The Chaotropic Effect as an Assembly Motif in Chemistry

Qualitative discrimination and quantitative prediction of salt in aqueous solution based on near-infrared spectroscopy

Freshwater resources have been gradually salinized in recent years, dramatically impacting the ecosystem and human health. Therefore, it is necessary to detect the salinity of freshwater resources. However, traditional detection methods make it difficult to check the type and concentration of salt quickly and accurately in solution. This paper uses a portable near-infrared spectrometer to qualitatively discriminate and quantitatively predict the salt in the solution. The study was carried out by adding ten salts of NaCl, KCl, MgCl2, CaCl2, Na2CO3, K2CO3, CaCO3, Na2SO4, K2SO4, MgSO4 to 2 mL of deionized water to prepare a single salt solution (0.02 %-1.00 %) totaling 100 sets. It was found that the Support vector machine (SVM) model was only effective in discriminating the class of salt anions in the solution. The Partial least squares-discriminant analysis (PLS-DA) model, on the other hand, can effectively discriminate the classes of salt in solution, and the accuracies of the optimal model prediction set and the interactive validation set are 98.86 % and 99.66 %, respectively. Furthermore, the Partial least squares regression (PLSR) models can accurately predict the concentration of NaCl, KCl, MgCl2, CaCl2, Na2CO3, K2CO3, CaCO3, Na2SO4, K2SO4, MgSO4 salt solutions. The coefficients of determination R2 of their model interactive validation sets were 0.99, 0.99, 0.99, 0.97, 0.99, 0.99, 0.98, 0.99, 0.98, and 0.98, respectively. This study shows that NIRS can achieve rapid and accurate qualitative and quantitative detection of salts in solution, which provides technical support for the utilization of safe water resources.

Tetraphenylborate enhanced enzymatic production of γ-cyclodextrin: System construction and its mechanism

γ-Cyclodextrin (γ-CD) is an attractive material among the natural cyclodextrins owing to its excellent properties. γ-CD is primarily produced from starch by γ-cyclodextrin glycosyltransferase (γ-CGTase) in a controlled system. However, difficulty in separation and low conversion rate leads to high production costs for γ-CD. In this study, γ-CGTase from Bacillus sp. G-825-6 STB17 was used in γ-CD production from cassava starch. With the introduction of sodium tetraphenylborate (NaBPh4), the total conversion rate was promoted from an initial 18.07 % to 50.49 % and the γ-CD ratio reached 78.81 % with a yield of 39.79 g/L. Furthermore, the mechanism was conducted via the determination of binding constant, which indicated that γ-CD exhibited much stronger binding strength with NaBPh4 than β-CD. The reformation of water molecules and the chaotropic effect might be the main driving forces for the interaction. Additionally, the conformations of CD complexes were depicted by NMR and molecular docking. The results further verified different binding patterns between CDs and tetraphenylborate ions, which might be the primary reason for the specific binding. This system not only guides γ-CD production with an efficient and easy-to-remove production aid but also offers a new perspective on the selection of complexing agents in CD production.

Controlling Protein Assembly with Superchaotropic Nano-Ions

In living systems, protein assemblies have essential functions, serving as structural supports, transport highways for molecular cargo, and containers of genetic material. The construction of protein assemblies, which involves control over space and time, remains a significant challenge in biotechnology. Here, we show that anionic boron clusters, 3,3'-commo-bis[closo-1,2-dicarba-3-cobaltadodecaborane] (COSAN-), and halogenated closo-dodecarboranes (B12X12 2-, X=H, Cl, or I), described as super-chaotropic nano-ions, induce the formation of 2D assemblies of model proteins, myoglobin, carbonic anhydrase, and trypsin inhibitor. We found that the nano-ion concentration reversibly controls the size of the protein assemblies. Furthermore, the secondary structures of the proteins are only slightly affected by assembly formation. For myoglobin, the formation of these assemblies even prevents temperature denaturation, highlighting a preservation effect of nano-ions. Our study reveals that inorganic boron-based nano-ions act as a reversible molecular glue for proteins, providing a potential starting point for the further development of controlled protein assemblies.

Redox-active polyoxovanadates as cofactors in the development of functional protein assemblies

The interactions of polyoxovanadates (POVs) with proteins have increasingly attracted interest in recent years due to their potential biomedical applications. This is especially the case because of their redox and catalytic properties, which make them interesting for developing artificial metalloenzymes. Organic-inorganic hybrid hexavanadates in particular offer several advantages over all-inorganic POVs. However, they have been scarcely investigated in biological systems even though, as shown in this work, hybrid hexavanadates are highly stable in aqueous solutions up to relatively high pH. Therefore, a novel bis-biotinylated hexavanadate was synthesized and shown to selectively interact with two biotin-binding proteins, avidin and streptavidin. Bridging interactions between multiple proteins led to their self-assembly into supramolecular bio-inorganic hybrid systems that have potential as artificial enzymes with the hexavanadate core as a redox-active cofactor. Moreover, the structure and charge of the hexavanadate core were determined to enhance the binding affinity and slightly alter the secondary structure of the proteins, which affected the size and speed of formation of the assemblies. Hence, tuning the polyoxometalate (POM) core of hybrid POMs (HPOMs) with protein-binding ligands has been demonstrated to be a potential strategy for controlling the self-assembly process while also enabling the formation of novel POM-based biomaterials that could be of interest in biomedicine.

Dynamic Dance of Chirality and Morphology: Interplay of Solvent-Sensitive Self-Assembly in Topological Evolution and Chirality Amplification

The building block Pyra-Chol has been designed and synthesized, which exhibits different achiral morphologies in good solvents, forming nanospheres in THF and nanoflowers in 1,4-dioxane. In the presence of water as a poor cosolvent, Pyra-Chol demonstrates an agnostic behavior, generating left-handed superhelices in the water:THF (80:20) solvent system. However, when the good solvent is switched to 1,4-dioxane, a change in chirality is observed in the water:1,4-dioxane (30:70) solvent system, resulting in the formation of fused nanospheres. Interestingly, when the poor cosolvent is changed from water to MCH in THF, the chiral pattern remains unchanged, but the morphology changes completely. Supported by the collective spectroscopic and microscopic analysis, the present study efficaciously demonstrates the remarkable control of hydrophobic building block over the chiral sense and also highlights the fascinating influence of good as well as poor cosolvent in supporting the distinct molecular packing.

