Ion-Hydrocarbon and/or Ion-Ion Interactions: Direct and Reverse Hofmeister Effects in a Synthetic Host

Hofmeister Effects in Supramolecular Chemistry for Anion-Modulation to Stabilize Zn Anode

Aqueous Zn-ion batteries (AZIBs) are considered as promising candidates for the next-generation large-scale energy storage, which, however, is facing the challenge of instable Zn anodes. The anion is pivotal in the stability of anodes, which are not being paid enough attention to. Herein, the modulation of anions is reported using the Hofmeister series in supramolecular chemistry to boost the stability of Zn anodes. It is found that the right-side anions in the Hofmeister series (e.g., OTf-) can enhance the Zn2+ transference number, increase the Coulombic efficiency, facilitate uniform Zn deposition, reduce the freezing point of electrolytes, and thereby stabilize the Zn anodes. More importantly, the right-side anions can form strong interaction with β-cyclodextrin (β-CD) compared to the left-side anions, and hence the addition of β-CD can further enhance the stability of Zn anodes in OTf--based electrolytes, showing enhancement of cycling lifespan in the Zn//Zn symmetric cells more than 45.5 times with β-CD compared with those without β-CD. On the contrary, the left-side anions show worse rate performance after the addition of β-CD. These results provide an effective and novel approach for choosing anions and matching additives to stabilize the anodes and achieve high-performance AZIBs through the Hofmeister effect.

Anion Binding to Ammonium and Guanidinium Hosts: Implications for the Reverse Hofmeister Effects Induced by Lysine and Arginine Residues

Anions have a profound effect on the properties of soluble proteins. Such Hofmeister effects have implications in biologics stability, protein aggregation, amyloidogenesis, and crystallization. However, the interplay between the important noncovalent interactions (NCIs) responsible for Hofmeister effects is poorly understood. To contribute to improving this state of affairs, we report on the NCIs between anions and ammonium and guanidinium hosts 1 and 2, and the consequences of these. Specifically, we investigate the properties of cavitands designed to mimic two prime residues for anion-protein NCIs─lysines and arginines─and the solubility consequences of complex formation. Thus, we report NMR and ITC affinity studies, X-ray analysis, MD simulations, and anion-induced critical precipitation concentrations. Our findings emphasize the multitude of NCIs that guanidiniums can form and how this repertoire qualitatively surpasses that of ammoniums. Additionally, our studies demonstrate the ease by which anions can dispense with a fraction of their hydration-shell waters, rearrange those that remain, and form direct NCIs with the hosts. This raises many questions concerning how solvent shell plasticity varies as a function of anion, how the energetics of this impact the different NCIs between anions and ammoniums/guanidiniums, and how this affects the aggregation of solutes at high anion concentrations.

Solvent effects in anion recognition

Anion recognition is pertinent to a range of environmental, medicinal and industrial applications. Recent progress in the field has relied on advances in synthetic host design to afford a broad range of potent recognition motifs and novel supramolecular structures capable of effective binding both in solution and at derived molecular films. However, performance in aqueous media remains a critical challenge. Understanding the effects of bulk and local solvent on anion recognition by host scaffolds is imperative if effective and selective detection in real-world media is to be viable. This Review seeks to provide a framework within which these effects can be considered both experimentally and theoretically. We highlight proposed models for solvation effects on anion binding and discuss approaches to retain strong anion binding in highly competitive (polar) solvents. The synthetic design principles for exploiting the aforementioned solvent effects are explored.

Selective recognition and discrimination of single isomeric changes in peptide strands with a host : guest sensing array

An indirect competitive binding mechanism can be exploited to allow a combination of cationic fluorophores and water-soluble synthetic receptors to selectively recognize and discriminate peptide strands containing a single isomeric residue in the backbone. Peptide isomerization occurs in long-lived proteins and has been linked with diseases such as Alzheimer's, cataracts and cancer, so isomers are valuable yet underexplored targets for selective recognition. Planar cationic fluorophores can selectively bind hydrophobic, Trp-containing peptide strands in solution, and when paired with receptors that provide a competitive host for the fluorophore, can form a differential sensing array that enables selective discrimination of peptide isomers. Residue variations such as D- and L-Asp, D- and L-isoAsp, D-Ser and D-Glu can all be recognized, simply by their effects on the folded structure of the flexible peptide. Molecular dynamics simulations were applied to determine the most favorable conformation of the peptide : fluorophore conjugate, indicating that favorable π-stacking with internal tryptophan residues in a folded binding pocket enables micromolar binding affinity.

Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions

Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM0.9-co-FPMAm0.1) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM0.9-co-FPMAm0.1) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM0.9-co-FPMAm0.1) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM0.9-co-FPMAm0.1) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM0.9-co-FPMAm0.1) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.

The temperature-dependence of host-guest binding thermodynamics: experimental and simulation studies

The thermodynamic parameters of host-guest binding can be used to describe, understand, and predict molecular recognition events in aqueous systems. However, interpreting binding thermodynamics remains challenging, even for these relatively simple molecules, as they are determined by both direct and solvent-mediated host-guest interactions. In this contribution, we focus on the contributions of water to binding by studying binding thermodynamics, both experimentally and computationally, for a series of nearly rigid, electrically neutral host-guest systems and report the temperature-dependent thermodynamic binding contributions ΔGb(T), ΔHb(T), ΔSb(T), and ΔCp,b. Combining isothermal titration calorimetry (ITC) measurements with molecular dynamics (MD) simulations, we provide insight into the binding forces at play for the macrocyclic hosts cucurbit[n]uril (CBn, n = 7-8) and β-cyclodextrin (β-CD) with a range of guest molecules. We find consistently negative changes in heat capacity on binding (ΔCp,b) for all systems studied herein - as well as for literature host-guest systems - indicating increased enthalpic driving forces for binding at higher temperatures. We ascribe these trends to solvation effects, as the solvent properties of water deteriorate as temperature rises. Unlike the entropic and enthalpic contributions to binding, with their differing signs and magnitudes for the classical and non-classical hydrophobic effect, heat capacity changes appear to be a unifying and more general feature of host-guest complex formation in water. This work has implications for understanding protein-ligand interactions and other complex systems in aqueous environments.

Large anion binding in water

Large water-soluble anions with chaotropic character display surprisingly strong supramolecular interactions in water, for example, with macrocyclic receptors, polymers, biomembranes, and other hydrophobic cavities and interfaces. The high affinity is traced back to a hitherto underestimated driving force, the chaotropic effect, which is orthogonal to the common hydrophobic effect. This review focuses on the binding of large anions with water-soluble macrocyclic hosts, including cyclodextrins, cucurbiturils, bambusurils, biotinurils, and other organic receptors. The high affinity of large anions to molecular receptors has been implemented in several lines of new applications, which are highlighted herein.

Hofmeister Effects Shine in Nanoscience

Hofmeister effects play a crucial role in nanoscience by affecting the physicochemical and biochemical processes. Thus far, numerous wonderful applications from various aspects of nanoscience have been developed based on the mechanism of Hofmeister effects, such as hydrogel/aerogel engineering, battery design, nanosynthesis, nanomotors, ion sensors, supramolecular chemistry, colloid and interface science, nanomedicine, and transport behaviors, etc. In this review, for the first time, the progress of applying Hofmeister effects is systematically introduced and summarized in nanoscience. It is aimed to provide a comprehensive guideline for future researchers to design more useful Hofmeister effects-based nanosystems.

"Salting out" in Hofmeister Effect Enhancing Mechanical and Electrochemical Performance of Amide-based Hydrogel Electrolytes for Flexible Zinc-Ion Battery

With the development of flexible and wearable electronic devices, it is a new challenge for polymer hydrogel electrolytes to combine high mechanical flexibility and electrochemical performance into one membrane. In general, the high content of water in hydrogel electrolyte membranes always leads to poor mechanical strength, and limits their applications in flexible energy storage devices. In this work, based on the "salting out" phenomenon in Hofmeister effect, a kind of gelatin-based hydrogel electrolyte membrane is fabricated with high mechanical strength and ionic conductivity by soaking pre-gelated gelatin hydrogel in 2 m ZnSO₄ aqueous. Among various gelatin-based electrolyte membranes, the gelatin-ZnSO₄ electrolyte membrane delivers the "salting out" property of Hofmeister effect, which improves both the mechanical strength and electrochemical performance of gelatin-based electrolyte membranes. The breaking strength reaches 1.5 MPa. When applied to supercapacitors and zinc-ion batteries, it can sustain over 7500 and 9300 cycles for repeated charging and discharging processes. This study provides a very simple and universal method to prepare polymer hydrogel electrolytes with high strength, toughness, and stability, and its applications in flexible energy storage devices provide a new idea for the construction of secure and stable flexible and wearable electronic devices.

