Ion‐Pair Recognition by a Heteroditopic Triazole‐Containing Receptor

A new heteroditopic calix[4]diquinone triazole containing receptor capable of recognising both cations and anions through Lewis base and C? H hydrogen‐bonding modes, respectively, of the triazole motif has been prepared. This ion‐pair receptor cooperatively binds halide/monovalent‐cation combinations in an aqueous mixture, with selectivity trends being established by ¹H NMR and UV/Vis spectroscopy. Cation binding by the calix[4]diquinone oxygen and triazole nitrogen donors enhances the strength of the halide complexation at the isophthalamide recognition site of the receptor. Conversely, anions bound in the receptor’s isophthalamide cavity enhance cation recognition. ¹H NMR investigations in solution suggest that the receptor’s triazole motifs are capable of coordinating simultaneously to both cation and anion guest species. Solid‐state X‐ray crystallographic structural analysis of a variety of receptor ion‐pair adducts further demonstrates the dual cation–anion binding role of the triazole group.     

A Fluorescent Ditopic Rotaxane Ion‐Pair Host

We report a rotaxane based on a simple urea motif that binds Cl^? selectively as a separated ion pair with H⁺ and reports the anion binding event through a fluorescence switch‐on response. The host selectively binds Cl^? over more basic anions, which deprotonate the framework, and less basic anions, which bind more weakly. The mechanical bond also imparts size selectivity to the ditopic host.     

Ion‐Pair Complexation with a Cavitand Receptor

The capability of resorcinarenes to bind anions within the alkyl feet at the lower rim has been exploited as the starting point for developing a new cavitand able to engulf contact ion pairs of primary ammonium salts in chlorinated solvents with association constants (K_ass) in the range of 10³–10⁴ M ^?1. Methylene bridges were introduced into the upper rim to freeze the resorcinarene in the cone conformation with the four H_down protons converging in the lower pocket, thereby maximizing the CH–anion interactions responsible for the anion binding. Four additional phosphate moieties were introduced into the lower rim in close proximity to the anionic site to provide hydrogen‐bonding‐acceptor P?O groups and promote cation complexation at the bottom of the cavitand. The binding ability of the synthesized ligands was analyzed by ¹H NMR spectroscopy and, when possible, by isothermal titration calorimetry (ITC); the data were in agreement when complementary techniques were used.     

Liquid‐Crystalline Supramolecular Polymers Formed through Complementary Nucleobase‐Pair Interactions

We report how the placement of nucleobase units, thymine, or N ⁶‐(4‐methoxybenzoyl)adenine, onto the ends of a mesogenic core, bis‐4‐alkoxy‐substituted bis(phenylethynyl)benzene, affects the properties of these materials. We show that addition of these bulky polar groups significantly reduces the range of liquid‐crystalline behavior of these compounds. However, mixing two complementary nucleobase‐containing AA‐ and BB‐type monomer units together does result in the formation of stable, thermotropic liquid‐crystalline (LC) phases. Hydrogen bonding is shown to play an important role in the formation of these LC phases, consistent with the formation of oligomeric or polymeric hydrogen‐bonded aggregates. X‐ray analyses of these mixed materials are consistent with the formation of smectic C phases.     

Frustrated Lewis Pair Chemistry of Chiral (+)‐Camphor‐Based Aminoboranes

Dimethylamino‐(+)‐camphorenamine reacted with an equimolar amount of Piers’ borane, HB(C₆F₅)₂, to give the corresponding iminium–hydroborate zwitterionic salt. Being in equilibrium with the parent enamine–HB(C₆F₅)₂ N–B pair, this salt was able to split hydrogen heterolytically, hydrogenating the iminium group in the molecule. Detailed studies revealed that the hydrogen splitting in this reaction proceeded through an intermolecular pathway leading to a bornylamine–HB(C₆F₅)₂ adduct. When the starting enamine is present in excess over HB(C₆F₅)₂, the produced bornylamine–HB(C₆F₅)₂ adduct breaks up, eliminating free bornylamine and forming the initial camphorenamine– HB(C₆F₅)₂ pair. This results in hydrogenation of the camphorenamine framework in a catalytic fashion.     

