synuclein (αSyn). PolyP, consisting of up to 1,000 phosphoanhydride bond-linked phosphate monomers, is one of the most ancient, enigmatic, and negatively charged molecules in biology. Amyloid fibril formation of both ß2m and αSyn could be accelerated by counter anion-binding and preferential hydration at relatively lower and higher concentrations of polyP, respectively, depending on the chain length of polyP. These bimodal concentration-dependent effects were also observed in salt- and heparin-induced amyloid fibril formation, indicating the generality of bimodal effects. We also address the effects of detergents, alcohols, and isoelectric point precipitation on amyloid fibril formation, in comparison with the effects of salts. Because polyP is present all around us, from cellular components to food additives, clarifying its effects and consequent biological roles will be important to further advance our understanding of amyloid fibrils. This review article is an extended version of the Japanese article, Linking Protein Folding to Amyloid Formation, published in SEIBUTSU BUTSURI Vol. 61, p. 358-365 (2021).

RESUMEN

The 'Nest' motif plays a functional role in protein owing to its ligand binding potential aided by geometric concavity. The presence of less favored left-handed conformation (L-state) in its structure makes this concavity possible and in shaping the native chemical environment amenable to stable binding interactions. To understand the persistent appearance of L-state torsion in the Nest motif, we analyzed 0.5µs Molecular Dynamics (MD) simulation trajectories of 35 six-residue peptides (out of a total of 50 large Nest sequences of ≥6 residues) identified in our previous study. Analysis of the MD trajectories of the individual peptides reveals initial L-state in 60% of the peptides persists for >40% of the trajectory. Further, Nests with different sequences appear to adopt a specific conformational state driven by the neighboring L-state residues. The sequences also possess short secondary structures and amino acid repeats, suggesting evolutionary conservation and the specific role of amino acids in locally predisposing the torsion angle to the L-state. These findings help us to understand how L-state conformation is an essential prerequisite in stabilizing the Nest motif and shed light on the sequence-structure-function paradigm in the rational design of peptides and peptidomimetics for therapeutics.Communicated by Ramaswamy H. Sarma.

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The globally widespread perfluorooctanoic acid (PFOA) is a concerning environmental contaminant, with a possible toxic long-term effects on the environment and human health The development of sensible, rapid, and low-cost detection systems is a current change in modern environmental chemistry. In this context, two triamine-based chemosensors, L1 and L2, containing a fluorescent pyrene unit, and their Zn(II) complexes are proposed as fluorescent probes for the detection of PFOA in aqueous media. Binding studies carried out by means of fluorescence and NMR titrations highlight that protonated forms of the receptors can interact with the carboxylate group of PFOA, thanks to salt bridge formation with the ammonium groups of the aliphatic chain. This interaction induces a decrease in the fluorescence emission of pyrene at neutral and slightly acidic pH values. Similarly, emission quenching has also been observed upon coordination of PFOA by the Zn(II) complexes of the receptors. These results evidence that simple polyamine-based molecular receptors can be employed for the optical recognition of harmful pollutant molecules, such as PFOA, in aqueous media.

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Exploiting non-covalent interactions to catalyze challenging ionic polymerizations is an ambitious goal but is in its infancy. We recently demonstrated non-covalent anion-binding catalysis as an effective methodology to enable living cationic polymerization (LCP) of vinyl ethers in an environmentally benign manner. Here, we further elucidate the structure-reactivity relationships of the elaborately designed seleno-cyclodiphosph(V)azanes catalysts and the roles of anion-binding interactions by a combined theoretical DFT study and experimental study. The investigation suggests that the distinct cis-cyclodiphosph(V)azane framework combined with "selenium effect" and electron-withdrawing 3,5-(CF3 )2 -Phenyl substitution pattern in catalyst enables a critical contribution to accessing excellent stability, anion affinity and solubility under polymerization conditions. Thus, the catalyst could leverage anion-binding interactions to precisely control reversible and transient dormant-active species equilibrium, allowing it to dynamically bind, recognize and pre-organize propagating ionic species and monomer, thereby facilitating efficient chain propagation and minimizing irreversible chain transfer events under mild conditions. The more in-depth understanding of the mechanism for anion-binding catalytic LCP reported herein should help to guide future catalyst design and to extend this concept to broader polymerization systems where ionic species serve as crucial intermediates.

Asunto(s)

RESUMEN

A new approach towards highly enantioselective halogen-bonding catalysis has been developed. To circumvent the intrinsic issues of the nature of the halogen-bond (XB) and the resultant unresolved limitations in asymmetric catalysis, fine-tuned halogen-halogen interactions between the substrate and XB-donor were designed to preorganize the substrate in the catalyst's cavity and boost enantiocontrol. The present strategy exploits both the electron cloud (Lewis base site) and the sigma (σ)-hole site of the halogen substituent of the substrates to form a tight catalyst-substrate-counteranion chiral complex, thus enabling a controlled induction of high levels of chirality transfer. Remarkable enantioselectivities of up to 95 : 5 e.r. (90 % ee) have been achieved in a model dearomatization reaction of halogen-substituted (iso)quinolines with tetrakis-iodotriazole multidentate anion-binding catalysts.