SPECIFIC ION EFFECTS ON COPPER ELECTROPLATING

The main goal of this work is to open new perspectives in the field of electrodeposition and provide green alternatives to the electroplating industry. The effect of different anions (SO₄^2-, ClO₃^-, NO₃^-, ClO₄^-, BF₄^-, PF₆^-) in solution on the electrodeposition of copper was investigated. The solutions, containing only the copper precursor and the background electrolyte, were tailored to minimize the environmental impact and reduce the use of organic additives and surfactants. The study is based on electrochemical measurements carried out to verify that no metal complexation takes place. We assessed the nucleation and growth mechanism, we performed a morphological characterization through scanning electron microscopy and deposition efficiency by measuring the film thickness through X-ray fluorescence spectroscopy. Significant differences in the growth mechanism and in the morphology of the electrodeposited films, were observed as a function of the background electrolyte.

The Influence of Small Biomolecules, Salts and Buffers on the Molecular Recognition of Amide Naphthotube in Aqueous Solutions

We found the binding affinities of amide naphthotube to neutral organic molecules in water are not influenced by most of small biomolecules, inorganic salts, and PBS and Tris buffers but are reduced in HEPES buffer through competitive binding. Nevertheless, salts do change the binding affinities of amide naphthotube to charged molecules through a screening effect.

Correlation coefficient-directed label-free characterization of native proteins by surface-enhanced Raman spectroscopy

Investigation of proteins in their native state is the core of proteomics towards better understanding of their structures and functions. Surface-enhanced Raman spectroscopy (SERS) has shown its unique advantages in protein characterization with fingerprint information and high sensitivity, which makes it a promising tool for proteomics. It is still challenging to obtain SERS spectra of proteins in the native state and evaluate the native degree. Here, we constructed 3D physiological hotspots for a label-free dynamic SERS characterization of a native protein with iodide-modified 140 nm Au nanoparticles. We further introduced the correlation coefficient to quantitatively evaluate the variation of the native degree, whose quantitative nature allows us to explicitly investigate the Hofmeister effect on the protein structure. We realized the classification of a protein of SARS-CoV-2 variants in 15 min, which has not been achieved before. This study offers an effective tool for tracking the dynamic structure of proteins and biomedical research.

Highly Selective Potentiometric Sensing of Biologically Relevant Pyrophosphate and Lysophosphatidic Acid Using N-Alkyl/Aryl Ammonium Resorcinarenes/Extended-Resorcinarenes as Ionophores

The ionophore properties of four kinds of N-alkyl/aryl ammonium resorcinarenes and extended-resorcinarenes were inspected for the first time to fabricate polymeric membrane electrodes for determination of biologically relevant pyrophosphate (PPi) and lysophosphatidic acid (LPA). The proposed ion selective electrodes (ISEs) showed significant preference for PPi and LPA with significant selectivity pattern differences from the Hofmeister series. To gain further insight into the performances of the developed ISEs, the binding constants of ionophore-anion complexes in the plasticized membrane phase were investigated, along with the optimized geometries and calculated electrostatic potential. Nernstian potential responses with good reversibility to target anions can be observed when shifting the optimized membranes in aqueous solutions in the concentration range from 10^-6.5 to 10^-2.3/10^-2.2 M. Moreover, potentiometric sensings of PPi and LPA in mineral water and artificial serum were achieved in low μM concentration range, demonstrating their promising real-world applications. These results provide a promising avenue for the development of polymeric membrane electrodes for biological relevant anions and will broaden the scope of potentiometric sensing.