A small-molecule carrier for the intracellular delivery of a membrane-impermeable protein with retained bioactivity

Intracellular protein delivery has the potential to revolutionize cell-biological research and medicinal therapy, with broad applications in bioimaging, disease treatment, and genome editing. Herein, we demonstrate successful delivery of a functional protein, cytochrome c (CYC), by using a boron cluster anion as molecular carrier of the superchaotropic anion type (B12Br11OPr2-). CYC was delivered into lipid bilayer vesicles as well as living cells, with a cellular uptake ratio approaching 90%. Mechanistic studies showed that CYC was internalized into cells through a permeation pathway directly into the cytoplasm, bypassing endosomal entrapment. Upon carrier-assisted internalization, CYC retained its bioactivity, as reflected by an induced cell apoptosis rate of 25% at low dose (1 µM). This study furbishes a direct protein delivery method by a molecular carrier with high efficiency, confirming the potential of inorganic cluster ions as protein transport vehicles with an extensive range of future cell-biological or biomedical applications.

An Update on Theoretical and Metrological Aspects of the Surface Hydrophobicity of Virus and Virus-Like Particles

Viruses are biological entities embodied in protein-based nanoparticles devoid of metabolic activity. Hence, the colloidal, interfacial, and chemical reactivity of virus particles (VPs) profoundly affects the fate of natural and artificial viruses in biotic or abiotic aqueous systems. These rely on the physical chemistry at the outer surface of VPs. In other words, whether wild or synthetic VPs and regardless of the scientific fields involved, taming viruses implies thus managing the physical chemistry at the VP external surface. The surface hydrophobicity (SH) of VPs is a critical feature that must be looked at. Still, the literature dealing with nanoscale hydrophobic domains at the proteinaceous surface of VPs underlying their global SH is like a fragmented puzzle. This article provides an overview of the topic from the perspective of modern protein biophysics for updating the classic physicochemical picture of outer VP/water interfaces hitherto accepted. Patterns of non-polar and "false-polar" patches, expressing variable hydrophobic degrees according to neighboring polar patches, are now drawn. The extensive discussion of reviewed data generates such fresh ideas to explore in the coming years for better modeling the SH of wild virions or engineered virus-based nanoparticles, paving the way for new directions in fundamental virology and virus-based chemistry.

Ion-Specific Interactions Engender Dynamic and Tailorable Properties in Biomimetic Cationic Polyelectrolytes

Biomaterials such as spider silk and mussel byssi are fabricated by the dynamic manipulation of intra- and intermolecular biopolymer interactions. Organisms modulate solution parameters, such as pH and ion co-solute concentration, to effect these processes. These biofabrication schemes provide a conceptual framework to develop new dynamic and responsive abiotic soft material systems. Towards these ends, the chemical diversity of readily available ionic compounds offers a broad palette to manipulate the physicochemical properties of polyelectrolytes via ion-specific interactions. In this study, we show for the first time that the ion-specific interactions of biomimetic polyelectrolytes engenders a variety of phase separation behaviors, creating dynamic thermal- and ion-responsive soft matter that exhibits a spectrum of physical properties, spanning viscous fluids to viscoelastic and viscoplastic solids. These ion-dependent characteristics are further rendered general by the merger of lysine and phenylalanine into a single, amphiphilic vinyl monomer. The unprecedented breadth, precision, and dynamicity in the reported ion-dependent phase behaviors thus introduce a broad array of opportunities for the future development of responsive soft matter; properties that are poised to drive developments in critical areas such as chemical sensing, soft robotics, and additive manufacturing.

Compelling DNA intercalation through 'anion-anion' anti-coulombic interactions: boron cluster self-vehicles as promising anticancer agents

Anticancer drugs inhibit DNA replication by intercalating between DNA base pairs, forming covalent bonds with nucleotide bases, or binding to the DNA groove. To develop safer drugs, novel molecular structures with alternative binding mechanisms are essential. Stable boron hydrides offer a promising alternative for cancer therapy, opening up additional options like boron neutron capture therapy based on 10B and thermal neutron beams or proton boron fusion therapy using 11B and proton beams. These therapies are more efficient when the boron compound is ideally located inside cancer cells, particularly in the nucleus. Current cancer treatments often utilize small, polycyclic, aromatic, planar molecules that intercalate between ds-DNA base pairs, requiring only a spacing of approximately 0.34 nm. In this paper, we demonstrate another type of intercalation. Notably, [3,3'-Fe(1,2-C2B9H11)2]-, ([o-FESAN]-), a compact 3D molecule measuring 1.1 nm × 0.6 nm, can as well intercalate by strong non-bonding interactions preferentially with guanine. Unlike known intercalators, which are positive or neutral, [o-FESAN]- is a negative species and when an [o-FESAN]- molecule approaches the negatively charged DNA phosphate chain an anion-anion interaction consistently anti-electrostatic via Ccluster-H⋯O-P bonds occurs. Then, when more molecules approach, an elongated outstandingly self-assembled structure of [o-FESAN]--[o-FESAN]- forms moving anions towards the interthread region to interact with base pairs and form aggregates of four [o-FESAN]- anions per base pair. These aggregates, in this environment, are generated by Ccluster-H⋯O-C, N-H⋯H-B and Ccluster-H⋯H-B interactions. The ferrabis(dicarbollide) boron-rich small molecules not only effectively penetrate the nucleus but also intercalate with ds-DNA, making them promising for cancer treatment. This amphiphilic anionic molecule, used as a carrier-free drug, can enhance radiotherapy in a multimodal perspective, providing healthcare professionals with improved tools for cancer treatment. This work demonstrates these findings with a plethora of techniques.

Probing Noncovalent Interaction Strengths of Host-Guest Complexes Using Negative Ion Photoelectron Spectroscopy

Noncovalent interactions (NCIs) are crucial for the formation and stability of host-guest complexes, which have wide-ranging implications across various fields, including biology, chemistry, materials science, pharmaceuticals, and environmental science. However, since NCIs are relatively weak and sensitive to bulk perturbation, direct and accurate measurement of their absolute strength has always been a significant challenge. This concept article aims to demonstrate the gas-phase electrospray ionization (ESI)-negative ion photoelectron spectroscopy (NIPES) as a direct and precise technique to measure the absolute interaction strength, probe nature of NCIs, and reveal the electronic structural information for host-guest complexes. Our recent studies in investigating various host-guest complexes that involve various types of NCIs such as anion-π, (di)hydrogen bonding, charge-separated ionic interactions, are overviewed. Finally, a summary and outlook are provided for this field.