SLAP: Small Labeling Pair for Single‐Molecule Super‐Resolution Imaging

Protein labeling with synthetic fluorescent probes is a key technology in chemical biology and biomedical research. A sensitive and efficient modular labeling approach (SLAP) was developed on the basis of a synthetic small‐molecule recognition unit (Ni‐trisNTA) and the genetically encoded minimal protein His_6‐10‐tag. High‐density protein tracing by SLAP was demonstrated. This technique allows super‐resolution fluorescence imaging and fulfills the necessary sampling criteria for single‐molecule localization‐based imaging techniques. It avoids masking by large probes, for example, antibodies, and supplies sensitive, precise, and robust size analysis of protein clusters (nanodomains).     

Nucleosome Immobilization Strategies for Single‐Pair FRET Microscopy

All genomic transactions in eukaryotes take place in the context of the nucleosome, the basic unit of chromatin, which is responsible for DNA compaction. Overcoming the steric hindrance that nucleosomes present for DNA‐processing enzymes requires significant conformational changes. The dynamics of these have been hard to resolve. Single‐pair Fluorescence Resonance Energy Transfer (spFRET) microscopy is a powerful technique for observing conformational dynamics of the nucleosome. Nucleosome immobilization allows the extension of observation times to a limit set only by photobleaching, and thus opens the possibility of studying processes occurring on timescales ranging from milliseconds to minutes. It is crucial however, that immobilization itself does not introduce artifacts in the dynamics. Here we report on various nucleosome immobilization strategies, such as single‐point attachment to polyethylene glycol (PEG) or surfaces coated with bovine serum albumin (BSA), and confinement in porous agarose or polyacrylamide gels. We compare the immobilization specificity and structural integrity of immobilized nucleosomes. A crosslinked star polyethylene glycol coating performs best with respect to tethering specificity and nucleosome integrity, and enables us to reproduce for the first time bulk nucleosome unwrapping kinetics in single nucleosomes without immobilization artifacts.     

Sequence‐Dependent Duplex Stabilization upon Formation of a Metal‐Mediated Base Pair

An artificial nucleoside surrogate with 1H‐imidazo[4,5‐f][1,10]phenanthroline ( P ) acting as an aglycone has been introduced into DNA oligonucleotide duplexes. This nucleoside surrogate can act as a bidentate ligand, and so is useful in the context of metal‐mediated base pairs. Several duplexes involving a hetero base pair with an imidazole nucleoside have been investigated. The stability of DNA duplexes incorporating the respective Ag^I‐mediated base pairs strongly depends on the sequence context. Quantum mechanical/molecular mechanical (QM/MM) calculations have been performed in order to gain insight into the factors determining this sequence dependence. The results indicated that, in addition to the stabilizing effect that results from the formation of coordinative bonds, destabilizing effects may occur when the artificial base pair does not fit optimally into the surrounding B‐DNA duplex.     

Asymmetric Dual‐Reagent Catalysis: Mannich‐type Reactions Catalyzed by Ion Pair

The combination of a new bifunctional phosphine and an acrylate generate a zwitterion in situ and it serves as an efficient catalyst for asymmetric reactions through a homogeneous ion‐pairing mode. This new catalytic system has been successfully applied to Mannich‐type reactions to give excellent results and it demonstrates a broad substrate scope. Such reactivity is not accessible with general organophosphine catalytic modes. Preliminary investigations into the mechanism are also presented.     

Additional Base‐Pair Formation in DNA Duplexes by a Double‐Headed Nucleotide

We have designed and synthesised a double‐headed nucleotide that presents two nucleobases in the interior of a dsDNA duplex. This nucleotide recognises and forms Watson–Crick base pairs with two complementary adenosines in a Watson–Crick framework. Furthermore, with judicious positioning in complementary strands, the nucleotide recognises itself through the formation of a T:T base pair. Thus, two novel nucleic acid motifs can be defined by using our double‐headed nucleotide. Both motifs were characterised by UV melting experiments, CD and NMR spectroscopy and molecular dynamics simulations. Both motifs leave the thermostability of the native dsDNA duplex largely unaltered. Molecular dynamics calculations showed that the double‐headed nucleotides are accommodated in the dsDNA by entirely local perturbations and that the modified duplexes retain an overall B‐type geometry with the dsDNA unwound by around 25 or 60°, respectively, in each of the modified motifs. Both motifs can be accommodated twice in a dsDNA duplex without incurring any loss of stability and extrapolating from this observation and the results of modelling, it is conceivable that both can be multiplied several times within a dsDNA duplex. These new motifs extend the DNA recognition repertoire and may form the basis for a complete series of double‐headed nucleotides based on all 16 base combinations of the four natural nucleobases. In addition, both motifs can be used in the design of nanoscale DNA structures in which a specific duplex twist is required.     