Unique Mechanisms of Ion Storage in Polyaniline Electrodes for Pseudocapacitive Energy Storage Devices Unraveled by EQCM-D Analysis

The optimal performance of organic electrodes for aqueous batteries requires their full compatibility with selected electrolyte solutions. Electrode materials having 1-3-dimensional structures of variable rigidity possess a confined space in their structure filled with water and electrolyte solutions. Depending on the rigidity and confined space geometry, insertion and extraction of ions into electrode structures are often coupled with incorporation/withdrawal of water molecules. Aside from the scientific interest in understanding the charging mechanism of such systems, co-insertion of solvent molecules affects strongly the charge storage capability of the electrodes for energy storage devices. We present herein in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) investigations of polyaniline (PANI) electrodes operating in various aqueous Na⁺-containing electrolytes, namely, Na₂SO₄, NaClO₄, NaBF₄, and NaPF₆. Careful analysis of the EQCM-D results provides a dynamic snapshot of the mixed anionic/protonic fluxes and the accompanying water molecules' insertion/extraction to/from the PANI electrodes. Based on our observations, it was found that the charging mechanism, as well as the capacity values, strictly depends on the electrolyte pH, the chaotropic/kosmotropic character of the anionic dopants, and the amount of the extracted water molecules. This study demonstrates the effectiveness of analysis by EQCM-D in selecting electrolytes for batteries comprising organic electrodes.

Nanopore Sensing Technique for Studying the Hofmeister Effect

The nanopore sensing technique is an emerging method of detecting single molecules, and extensive research has gone into various fields, including nanopore sequencing and other applications of single-molecule studies. Recently, several researchers have explored the specific ion effects in nanopore channels, enabling a unique understanding of the Hofmeister effect at the single-molecule level. Herein, the recent advances of using nanopore sensing techniques are reviewed to study the Hofmeister effect and the physicochemical mechanism of this process is attempted. The challenges and goals are also discussed for the future in this field.

Embedding of Functionalized Coordination Cages and a Molecular Knot in a Polymeric Membrane for Potentiometric Sensing of Environmentally Important Oxyanions and Halides

Three kinds of coordination cages and a molecular knot with inductively activated ⁺P-H, N-H, or C-H hydrogen bond donors anchoring in the functionalized cavities were inspected as ionophores to develop polymeric membrane ISEs for potentiometric sensing of environmentally important oxyanions and halides. The proposed ISEs displayed significant preference for perrhenate, phosphate, or chloride with a selectivity pattern distinctively different from the sequence depending on the Gibbs free energy of hydration owing to the high degree of shape, charge, and size selectivity originating from the rigidity and complementarity of the binding cavities. To gain further insight into the response characters of the proposed ISEs, the binding constants of ionophore-anion complexes in the membrane phase were investigated, and the binding affinity, together with the Hofmeister series, correlates well with the determined selectivity pattern of the proposed ISEs. Optimizing the composition of the membrane such as lipophilic additives and plasticizers produced ISEs displaying Nernstian/near-Nernstian potentiometric responses to primary anions with a wide linear range, improved detection limits, good reversibility, and satisfying lifetime. Potentiometric determination of perrhenate, phosphate, and chloride in river water, mineral water, and artificial serum samples was achieved with good recovery and accuracy using the proposed ISEs, demonstrating their potential for real-life applications. These results will shed new light on how novel ionophores could be designed for potentiometric sensing and broaden the scope of host-guest chemistry of coordination cages and molecular knots.

Ion-Specific Effects on Vesicle-to-Micelle Transitions of an Amino Acid Surfactant Probed by Chemical Trapping