Boron Cluster Anions Dissolve En Masse in Lipids Causing Membrane Expansion and Thinning

Boron clusters are applied in medicinal chemistry because of their high stability in biological environments and intrinsic ability to capture neutrons. However, their intermolecular interactions with lipid membranes, which are critical for their cellular delivery and biocompatibility, have not been comprehensively investigated. In this study, we combine different experimental methods - Langmuir monolayer isotherms at the air-water interface, calorimetry (DSC, ITC), and scattering techniques (DLS, SAXS) - with MD simulations to evaluate the impact of closo-dodecaborate clusters on model membranes of different lipid composition. The cluster anions interact strongly with zwitterionic membranes (POPC and DPPC) via the chaotropic effect and cause pronounced expansions of lipid monolayers. The resulting lipid membranes contain up to 33 mol % and up to 52 weight % of boron cluster anions even at low aqueous cluster concentrations (1 mM). They show high (μM) affinity to the hydrophilic-hydrophobic interface, affecting the structuring of the lipid chains, and therefore triggering a sequence of characteristic effects: (i) an expansion of the surface area per lipid, (ii) an increase in membrane fluidity, and (iii) a reduction of bilayer thickness. These results aid the design of boron cluster derivatives as auxiliaries in drug design as well as transmembrane carriers and help rationalize potential toxicity effects.

Cyclic Oxothiomolybdates: Building Blocks for Cyclodextrin-Based Open Frameworks

Desolvation processes, though common in self-assembled biological structures, are rarely evidenced and utilized in the design of crystalline architectures. In this study, we introduce a novel approach using the [Mo8S8O8(OH)8(guest)]2- complex, formed by the self-condensation of four [MoV 2O2S2]2- fragments around a guest unit (MoVIO6H4 or oxalate), as a chaotropic scaffold for crystallizing hybrid organic-inorganic systems with natural cyclodextrins. Our findings reveal that β-cyclodextrin (β-CD) facilitates the formation of host-guest complexes, while α-cyclodextrin (α-CD) induces the formation of a Kagome-type structure with significant voids. These new compounds were thoroughly characterized using X-ray diffraction (both powder and single-crystal), N2 adsorption, elemental and thermogravimetric analysis. Additionally, solution studies using 1H NMR titration and small-angle X-ray scattering (SAXS) demonstrated pre-association of the building units in solution. These results enhance our understanding of the design principles for supramolecular structures composed of inorganic polyanions and cyclodextrins.

Nanoconfinement of polyoxometalates in cyclodextrin: computational inspections of the binding affinity and experimental demonstrations of reactivity modulation

Chaotropic polyoxometalates (POMs) form robust host-guest complexes with γ-cyclodextrin (γ-CD), offering promising applications in catalysis, electrochemical energy storage, and nanotechnology. In this article, we provide the first computational insights on the supramolecular binding mechanisms using density-functional theory and classical molecular dynamics simulations. Focusing on the encapsulation of archetypal Keggin-type POMs (PW12O40 3-, SiW12O40 4- and BW12O40 5-), our findings reveal that the lowest-charged POM, namely PW12O40 3- spontaneously confines within the wider rim of γ-CD, but BW12O40 5- does not exhibit this behaviour. This striking affinity for the hydrophobic pocket of γ-CD originates from the structural characteristics of water molecules surrounding PW12O40 3-. Moreover, through validation using 31P NMR spectroscopy, we demonstrate that this nanoconfinement regulates drastically the POM reactivity, including its capability to undergo electron transfer and intermolecular metalate Mo/W exchanges. Finally, we exploit this nanoconfinement strategy to isolate the elusive mixed addenda POM PW11MoO40 3-.

Photoelectron Spectroscopy and Computational Study on Microsolvated [B10H10]2- Clusters and Comparisons to Their [B12H12]2- Analogues

Microhydrated closo-boranes have attracted great interest due to their superchaotropic activity related to the well-known Hofmeister effect and important applications in biomedical and battery fields. In this work, we report a combined negative ion photoelectron spectroscopy and quantum chemical investigation on hydrated closo-decaborate clusters [B10H10]2-·nH2O (n = 1-7) with a direct comparison to their analogues [B12H12]2-·nH2O and free water clusters. A single H2O molecule is found to be sufficient to stabilize the intrinsically unstable [B10H10]2- dianion. The first two water molecules strongly interact with the solute forming B-H···H-O dihydrogen bonds while additional water molecules show substantially reduced binding energies. Unlike [B12H12]2-·nH2O possessing a highly structured water network with the attached H2O molecules arranged in a unified pattern by maximizing B-H···H-O dihydrogen bonding, distinct structural arrangements of the water clusters within [B10H10]2-·nH2O are achieved with the water cluster networks from trimer to heptamer resembling free water clusters. Such a distinct difference arises from the variations in size, symmetry, and charge distributions between these two dianions. The present finding again confirms the structural diversity of hydrogen-bonding networks in microhydrated closo-boranes and enriches our understanding of aqueous borate chemistry.

Intercluster B-H and B-B aggregation in iso- and trans-[B20H18]2-. Spherical aromaticity in borane dimers

The formation of molecular-based functional materials is a key step towards the development of technologies at the nanoscale. Recently, it has been shown that after oxidation of closo-[B10H10]2- anions, an induced aggregation of two cluster units is achieved, retaining their parent B10 backbones as persistent building blocks. Such characterization provides an interesting scenario to further understand relevant factors leading to aggregation in a minimal structure involving two units. Here, we explore the interaction between closo-[B10H10]2- units in two isomers, namely, iso- and trans-[B20H18]2-, involving different intercluster contacts based on B-B and B-H interactions, respectively. Our results show that the inherent spherical aromatic characteristics of the parent closo-[B10H10]2- cluster are persistent in both iso- and trans-[B20H18]2- isomers as an interplay between the spherical aromatic properties from both B10 motifs, leading to an overlap of the shielding regions from shielding cone properties, ascribed as a dual spherical-spherical aromatic cluster. From 11B-NMR features, it came out that trans-[B20H18]2- involves larger differences in comparison to closo-[B10H10]2-, owing to the variation of the B10-B10 backbone provided by the intercluster B-interaction, thus resulting in a more effective aggregation connecting such building units, towards boron-based cluster materials.