Modelling Ion‐Pair Geometries and Dynamics in a 1‐Ethyl‐1‐methylpyrrolidinium‐Based Ion‐Conductive Crystal

Full conformational and energy explorations are conducted on an organic ionic plastic crystal, 1‐ethyl‐1‐methylpyrrolidium tetrafluoroborate [C₂mpyr][BF₄]. The onsets of various stages of dynamic behaviour, which appear to account for low‐temperature solid–solid phase transitions, are investigated by using quantum‐chemical simulations. It is suggested that pseudorotation of the pyrrolidine ring occurs in the first instance; the partial rotation of the entire cation subsequently occurs and may be accompanied by reorientation of the ethyl chain as the temperature increases further. A cation–anion configuration, whereby BF₄^? interacts with the C₂mpy cation from the side of the ring, is the most likely structure in the low‐temperature phase IV region. These interpretations are supported by ¹³C nuclear magnetic resonance chemical‐shift analysis.     

Non‐Innocent Methylene Linker in Bridged Lewis Pair Initiators

Deprotonation usually occurs as an unwanted side reaction in the Lewis pair polymerization of Michael acceptors, for which the conjugated addition of the Lewis base to the acid‐activated monomer is the commonly accepted initiation mechanism. This has also been reported for B?P‐based bridged Lewis pairs (BLPs) that form macrocyclic addition products. We now show that the formerly unwanted deprotonation is the likely initiation pathway in the case of Al?P‐based BLPs. In a detailed study of a series of Al?P‐based BLPs, using a combination of single‐crystal diffraction experiments (X‐ray and neutron) and mechanistic investigations (experimental and computational), an active role of the methylene bridge was revealed, acting as a base towards the α‐acidic monomers. Additionally, the polymerization studies proved a living behavior combined with significantly high activities, narrow molecular mass distributions, and the possibility of copolymerization.     

Ion‐Pair Recognition of Tetramethylammonium Salts by Halogenated Resorcinarenes

The non‐covalent interactions of different upper‐rim‐substituted C₂‐resorcinarenes with tetramethylammonium salts were analyzed in the gas phase in an Electrospray Ionization Fourier‐transform ion‐cyclotron‐resonance (ESI‐FTICR) mass spectrometer and by ¹H NMR titrations. The order of binding strengths of the hosts towards the tetramethylammonium cation in the gas phase reflects the electronic nature of the substituents on the upper rim of the resorcinarene. In solution, however, a different trend with particularly high binding constants for halogenated resorcinarenes has been observed. This trend can be explained by a synergetic effect originating from the interaction of the halogenated resorcinarenes with the counter anions through hydrogen bonding. This study highlights the importance of weak interactions in recognition processes and points out the benefits of comparing the gas‐phase data with results obtained from solution experiments.     

Chalcogen Bond Mediated Enhancement of Cooperative Ion‐Pair Recognition

A series of heteroditopic receptors containing halogen bond (XB) and unprecedented chalcogen bond (ChB) donors integrated into a 3,5‐bis‐triazole pyridine structure covalently linked to benzo‐15‐crown‐5 ether motifs exhibit remarkable cooperative recognition of halide anions. Multi‐nuclear ¹H, ¹³C, ¹²⁵Te and ¹⁹F NMR, ion pair binding investigations reveal sodium cation–benzo‐crown ether binding dramatically enhances the recognition of bromide and iodide halide anions, with the chalcogen bonding heteroditopic receptor notably displaying the largest enhancement of halide binding strength of over two hundred‐fold, in comparison to the halogen bonding and hydrogen bonding heteroditopic receptor analogues. DFT calculations suggest crown ether sodium cation complexation induces a polarisation of the sigma hole of ChB and XB heteroditopic receptor donors as a significant contribution to the origin of the unique cooperativity exhibited by these systems.     