Ion-specific effects widely exist in biological and chemical systems and cannot be explained by classical theories. The complexity of ion-specific effects in protein systems at the molecular level necessitates the use of mimetic models involving smaller molecules, such as amino acids, oligopeptides, and other organic molecules bearing amide bonds. Therefore, it is of theoretical value to determine the effect of additional salts on the aggregation transitions of acyl amino acid surfactants. Herein, the effects of specific tetraalkylammonium ions (TAA⁺) on sodium lauroyl glycinate (SLG) aggregation were studied by dynamic light scattering (DLS) and transmission electron microscopy. Although previous studies have shown that the kosmotropic TAA⁺ ions tend to induce micellar growth or micelle-to-vesicle transitions of some anionic surfactants, TAA⁺ addition in the present study induced partial vesicle-to-micelle transitions in SLG solutions. The chemical trapping (CT) method was employed to estimate changes in the interfacial molarities of water, amide bonds, and carboxylate groups during such transitions. The vesicle-to-micelle transitions were accompanied by a marked rise in interfacial water molarity and a decline in interfacial amide bonds molarity, suggesting that the hydrated TAA⁺ entered the interfacial region and disrupted hydrogen bonding, thus preventing the SLG monomers from packing tightly. Molecular dynamic simulation was also performed to demonstrate the salt-induced cleavage of amide-amide bonds between SLG headgroups. Furthermore, both CT and DLS results show that the ability of tetraalkylammonium cations to induce such transitions increased with increasing size and hydrophobicity of the cation, which follows the Hofmeister series. The current study offers critical molecular-level evidence for understanding the specific effects of tetraalkylammonium ions on the aggregation transitions of an acyl amino acid surfactant.

Interaction among bovine serum albumin (BSA) molecules in the presence of anions: a small-angle neutron scattering study

Protein-protein interaction in solution strongly depends on dissolved ions and solution pH. Interaction among globular protein (bovine serum albumin, BSA), above and below of its isoelectric point (pI ≈ 4.8), is studied in the presence of anions (Cl^-, Br^-, I^-, F^-, SO₄^2-) using small-angle neutron scattering (SANS) technique. The SANS study reveals that the short-range attraction among BSA molecules remains nearly unchanged in the presence of anions, whereas the intermediate-range repulsive interaction increases following the Hofmeister series of anions. Although the interaction strength modifies below and above the pI of BSA, it nearly follows the series.

Buffer and Salt Effects in Aqueous Host-Guest Systems: Screening, Competitive Binding, or Both?

There are many open questions regarding the supramolecular properties of ions in water, a fact that has ramifications within any field of study involving buffered solutions. Indeed, as Pielak has noted (Buffers, Especially the Good Kind, Biochemistry, 2021, in press. DOI:10.1021/acs.biochem.1c00200) buffers were conceived of with little regard to their supramolecular properties. But there is a difficulty here; the mathematical models supramolecular chemists use for affinity determinations do not account for screening. As a result, there is uncertainty as to the magnitude of any screening effect and how this compares to competitive salt/buffer binding. Here we use a tetra-cation cavitand to compare halide affinities obtained using a traditional unscreened model and a screened (Debye-Hückel) model. The rule of thumb that emerges is that if ionic strength is changed by >1 order of magnitude─either during a titration or if a comparison is sought between two different buffered solutions─screening should be considered. We also build a competitive mathematical model showing that binding attenuation in buffer is largely due to competitive binding to the host by said buffer. For the system at hand, we find that the effect of competition is approximately twice that of the effect of screening (∼RT at 25 °C). Thus, for strong binders it is less important to account for screening than it is to account for competitive complexation, but for weaker binders both effects should be considered. We anticipate these results will help supramolecular chemists unravel the properties of buffers and so help guide studies of biomacromolecules.

Electrostatic Potential Field Effects on Amine Macrocyclizations within Yoctoliter Spaces: Supramolecular Electron Withdrawing/Donating Groups