Detailed Study on the Interfacial Interaction between Different Polyoxometalates and Tetronic Block Copolymers Exploring the Langmuir-Blodgett Technique

Polyoxometalates (POMs) interact with various biologically relevant entities. A basic understanding of this interaction is very important for various applications in the biological field. In this work, the focus is on the study of the interaction between tetronics and Keggin POMs. T701 and T90R4 are the two tetronics considered here; they have different solubilities in water due to different PPO/PEO ratios. The arrangement of PPO and PEO is also different with respect to the central ethylenediamine groups. Three different Keggin-type POMs, phosphomolybdic acid (PMA), phosphotungstic acid (PTA), and silicotungstic acid (STA), with different charge densities are chosen for an elaborate investigation using Langmuir-Blodgett technique. The observation is analyzed thoroughly, which shows both electrostatic interaction and adsorption of POMs on the PPO blocks of the tetronics due to the chaotropic effect, which is responsible for the binding of POMs (in subphase) with the tetronic monolayer. This interaction results in an expanded yet rigid monolayer for POM-tetronic association on the surface. Surface pressure vs mean molecular area isotherm is the key characterization to reach the conclusion. UV-vis spectroscopy, NMR, ITC, ellipsometric studies, FTIR, and SEM also serve as supportive characterization techniques.

The effect of host size on binding in host-guest complexes of cyclodextrins and polyoxometalates

Harnessing flexible host cavities opens opportunities for the design of novel supramolecular architectures that accommodate nanosized guests. This research examines unprecedented gas-phase structures of Keggin-type polyoxometalate PW12O40 3- (WPOM) and cyclodextrins (X-CD, X = α, β, γ, δ, ε, ζ) including previously unexplored large, flexible CDs. Using ion mobility spectrometry coupled to mass spectrometry (IM-MS) in conjunction with molecular dynamics (MD) simulations, we provide first insights into the binding modes between WPOM and larger CD hosts as isolated structures. Notably, γ-CD forms two distinct structures with WPOM through binding to its primary and secondary faces. We also demonstrate that ε-CD forms a deep inclusion complex, which encapsulates WPOM within its annular inner cavity. In contrast, ζ-CD adopts a saddle-like conformation in its complex with WPOM, which resembles its free form in solution. More intriguingly, the gas-phase CD-WPOM structures are highly correlated with their counterparts in solution as characterized by nuclear magnetic resonance (NMR) spectroscopy. The strong correlation between the gas- and solution phase structures of CD-WPOM complexes highlight the power of gas-phase IM-MS for the structural characterization of supramolecular complexes with nanosized guests, which may be difficult to examine using conventional approaches.

Size and Polarizability of Boron Cluster Carriers Modulate Chaotropic Membrane Transport

Perhalogenated closo-borates represent a new class of membrane carriers. They owe this activity to their chaotropicity, which enables the transport of hydrophilic molecules across model membranes and into living cells. The transport efficiency of this new class of cluster carriers depends on a careful balance between their affinity to membranes and cargo, which varies with chaotropicity. However, the structure-activity parameters that define chaotropic transport remain to be elucidated. Here, we have studied the modulation of chaotropic transport by decoupling the halogen composition from the boron core size. The binding affinity between perhalogenated decaborate and dodecaborate clusters carriers was quantified with different hydrophilic model cargos, namely a neutral and a cationic peptide, phalloidin and (KLAKLAK)2. The transport efficiency, membrane-lytic properties, and cellular toxicity, as obtained from different vesicle and cell assays, increased with the size and polarizability of the clusters. These results validate the chaotropic effect as the driving force behind the membrane transport propensity of boron clusters. This work advances our understanding of the structural features of boron cluster carriers and establishes the first set of rational design principles for chaotropic membrane transporters.

How Polyoxometalate Promote the Aggregation of a Cyanine Dye in the Aqueous Solution of Non-Ionic Copolymers of Varying Hydrophilic-Lipophilic Balance?

The nonradiative pathway leading to the photoisomerization of a cyanine dye is well-established information. However, the modulations induced in the photoisomerization pathway by a Keggin-type polyoxometalate in a confined media is new. Our study reveals that, in the presence of pluronic block copolymers F-108 and P-123, phosphomolybdic acid hydrate (PMA) promotes the aggregation of 3,3'-diethylthiadicarbocyanine iodide (DTDCI). The absorption spectra show a clear indication of a red-shifted trimer band in F-108 and P-123, whereas it is absent in F-127 and P-84. Fluorescence emission studies suggest that, in the presence of PMA, the rate of photoisomerization is accelerated in F-108, P-123, and P-84 micelles, whereas it is retarded in F-127 micelles. Time-resolved studies in the presence of PMA indicate the preference of F-108, P-84, and P-123 toward the trapped conformer of DTDCI, whereas F-127 favors the formation of photoisomer of DTDCI. Our findings imply the importance of the interplay between the hydrophobic and electrostatic interactions between the DTDCI cations and the PMA anions in nonionic micelles of varying hydrophilic-lipophilic balance (HLB). Dynamic light scattering (DLS) data suggest a modulation by PMA in the intermicellar electrostatic repulsions of a hydrophilic copolymer micelle, whereas its unaffected in a hydrophobic copolymeric micelle.

Do Specific Ion Effects on Collective Relaxation Arise from Perturbation of Hydrogen-Bonding Network Structure?

The change in the transport properties (i.e., water diffusivity, shear viscosity, etc.) when adding salts to water has been used to classify ions as either being chaotropic or kosmotropic, a terminology based on the presumption that this phenomenon arises from respective breakdown or enhancement of the hydrogen-bonding network structure. Recent quasi-elastic neutron scattering measurements of the collective structural relaxation time, τC, in aqueous salt solutions were interpreted as confirming this proposed origin of ion effects on the dynamics of water. However, we find similar changes in τC in the same salt solutions based on molecular dynamics (MD) simulations using a coarse-grained water model in which no hydrogen bonding exists, challenging this conventional interpretation of mobility change resulting from the addition of salts to water. A thorough understanding of specific ion effects should be useful in diverse material manufacturing and biomedical applications, where these effects are prevalent, but poorly understood.