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The supramolecular recognition of anions is increasingly harnessed to achieve the self-assembly of supramolecular architectures, ranging from cages and polymers to (pseudo)rotaxanes. The cyanostar (CS) macrocycle has previously been shown to form 2 : 1 complexes with organophosphate anions that can be turned into [3]rotaxanes by stoppering. Here we achieved steric control over the assembly of pseudorotaxanes comprising the cyanostar macrocycle and a thread that is based, for the first time, on organo-pyrophosphonates. Subtle differences in steric bulk on the threads allowed formation of either [3]pseudorotaxanes or [2]pseudorotaxanes. We demonstrate that the threading kinetics are governed by the steric demand of the organo-pyrophosphonates and in one case, slows down to the timescale of minutes. Calculations show that the dianions are sterically offset inside the macrocycles. Our findings broaden the scope of cyanostar-anion assemblies and may have relevance for the design of molecular machines whose directionality is a result of relatively slow slipping.

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We present a ligand platform featuring appended ditopic Lewis acids to facilitate capture/activation of diatomic substrates. We show that incorporation of two 9-borabicyclo[3.3.1]nonane (9-BBN) units on a single carbon tethered to a pyridine pyrazole scaffold maintains a set of unquenched nitrogen donors available to coordinate FeII , ZnII , and NiII . Using hydride ion affinity and competition experiments, we establish an additive effect for ditopic secondary sphere boranes, compared to the monotopic analogue. These effects are exploited to achieve high selectivity for binding NO2 - in the presence of competitive anions such as F- and NO3 - . Finally, we demonstrate hydrazine capture within the second-sphere of metal complexes, followed by unique activation pathways to generate hydrazido and diazene ligands on Zn and Fe, respectively.

RESUMEN

Appending functional groups to the exterior of Zn4 L4 self-assembled cages allows gated control of anion binding. While the unfunctionalized cages contain aryl groups in the ligand that can freely rotate, attaching inert functional groups creates a "doorstop", preventing rotation and slowing the guest exchange rate, even though the interiors of the host cavities are identically structured. The effects on anion exchange are subtle and depend on multiple factors, including anion size, the nature of the leaving anion, and the electron-withdrawing ability and steric bulk of the pendant groups. Multiple exchange mechanisms occur, and the nature of the external groups controls associative and dissociative exchange processes: these bulky groups affect both anion egress and ingress, introducing an extra layer of selectivity to the exchange. Small changes can have large effects: affinities for anions as similar as PF6 - and SbF6 - can vary by as much as 400-fold between identically sized cavities.

RESUMEN

Exploring new noncovalent bonding motifs with reversibly tunable binding affinity is of fundamental importance in manipulating the properties and functions of supramolecular self-assembly systems and materials. Herein, for the first time, we demonstrate a unique visible-light-switchable telluro-triazole/triazolium-based chalcogen bonding (ChB) system in which the Te moieties are connected by azobenzene cores. The binding strengths between these azo-derived ChB receptors and the halide anions (Cl- , Br- ) could be reversibly regulated upon irradiation by visible light of different wavelengths. The cis-bidentate ChB receptors exhibit enhanced halide anion binding ability compared to the trans-monodentate receptors. In particular, the telluro-triazolium-based ChB receptor can achieve both high and significantly photoswitchable binding affinities for halide anions, which enable it to serve as an efficient photocontrolled organocatalyst for ChB-assisted halide abstraction in a Friedel-Crafts alkylation benchmark reaction.

RESUMEN

We describe the synthesis, crystallographic characterization of a new Cu-Salphen compound and its use as a host Lewis-acid against guest anions in two versions: a) free molecule, b) copolymerized with methyl methacrylate:n-butyl acrylate (1 : 4-wt.) as protective co-monomers. Higher contents in Cu-Salphen yielded larger and more homogeneous polymer sizes. Polymer size together with glass transitions, heat capacity, thermal degradation, guest-saturation degrees and host-guest species distribution profiles from spectrophotometric titrations explained growths of up to 630-fold in K11 and 180000-fold in K12 for the host's binding site attributable to a solvophobic protection from the macromolecular structure. Spectrofluorimetry revealed blue-shifted×13-16 larger luminescence for Cu-Salphen in the polymers (λem =488-498 nm) than that of the non-polymerized counterpart (λem =510-543 nm) and "turn-on" blue-shifted enhanced fluorescence upon guest association. We propose a cooperative incorporation of the guests occurring from the outer medium toward internally protected binding site pockets in the random coil polymer conformations.

RESUMEN

Dicyanoaurate(I) anion, [Au(CN)2 ]- , plays a central role in the current industrial production of gold, as its extraction from crude ore samples represents the most money-consuming step. Herein, we present the strongest host-guest recognition of dicyanoaurate anion using the bambusuril receptor in water, a highly competitive solvent. The micromolar stability of such a complex facilitated the up to date most efficient supramolecular stripping of dicyanoaurate from activated carbon at ambient temperature. Thermodynamic characteristics of bambusuril binding with [Au(CN)2 ]- differing from binding of other inorganic chaotropic anions are rationalized, as well as the bambusuril selectivity for [Au(CN)2 ]- over [Ag(CN)2 ]- .

RESUMEN

Supramolecular anion binding to squaramide crosslinkers in poly(N,N-dimethylacrylamide) gel networks enhances swelling and allows reversible chemically driven actuation. The volume swelling ratio of the gels is shown to depend on both the type of anion and its concentration. 1H NMR and UV-vis titrations with the squaramide crosslinkers reveal a relationship between anion binding affinity and the concentration-dependent swelling behavior. Gel swelling is shown to be reversible, and by embedding a solid support into rod-shaped gels, soft actuators are fabricated that undergo forward and backward bending motion in response to changing anion concentration. The swelling and bending process, which is accompanied by intense green coloration of the gel, is achieved by using only low amounts of crosslinker. This macroscopic actuation achieved by anion binding to specific molecular entities in the polymer network will open new opportunities in the field of chemically responsive materials.