The central role of Coulombic interactions in enzyme catalysis has inspired multiple approaches to sculpting electrostatic potential fields (EPFs) for controlling chemical reactivity, including ion gradients in water microdroplets, the tips of STMs, and precisely engineered crystals. These are powerful tools because EPFs can affect all reactions, even those whose mechanisms do not involve formal charges. For some time now, supramolecular chemists have become increasingly proficient in using encapsulation to control stoichiometric and catalytic reactions. However, the field has not taken advantage of the broad range of nanocontainers available to systematically explore how EPFs can affect reactions within their inner-spaces. With that idea in mind, previously, we reported on how positively and negatively charged supramolecular capsules can modulate the acidity and reactivity of thiol guests bound within their inner, yoctoliter spaces (Cai, X.; Kataria, R.; Gibb, B. C. J. Am. Chem. Soc. 2020, 142, 8291–8298; Wang, K.; Cai, X.; Yao, W.; Tang, D.; Kataria, R.; Ashbaugh, H. S.; Byers, L. D.; Gibb, B. C. J. Am. Chem. Soc.2019, 141, 6740–6747). Building on this, we report here on the cyclization of 14-bromotetradecan-1-amine inside these yoctoliter containers. We examine the rate and activation thermodynamics of cyclization (Eyring analysis), both in the absence and presence of exogenous salts whose complementary ion can bind to the outside of the capsule and hence attenuate its EPF. We find the cyclization rates and activation thermodynamics in the two capsules to be similar, but that for either capsule attenuation of the EPF slows the reaction down considerably. We conclude the capsules behave in a manner akin to covalently attached electron donating/withdrawing groups in a substrate, with each capsule enforcing their own deviations from the idealized S_N2 mechanism by moving electron density and charge in the activated complex and TS, and that the idealized S_N2 mechanism inside the theoretical neutral host is relatively difficult because of the lack of solvation of the TS.

Design and Applications of Water-Soluble Coordination Cages

Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

Anion binding to ubiquitin and its relevance to the Hofmeister effects

Although the non-covalent interactions between proteins and salts contributing to the Hofmeister effects have been generally mapped, there are many questions regarding the specifics of these interactions. We report here studies involving the small protein ubiquitin and salts of polarizable anions. These studies reveal a complex interplay between the reverse Hofmeister effect at low pH, the salting-in Hofmeister effect at higher pH, and six anion binding sites in ubiquitin at the root of these phenomena. These sites are all located at protuberances of preorganized secondary structure, and although stronger at low pH, are still apparent when ubiquitin possesses no net charge. These results demonstrate the traceability of these Hofmeister phenomena and suggest new strategies for understanding the supramolecular properties of proteins.

Studying the supramolecular properties of Ubiquitin reveals six anion binding sites that contribute to the reverse Hofmeister effect at low pH and the salting-in Hofmeister effect at higher pH.

Molecular Mechanism for the Interactions of Hofmeister Cations with Macromolecules in Aqueous Solution

Ion identity and concentration influence the solubility of macromolecules. To date, substantial effort has been focused on obtaining a molecular level understanding of specific effects for anions. By contrast, the role of cations has received significantly less attention and the underlying mechanisms by which cations interact with macromolecules remain more elusive. To address this issue, the solubility of poly(N-isopropylacrylamide), a thermoresponsive polymer with an amide moiety on its side chain, was studied in aqueous solutions with a series of nine different cation chloride salts as a function of salt concentration. Phase transition temperature measurements were correlated to molecular dynamics simulations. The results showed that although all cations were on average depleted from the macromolecule/water interface, more strongly hydrated cations were able to locally accumulate around the amide oxygen. These weakly favorable interactions helped to partially offset the salting-out effect. Moreover, the cations approached the interface together with chloride counterions in solvent-shared ion pairs. Because ion pairing was concentration-dependent, the mitigation of the dominant salting-out effect became greater as the salt concentration was increased. Weakly hydrated cations showed less propensity for ion pairing and weaker affinity for the amide oxygen. As such, there was substantially less mitigation of the net salting-out effect for these ions, even at high salt concentrations.

Hofmeister Effect in Self-Organized Chemical Systems

We studied the effect of spectator ions in the prototype of far-from-equilibrium self-organized chemical systems, the Belousov-Zhabotinsky (BZ) reaction. In particular, we investigated the specific ion effect of alkali metal cations, connoted for their kosmotropic and chaotropic properties. By means of combined experimental and numerical approaches, we could show a neat and robust evidence for the Hofmeister effect in this system. Spectator cations induce a marked increment of the induction period that preludes regular oscillations and decrease the oscillation amplitude following the sequence Li⁺ < Na⁺ ≪ K⁺ ∼ Cs⁺. These ions affect the system kinetics by interfering in the interaction between the oxidized form of the catalyst and the organic substrate, responsible for resetting the BZ system to pre-autocatalytic (reduced) conditions. The specific ion effect on these key reactive steps is systematically characterized and correlated with different parameters which describe the interaction of the cations with the solvent.