Unravelling Competitive Interactions between Polymer Side Chains and End Groups with β-Cyclodextrin

This study investigates unexpected competitive host-guest interactions of β-cyclodextrin (β-CD), which can occur with polymers in aqueous solution, using the examples of the two polymers poly(oligo(ethylene glycol) methyl ether methacrylate) and poly(glycerol mono methacrylate). Systematic structural modifications of the polymer provide insight into the host-guest interaction with β-CD and the competition between side chains and end groups such as hydrophobic end groups remaining from reversible addition fragmentation chain transfer polymerization or intentionally implemented molecular recognition units such as arylazopyrazole photoswitches.

Greasy Cations Bind to Neutral Macromolecules in Aqueous Solution

Ions influence the solution properties of macromolecules. Although much is known about anions, cationic effects are considered mostly in terms of weak interactions or exclusion from neutral interfaces. Herein, we have systematically studied the effect of quaternary tetraalkylammonium cations (NH4+, NMe4+, NEt4+, NPr4+, NBu4+) on the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in aqueous solution. Solubility measurements were coupled to 1H NMR and ATR-FTIR spectroscopic measurements. The solubility and NMR measurements revealed a direct binding between the greasiest cations and the isopropyl group of the macromolecule, evidenced from the nonlinear, Langmuir-type chemical shift response only at the isopropyl NMR signals with increasing salt concentrations. The ATR-FTIR measurements focusing on the amide oxygen showed that it is not the main direct-binding site. Additionally, the salting-out effects of the greasier cations correlate with their hydration entropies. These results demonstrate that the most weakly hydrated cations can bind to macromolecules as strongly as the weakly hydrated Hofmeister anions.

Metallacarboranes in Medicinal Chemistry: Current Advances and Future Perspectives

Metallacarboranes, exemplified by cobalt bis(dicarbollide) ([COSAN]-), have excelled their historical metallocene analogue label to become promising in drug design, medical studies, and fundamental biological research. Serving as a unique platform for conjugation with biomolecules, they also constitute an auspicious building block for biologically active derivatives and a carrier for cellular transport of membrane-impermeable cargos. Modified [COSAN]- exhibits specific antimicrobial, antiviral, and anticancer actions showing promise for preclinical trials. Contributing to the ongoing development in medicinal chemistry, metallacarboranes offer desirable physicochemical properties and low acute toxicity. This article presents a critical look at metallacarboranes in the context of their application in medicinal chemistry, emphasizing [COSAN]- as a potential game-changer in drug design and biomedical sciences. As medicinal chemistry seeks innovative building blocks, metallacarboranes emerge as an important novelty with versatile solutions and promising implications.

Role of Metal-Organic Framework Topology on Thermodynamics of Polyoxometalate Encapsulation

Polyoxometalates (POMs) are discrete anionic clusters whose rich redox properties, strong Bro̷nsted acidity, and high availability of active sites make them potent catalysts for oxidation reactions. Metal-organic frameworks (MOFs) have emerged as tunable, porous platforms to immobilize POMs, thus increasing their solution stability and catalytic activity. While POM@MOF composite materials have been widely used for a variety of applications, little is known about the thermodynamics of the encapsulation process. Here, we utilize an up-and-coming technique in the field of heterogeneous materials, isothermal titration calorimetry (ITC), to obtain full thermodynamic profiles (ΔH, ΔS, ΔG, and Ka) of POM binding. Six different 8-connected hexanuclear Zr-MOFs were investigated to determine the impact of MOF topology (csq, scu, and the) on POM encapsulation thermodynamics.

Boron Cluster Renders Organic Radicals Water-Stable for Photothermal Anti-Infections

Water-stable organic radicals are promising photothermal conversion candidates for photothermal therapy (PTT). However, organic radicals are usually unstable in biological environments, which greatly hinders their wide application. Here, we have developed a chaotropic effect-based and photoinduced water-stable supramolecular radical (MB12-2) for efficient antibacterial PTT. The supramolecular radical precursor MB12-1 was constructed by the chaotropic effect between closo-dodecaborate cluster (B12H122-) and N,N'-dimethylated dipyridinium thiazolo [5,4-d] thiazole (MPT2+). Subsequently, with triethanolamine (TEOA) serving as an electron donor, MB12-1 could transform to its radical form MB12-2 through photoinduced electron transfer (PET) under 435-nm laser irradiation. The N2 adsorption-desorption analysis confirmed that MB12-2 was tightly packed through the introduction of B12H122-, which effectively enhanced its stability via a spatial site-blocked effect. Moreover, the half-life of MB12-2 in water was calculated through ultraviolet-visible light (UV-vis) absorption spectra results for periods as long as 20 days. In addition, in the skin infection model, MB12-2, as a wound dressing, showed remarkable photothermal antibacterial activity (>97%) under 660-nm laser irradiation and promoted wound healing. This study presents a simple method for designing long-term water-stable supramolecular radicals, offering a novel avenue for noncontact treatments for bacterial infections.

Assessing Weak Anion Binding to Small Peptides

Since Hofmeister's seminal studies in the late 19th century, it has been known that salts and buffers can drastically affect the properties of peptides and proteins. These Hofmeister effects can be conceived of in terms of three distinct phenomena/mechanisms: water-salt interactions that indirectly induce the salting-out of a protein by water sequestration by the salt, and direct salt-protein interactions that can either salt-in or salt-out the protein. Unfortunately, direct salt-protein interactions responsible for Hofmeister effects are weak and difficult to quantify. As such, they are frequently construed of as being nonspecific. Nevertheless, there has been considerable effort to better specify these interactions. Here, we use pentapeptides to demonstrate the utility of the H-dimension of nuclear magnetic resonance (NMR) spectroscopy to assess anion binding using N-H signal shifts. We qualify binding using these, demonstrating the upfield shifts induced by anion association and revealing how they are much larger than the corresponding downfield shifts induced by magnetic susceptibility and other ionic strength change effects. We also qualify binding in terms of how the pattern of signal shifts changes with point mutations. In general, we find that the observed upfield shifts are small compared with those induced by anion binding to amide-based hosts, and MD simulations suggest that this is so. Thus, charge-diffuse anions associate mostly with the nonpolar regions of the peptide rather than directly interacting with the amide N-H groups. These findings reveal the utility of 1H NMR spectroscopy for qualifying affinity to peptides─even when affinity constants are very low─and serve as a benchmark for using NMR spectroscopy to study anion binding to more complex systems.