Frustrated Lewis Pair Mediated 1,2‐Hydrocarbation of Alkynes

Frustrated Lewis pair (FLP) chemistry enables a rare example of alkyne 1,2‐hydrocarbation with N‐methylacridinium salts as the carbon Lewis acid. This 1,2‐hydrocarbation process does not proceed through a concerted mechanism as in alkyne syn‐hydroboration, or through an intramolecular 1,3‐hydride migration as operates in the only other reported alkyne 1,2‐hydrocarbation reaction. Instead, in this study, alkyne 1,2‐hydrocarbation proceeds by a novel mechanism involving alkyne dehydrocarbation with a carbon Lewis acid based FLP to form the new C−C bond. Subsequently, intermolecular hydride transfer occurs, with the Lewis acid component of the FLP acting as a hydride shuttle that enables alkyne 1,2‐hydrocarbation.     

Development of a Metal‐Ion‐Mediated Base Pair for Electron Transfer in DNA

A new C‐nucleoside structurally based on the hydroxyquinoline ligand was synthesized that is able to form stable pairs in DNA in both the absence and the presence of metal ions. The interactions between the metal centers in adjacent Cu^II‐mediated base pairs in DNA were probed by electron paramagnetic resonance (EPR) spectroscopy. The metal–metal distance falls into the range of previously reported values. Fluorescence studies with a donor–DNA–acceptor system indicate that photoinduced charge‐transfer processes across these metal‐ion‐mediated base pairs in DNA occur more efficiently than over natural base pairs.     

New Ion‐Pair Based All‐Solid‐State Surfactant Sensitive Sensor for Potentiometric Determination of Cationic Surfactants

Cationic surfactants of different types were determined using a few potentiometric sensors based on ion‐pair complexes (dodecyldimethylbenzylammonium dodecylsulfate, dodecylmethylbenzylammonium dodecylbenzensulfonate, tetrahexadecylammonium dodecylsulfate and Hyamine (benzethonium dodecylsulfate)) as sensing materials. The response of the all‐solid state surfactant sensitive electrode based on a Teflonized graphite conducting substrate, coated with a PVC membrane containing sensing material, was investigated in the solutions of Hyamine and hexadecyltrimethylammonium ion in the concentration range from 1 μM to 10 mM. Potentiometric surfactant cation titration has been performed using sodium dodecylsulfate as titrant and an ion‐pair‐based surfactant sensitive electrode as a potentiometric indicator. Several commercial surfactant products have also been titrated and the results were compared with those obtained with two‐phase standard titration method.     

Novel Coronene–Naphthalene Dimide‐Based Donor–Acceptor Pair for Tunable Charge‐Transfer Nanostructures

Charge‐transfer (CT) assemblies of aromatic donor (D) and acceptor (A) molecules have recently gained attention as a promising material for organic electronics and ferroelectrics. Two major factors which govern their functions are the strength of CT interaction and their supramolecular nanostructuring. Here we present coronene‐naphthalenediimide (NDI)‐based novel D‐A pairs that form alternately stacked CT assemblies. Through systematic substitution of the NDI derivatives and studying their CT interactions with coronene, a clear understanding of the secondary forces responsible for controlling their association is gained. Finally, the use of CT‐based supramolecular amphiphiles for their nanostructural engineering into ordered one‐dimensional (1‐D) assemblies is demonstrated.     

Calix[4]pyrrole‐Based Heteroditopic Ion‐Pair Receptor That Displays Anion‐Modulated,Cation‐Binding Behavior

A new ditopic ion‐pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl₃. Treatment of the halide‐ion complexes of 1 with Group I and II metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺; studied as their perchlorate salts in CD₃CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [ 1? F]^? (preformed as the tetrabutylammonium (TBA⁺) complex), little evidence of interaction with the K⁺ ion was seen. In contrast, when this same complex (i.e., [ 1? F]^? as the TBA⁺ salt) was treated with the Li⁺ or Na⁺ ions, complete decomplexation of the receptor‐bound fluoride ion was observed. In sharp contrast to what was seen with Li⁺, Na⁺, and K⁺, treating complex [ 1? F]^? with the Cs⁺ ion gave rise to a stable, receptor‐bound ion‐pair complex [Cs ?1? F] that contains the Cs⁺ ion complexed within the cup‐like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [ 1? Cl]^?. In this case, no appreciable interaction was observed with Na⁺ or K⁺. In addition, treating [ 1? Cl]^? with Li⁺ produces a tightly hydrated dimeric ion‐pair complex [ 1? LiCl(H₂O)]₂ in which two Li⁺ ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [ 1? F]^?, exposure of [ 1? Cl]^? to the Cs⁺ ion gives rise to an ion‐pair complex [Cs ?1? Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.