Intrinsic and Extrinsic Control of the pKa of Thiol Guests inside Yoctoliter Containers

Despite decades of research, there are still many open questions surrounding the mechanisms by which enzymes catalyze reactions. Understanding all the noncovalent forces involved has the potential to allow de novo catalysis design, and as a step toward this, understanding how to control the charge state of ionizable groups represents a powerful yet straightforward approach to probing complex systems. Here we utilize supramolecular capsules assembled via the hydrophobic effect to encapsulate guests and control their acidity. We find that the greatest influence on the acidity of bound guests is the location of the acidic group within the yoctoliter space. However, the nature of the electrostatic field generated by the (remote) charged solubilizing groups also plays a significant role in acidity, as does counterion complexation to the outer surfaces of the capsules. Taken together, these results suggest new ways by which to affect reactions in confined spaces.

Effect of solvent and ions on the structure and dynamics of a hyaluronan molecule

Hyaluronic acid (hyaluronan, HA) is a negatively charged polysaccharide forming highly swollen random coils in aqueous solutions. Their size decreases along with growing salt concentration, but the mechanism of this phenomenon remains unclear. We carry out molecular-dynamics simulations of a 48-monosaccharide HA oligomer in varying salt concentration and temperature. They identify the interaction points of Na⁺ ions with the HA chain and reveal their influence on the HA solvation-shell structure. The salt-dependent variation of the molecular size does not consist in the distribution of the dihedral angles of the glycosidic connections but is driven by the random flips of individual dihedral angles, which cause the formation of temporary hairpin-like structures effectively shortening the chain. They are induced by the frequency of cation-chain interactions that grow with the salt concentration, but is reduced by the simultaneous decrease of ions' activities. This leads to an anomalous random-coil shrinkage at 0.6 M salt concentration.

Probing the glycopolymer–ion interaction via specific ion effects

Specific ion effects were used to probe the interactions between thermoresponsive glycopolymers and different ions.

Dynamic Light Scattering - an all-purpose guide for the supramolecular chemist

Dynamic light scattering (DLS) is a useful tool for the study of the solution-based behavior of colloids and molecular assemblies. The aim of this methods paper is to provide perspective on how this technique can be used by supramolecular chemists. As this technique is not extensively used within the field, we also provide a historical background of its development, summarize data interpretation and the strengths and limitations of the technique, and provide a perspective on some of the important features for supramolecular chemists that can be found in an instrument.

Binding Properties and Supramolecular Polymerization of a Water-Soluble Resorcin[4]arene

Controlling the self-assembly of molecules in water is difficult because the small size, polarity, and hydrogen bond donating and accepting properties of water attenuate most non-covalent interactions. Here we describe how resorcinarene 1, with pyridinium pendent groups, assembles in water to form head-to-tail assemblies. These small supramolecular polymers form because they offer greater stabilization than any latent head-to-head assembly of resorcinarenes to form dimeric (or hexameric) containers. Instead, the resorcinarene bowl - particularly if negatively charged - is a good host for the pyridinium pendent groups of a second resorcinarene. Alternatively, resorcinarene 1 is also a good host for complexing anions and cations of any added salt. In combination therefore, host 1 possesses a rich repertoire of supramolecular properties that is dependent on the ionic strength and the nature of salts, pH, and the concentration of the host. These findings provide new information about controlling the self-assembly of resorcinarenes in water.

Using reversible non-covalent and covalent bonds to create assemblies and equilibrating molecular networks that survive 5 molar urea

The limits of self-assembly and host-guest chemistry in water solutions containing competitive solutes are largely unexplored. We report here a new family of self-assembling systems that are stitched together at two levels by reversible hydrazone bonds and by non-covalent self-assembly in strongly denaturing conditions. Three different hydrazides of various charge and hydrophobicity are combined with an aldehyde-containing calixarene, and each system spontaneously forms AB hydrazones that subsequently self-assemble into four-component (AB)2 structures in water. The assemblies display varying responses to added NaCl and/or urea. The most robust assembly survives completely intact in solution up to 5 M urea. We also combine the aldehyde calixarene with two different hydrazides in the same tube to create complex, competitive dynamic libraries. We report experiments in which the composition of the dynamic equilibrating library is under the control of self-assembly, allowing the systems to choose the components that form the most stable assemblies under a variety of competitive solutions conditions. These dynamic networks of equilibrating molecules maintain remarkably similar equilibrium positions under widely varying concentrations of urea and NaCl.