Chaotropic Effect-Induced Sol-Gel Transition and Radical Stabilization for Bacterially Sensitive Near-Infrared Photothermal Therapy

Deep-seated bacterial infections (DBIs) are stubborn and deeply penetrate tissues. Eliminating deep-seated bacteria and promoting tissue regeneration remain great challenges. Here, a novel radical-containing hydrogel (SFT-B Gel) cross-linked by a chaotropic effect was designed for the sensing of DBIs and near-infrared photothermal therapy (NIR-II PTT). A silk fibroin solution stained with 4,4',4″-(1,3,5-triazine-2,4,6-triyl)tris(1-methylpyridin-1-ium) (TPT3+) was employed as the backbone, which could be cross-linked by a closo-dodecaborate cluster (B12H122-) through a chaotropic effect to form the SFT-B Gel. More interestingly, the SFT-B Gel exhibited the ability to sense DBIs, which could generate a TPT2+• radical with obvious color changes in the presence of bacteria. The radical-containing SFT-B Gel (SFT-B★ Gel) possessed strong NIR-II absorption and a remarkable photothermal effect, thus demonstrating excellent NIR-II PTT antibacterial activity for the treatment of DBIs. This work provides a new approach for the construction of intelligent hydrogels with unique properties using a chaotropic effect.

Dilute polyelectrolyte solutions: recent progress and open questions

Polyelectrolytes are a class of polymers possessing ionic groups on their repeating units. Since counterions can dissociate from the polymer backbone, polyelectrolyte chains are strongly influenced by electrostatic interactions. As a result, the physical properties of polyelectrolyte solutions are significantly different from those of electrically neutral polymers. The aim of this article is to highlight key results and some outstanding questions in the polyelectrolyte research from recent literature. We focus on the influence of electrostatics on conformational and hydrodynamic properties of polyelectrolyte chains. A compilation of experimental results from the literature reveals significant disparities with theoretical predictions. We also discuss a new class of polyelectrolytes called poly(ionic liquid)s that exhibit unique physical properties in comparison to ordinary polyelectrolytes. We conclude this review by listing some key research challenges in order to fully understand the conformation and dynamics of polyelectrolytes in solutions.

The Rise of Boron-Containing Compounds: Advancements in Synthesis, Medicinal Chemistry, and Emerging Pharmacology

Boron-containing compounds (BCC) have emerged as important pharmacophores. To date, five BCC drugs (including boronic acids and boroles) have been approved by the FDA for the treatment of cancer, infections, and atopic dermatitis, while some natural BCC are included in dietary supplements. Boron's Lewis acidity facilitates a mechanism of action via formation of reversible covalent bonds within the active site of target proteins. Boron has also been employed in the development of fluorophores, such as BODIPY for imaging, and in carboranes that are potential neutron capture therapy agents as well as novel agents in diagnostics and therapy. The utility of natural and synthetic BCC has become multifaceted, and the breadth of their applications continues to expand. This review covers the many uses and targets of boron in medicinal chemistry.

Observation of a super-tetrahedral cluster of acetonitrile-solvated dodecaborate dianion via dihydrogen bonding

We launched a combined negative ion photoelectron spectroscopy and multiscale theoretical investigation on the geometric and electronic structures of a series of acetonitrile-solvated dodecaborate clusters, i.e., B12H122-·nCH3CN (n = 1-4). The electron binding energies of B12H122-·nCH3CN are observed to increase with cluster size, suggesting their enhanced electronic stability. B3LYP-D3(BJ)/ma-def2-TZVP geometry optimizations indicate each acetonitrile molecule binds to B12H122- via a threefold dihydrogen bond (DHB) B3-H3 ⁝⁝⁝ H3C-CN unit, in which three adjacent nucleophilic H atoms in B12H122- interact with the three methyl hydrogens of acetonitrile. The structural evolution from n = 1 to 4 can be rationalized by the surface charge redistributions through the restrained electrostatic potential analysis. Notably, a super-tetrahedral cluster of B12H122- solvated by four acetonitrile molecules with 12 DHBs is observed. The post-Hartree-Fock domain-based local pair natural orbital- coupled cluster singles, doubles, and perturbative triples [DLPNO-CCSD(T)] calculated vertical detachment energies agree well with the experimental measurements, confirming the identified isomers as the most stable ones. Furthermore, the nature and strength of the intermolecular interactions between B12H122- and CH3CN are revealed by the quantum theory of atoms-in-molecules and the energy decomposition analysis. Ab initio molecular dynamics simulations are conducted at various temperatures to reveal the great kinetic and thermodynamic stabilities of the selected B12H122-·CH3CN cluster. The binding motif in B12H122-·CH3CN is largely retained for the whole halogenated series B12X122-·CH3CN (X = F-I). This study provides a molecular-level understanding of structural evolution for acetonitrile-solvated dodecaborate clusters and a fresh view by examining acetonitrile as a real hydrogen bond (HB) donor to form strong HB interactions.

Polyoxometalate-Organic Hybrid "Calixarenes" as Supramolecular Hosts

Calixarenes are among the most useful and versatile macrocycles in supramolecular chemistry. The one thing that has not changed in the 80 years since their discovery, despite numerous derivatizations, is their fully organic, covalent scaffolds. Here, we report a new type of organic-inorganic hybrid "calixarenes" constructed by means of coordination-driven assembly. Replacing acetate ligands on the {SiW10 Cr2 (OAc)2 } clusters with 5-hydroxyisophthalates allows these 95° inorganic building blocks to be linked into bowl-shaped, hybrid "calix[n]arenes" (n=3, 4). With a large concave cavity, the metal-organic calix[4]arene can accommodate nanometer-sized polyoxoanions in an entropically driven process. The development of hybrid variants of calixarenes is expected to expand the scope of their physicochemical properties, guest/substrate binding, and applications on multiple fronts.

Cooperative Self-Assembly Process Involving Giant Toroidal Polyoxometalate as a Membrane Building Block in Nanoscale Vesicles

The self-assembly of organic amphiphilic species into various aggregates such as spherical or elongated micelles and cylinders up to the formation of lyotropic hexagonal or lamellar phases results from cooperative processes orchestrated by the hydrophobic effect, while those involving ionic inorganic polynuclear entities and nonionic organic components are still intriguing. Herein, we report on the supramolecular behavior of giant toroidal molybdenum blue-type polyoxometalate, namely, the {Mo154} species in the presence of n-octyl-β-glucoside (C8G1), widely used as a surfactant in biochemistry. Structural investigations were carried out using a set of complementary multiscale methods including single-crystal X-ray diffraction analysis supported by molecular modeling, small-angle X-ray scattering and cryo-TEM observations. In addition, liquid NMR, viscosimetry, surface tension measurement, and isothermal titration calorimetry provided further information to decipher the complex aggregation pathway. Elucidation of the assembly process reveals a rich scenario where the presence of the large {Mo154} anion disrupts the self-assembly of the C8G1, well-known to produce micelles, and induces striking successive phase transitions from fluid-to-gel and from gel-to-fluid. Herein, intimate organic-inorganic primary interactions arising from the superchaotropic nature of the {Mo154} lead to versatile nanoscopic hybrid C8G1-{Mo154} aggregates including crystalline discrete assemblies, smectic lamellar liquid crystals, and large uni- or multilamellar vesicles where the large torus {Mo154} acts a trans-membrane component.

All-Inorganic Polyoxometalates Act as Superchaotropic Membrane Carriers

Polyoxometalates (POMs) are known antitumoral, antibacterial, antiviral, and anticancer agents and considered as next-generation metallodrugs. Herein, a new biological functionality in neutral physiological media, where selected mixed-metal POMs are sufficiently stable and able to affect membrane transport of impermeable, hydrophilic, and cationic peptides (heptaarginine, heptalysine, protamine, and polyarginine) is reported. The uptake is observed in both, model membranes as well as cells, and attributed to the superchaotropic properties of the polyoxoanions. In view of the structural diversity of POMs these findings pave the way toward their biomedical application in drug delivery or for cell-biological uptake studies with biological effector molecules or staining agents.

Applications of Boron Cluster Supramolecular Frameworks as Metal-Free Chemodynamic Therapy Agents for Melanoma

Chemodynamic therapy (CDT) is a highly targeted approach to treat cancer since it converts hydrogen peroxide into harmful hydroxyl radicals (OH·) through Fenton or Fenton-like reactions. However, the systemic toxicity of metal-based CDT agents has limited their clinical applications. Herein, a metal-free CDT agent: 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT)/ [closo-B12 H12 ]2- (TPT@ B12 H12 ) is reported. Compared to the traditional metal-based CDT agents, TPT@B12 H12 is free of metal avoiding cumulative toxicity during long-term therapy. Density functional theory (DFT) calculation revealed that TPT@B12 H12 decreased the activation barrier more than 3.5 times being a more effective catalyst than the Fe2+ ion (the Fenton reaction), which decreases the barrier about twice. Mechanismly, the theory calculation indicated that both [B12 H12 ]-· and [TPT-H]2+ have the capacity to decompose hydrogen into 1 O2 , OH·, and O2 -· . With electron paramagnetic resonance and fluorescent probes, it is confirmed that TPT@B12 H12 increases the levels of 1 O2 , OH·, and O2 -· . More importantly, TPT@B12 H12 effectively suppress the melanoma growth both in vitro and in vivo through 1 O2 , OH·, and O2 -· generation. This study specifically highlights the great clinical translational potential of TPT@B12 H12 as a CDT reagent.

Switchable Redox and Thermo-Responsive Supramolecular Polymers Based on Cyclodextrin-Polyoxometalate Tandem

Supramolecular polymers built from stimuli-responsive host-guest interactions represent an attractive way of tailoring smart materials. Herein, we exploit the chaotropic effect of polyoxometalates and related host-guest properties to design unconventional polymer systems with reversible redox and thermo-responsive sol-gel transition. These supramolecular networks result from the association of cyclodextrin-based oligomers and Keggin-type POMs acting as electro-active crosslinking agents. The structure and the dynamics of such self-assembly systems have been investigated using a multiscale approach involving MALDI-TOF, viscosity measurements, cyclic voltammetry, 1 H-NMR (1D and DOSY), and Small-Angle X-ray Scattering. Our results reveal that the chaotropic effect corresponds to a powerful and efficient force that can be used to induce responsiveness in hybrid supramolecular oligomeric systems.

Chaotropic Effect-Induced Self-Assembly of the Malachite Green and Boron Cluster for Toxicity Regulation and Photothermal Therapy

Malachite green (MG), a toxic antibacterial agent, is widely used in the farming industry. Effectively regulating the biotoxicity of this highly water-soluble cationic dye is challenging. Here, we present a novel strategy to reduce the biotoxicity of MG through the self-assembly of MG and the closo-dodecaborate cluster ([B12H12]2-) driven by the chaotropic effect. [B12H12]2- and MG in an aqueous solution can rapidly form an insoluble cubic-type supramolecular complex (B12-MG), and the original toxicity of MG is completely suppressed. Surprisingly, this supramolecular complex, B12-MG, has a strong UV-vis absorption peak at 600-800 nm and significant photothermal conversion efficiency under 660 nm laser irradiation. On this basis, B12-MG, the supramolecular complex, can be used as an efficient photothermal agent for antimicrobial photothermal therapy (PTT) both in vitro and in vivo. As a molecular chaperone of MG, [B12H12]2- not only can be applied as an antidote to regulate the biotoxicity of MG but also provides a novel method for the construction of photothermal agents for PTT based on the chaotropic effect.