Anion Recognition in Water by Charge-Neutral Halogen and Chalcogen Bonding Foldamer Receptors

A novel strategy for the recognition of anions in water using charge-neutral σ-hole halogen and chalcogen bonding acyclic hosts is demonstrated for the first time. Exploiting the intrinsic hydrophobicity of halogen and chalcogen bond donor atoms integrated into a foldamer structural molecular framework containing hydrophilic functionalities, a series of water-soluble receptors was constructed for an anion recognition investigation. Isothermal titration calorimetry (ITC) binding studies with a range of anions revealed the receptors to display very strong and selective binding of large, weakly hydrated anions such as I^- and ReO₄^-. This is achieved through the formation of 2:1 host-guest stoichiometric complex assemblies, resulting in an encapsulated anion stabilized by cooperative, multidentate, convergent σ-hole donors, as shown by molecular dynamics simulations carried out in water. Importantly, the combination of multiple σ-hole-anion interactions and hydrophobic collapse results in I^- affinities in water that exceed all known σ-hole receptors, including cationic systems (β₂ up to 1.68 × 10¹¹ M^-2). Furthermore, the anion binding affinities and selectivity trends of the first example of an all-chalcogen bonding anion receptor in pure water are compared with halogen bonding and hydrogen bonding receptor analogues. These results further advance and establish halogen and chalcogen bond donor functions as new tools for overcoming the challenging goal of anion recognition in pure water.

A fluoride selective water-soluble anion receptor based on a 1,2-phenylenediacetic acid and calcium ion dimer

The dimeric receptor 1 from 1,2-phenylenediacetic acid and calcium ions recognized fluoride ions almost exclusively in 100% water.

The Chaotropic Effect as an Assembly Motif in Chemistry

Following up on scattered reports on interactions of conventional chaotropic ions (for example, I- , SCN- , ClO4- ) with macrocyclic host molecules, biomolecules, and hydrophobic neutral surfaces in aqueous solution, the chaotropic effect has recently emerged as a generic driving force for supramolecular assembly, orthogonal to the hydrophobic effect. The chaotropic effect becomes most effective for very large ions that extend beyond the classical Hofmeister scale and that can be referred to as superchaotropic ions (for example, borate clusters and polyoxometalates). In this Minireview, we present a continuous scale of water-solute interactions that includes the solvation of kosmotropic, chaotropic, and hydrophobic solutes, as well as the creation of void space (cavitation). Recent examples for the association of chaotropic anions to hydrophobic synthetic and biological binding sites, lipid bilayers, and surfaces are discussed.

Dynamic light scattering studies of the effects of salts on the diffusivity of cationic and anionic cavitands

Although alkali halide salts play key roles in all living systems, the physical models used to describe the properties of aqueous solutions of salts do not take into account specific ion–ion interactions. To identify specific ion–ion interactions possibly contributing to the aggregation of proteins, we have used dynamic light scattering (DLS) to probe the aggregation of charged cavitands. DLS measurements of negatively charged 1 in the presence of a range of alkali metal halides reveal no significant aggregation of host 1 as a function of the nature of the cation of the added salt. Only at high concentrations could trace amounts of aggregation be detected by ¹H NMR spectroscopy. Contrarily, 1 was readily aggregated and precipitated by ZnCl₂. In contrast, although fluoride and chloride did not induce aggregation of positively charged host 2, this cavitand exhibited marked aggregation as a function of bromide and iodide concentration. Specifically, bromide induced small but significant amounts of dimerization, whilst iodide induced extreme aggregation. Moreover, in these cases aggregation of host 2 also exhibited a cationic dependence, with an observed trend Na⁺ > Li⁺ > K⁺ ≈ Cs⁺. In combination, these results reveal new details of specific ion pairings in aqueous solution and how this can influence the properties of dissolved organics.