Dispersion Interactions in Condensed Phases and inside Molecular Containers

ConspectusThe past decade has seen significant progress in the understanding and appreciation of the importance of London dispersion interactions (LDIs) in supramolecular systems and solutions. The Slater-Kirkwood formula relates LDIs to the molecular polarizabilities of the two interacting molecular species (α) and their interaction distance (a dependence of R-6). When advancing arguments related to intermolecular interactions, it is frequently assumed that molecules with larger molecular polarizabilities are more amenable to larger LDIs. However, arguments related to molecular polarizabilities are not always transferable to the condensed phase. In fact, the underlying bulk and molecular polarizabilities of common solvents show opposing trends. The intuitive concept that aromatic molecules are more polarizable than saturated hydrocarbons and that perfluorinated molecules are less polarizable than saturated hydrocarbons applies to the condensed phase only. When treating association phenomena in solution, where LDIs are generally very attenuated, the use of bulk polarizabilities is recommended, which are experimentally accessible through either refractive index measurements or suitable solvatochromic probes. Such probes can also be used to assess polarizabilities inside molecular container compounds, such as cucurbit[n]urils (CBn), cyclodextrins, calixarenes, and hemicarcerands. These macrocyclic cavities can have extreme microenvironments. For example, the inner concave phase of CB7 has been shown to be weakly polarizable, falling in between the gas phase and perfluorohexane; those of β-cyclodextrin and p-sulfonatocalix[4]arene have been found to be similarly polarizable as water and alkanes, respectively, and the inside of hemicarcerands displays a very large bulk polarizability, exceeding that of diiodomethane. CBn compounds are privileged molecular container compounds, which we exemplify in this Account through case studies. (1) CBn macrocycles are prime water-soluble receptors for hydrocarbons, allowing for the reduction of the binding free energies to two components: the hydrophobic effect and dispersion interactions. To understand hydrocarbon binding, we initiated the HYDROPHOBE challenge, which revealed the shortcomings of both quantum-chemical and molecular dynamics approaches. (2) The smallest CBn receptor, CB5, is uniquely suited to bind the entire noble gas series, where hydrophobic effects and dispersion interactions operate in opposite directions. CB5 was revaled to be a unique synthetic receptor for noble gases, with the dominant driving force being the recovery of the cavitation energies for the hydration of noble gases in aqueous solution. Computational methods that encounter challenges in predicting hydrocarbon affinities and trends for CB6 and CB7 perform well for noble gases binding to CB5. (3) The larger homologue, CB8, allows one to set up intermolecular interaction chambers by the encapsulation of a (first) aromatic guest, thereby tuning LDIs inside the receptor cavity. In this manner, CB8 can be modulated to preferentially bind unsaturated and aromatic rather than saturated hydrocarbons, while the unmodified cavities of the smaller macrocycles CB6 and CB7 show selective binding of saturated hydrocarbons. (4) The (charged) host-guest complexes of CBn hosts are sufficiently stable in the gas phase, allowing for the study of the influence of LDIs on inner-phase chemical reactions. These studies are particularly interesting for the theoretical analysis of isolated host-guest LDIs, as experimental and computational data are directly comparable in the gas phase due to the absence of the solvation effect.

Stabilization of Guest Molecules inside Cation-Lidded Cucurbiturils Reveals that Hydration of Receptor Sites Can Impede Binding

Docking of alkali metal ions to water-soluble macrocyclic receptors generally reduces the affinity of guest molecules due to competitive binding. The idea that solvation water molecules could display a larger steric hindrance towards guest binding than cations has not been considered to date. We show that the docking of large cations to cucurbit[5]uril (CB5) unexpectedly increases (by a factor of 5-8) the binding of hydrophobic guests, methane and ethane. This is due to the removal of water molecules from the carbonyl portals of CB5 during cation binding, which frees up space for hydrophobe encapsulation. In contrast, smaller cations like sodium protrude deeply into the cavity of CB5 and cause the expected decrease in binding, such that the rational selection of alkali cations allows for a variation of up to a factor of 20 in binding of methane and ethane. The statistical analysis of crystallographic data shows that the cavity volume of CB5 can be enlarged by placing large alkali ions (Rb+ and Cs+ ) centro-symmetrically at the portals. The results reveal a hitherto elusive steric hindrance of solvation water molecules near receptor binding sites, which is pertinent for the design of supramolecular catalysts and the understanding of biological receptors.

Functionalization of Dodecaborates by Mild and Efficient Pd-Catalyzed Formation of B-C Bonds with Boronic Acids

Hybrid organic-inorganic molecules have recently received great interest due to their unique properties, which give access to their implementation in biological and material sciences. Herein, a new synthetic approach for the direct-linkage of the purely inorganic dodecaborate cluster to organic building blocks through B-C bond is established, using boronic acids as functional groups on the organic moiety, reacting under Suzuki-Miyaura coupling conditions with iodo-undecahydridododecaborate. The choices of ligand (DavePhos) and solvent (N-methylpyrrolidone for electron-poor, CD3 CN for electron-rich groups) are essential for the successful coupling. Ultimately, the newly described methodology is found to be functional-group tolerant covering a wide spectrum of substrates including electron-poor arenes.

Surprising Nanomechanical and Conformational Transition of Neutral Polyacrylamide in Monovalent Saline Solutions

Polyacrylamide (PAM) is one of the most important water-soluble polymers that has been extensively applied in water treatment, drug delivery, and flexible electronic devices. The basic properties, e.g., microstructure, nanomechanics, and solubility, are deeply involved in the performance of PAM materials. Current research has paid more attention to the development and expansion of the macroscopic properties of PAM materials, and the study of the mechanism involved with the roles of water and ions on the properties of PAM is insufficient, especially for the behaviors of neutral amide side groups. In this study, single molecule force spectroscopy was combined with molecular dynamic (MD) simulations, atomic force microscope imaging, and dynamic light scattering to investigate the effects of monovalent ions on the nanomechanics and molecular conformations of neutral PAM (NPAM). These results show that the single-molecule elasticity and conformation of NPAM exhibit huge variation in different monovalent salt solutions. NPAM adopts an extended conformation in aqueous solutions of strong hydrated ion (acetate), while transforms into a collapse globule in the existence of weakly hydrated ion (SCN-). It is believed that the competition between intramolecular and intermolecular weak interactions plays a key role to adjust the molecular conformation and elasticity of NPAM. The competition can be largely influenced by the type of monovalent ions through hydration or a chaotropic effect. Methods utilized in this study provide a means to better understand the Hofmeister effect of ions on other macromolecules containing amide groups at the single-molecule level.

Structural Patterns Enhancing the Antibacterial Activity of Metallacarborane-Based Antibiotics

Healthcare systems heavily rely on antibiotics to treat bacterial infections, but the widespread presence of multidrug-resistant bacteria puts this strategy in danger. Novel drugs capable of overcoming current resistances are needed if our ability to treat bacterial infections is to be maintained. Boron clusters offer a valuable possibility to create a new class of antibiotics and expand the chemical space of antibiotics beyond conventional carbon-based molecules. In this work, we identified two promising structural patterns providing cobalta bis(dicarbollide)(COSAN)-based compounds with potent and selective activity toward Staphylococcus aureus (including clinical strains): introduction of the α-amino acid amide and addition of iodine directly to the metallacarborane cage. Furthermore, we found that proper hydrophilic-lipophilic balance is crucial for the selective activity of the tested compounds toward S. aureus over mammalian cells. The patterns proposed in this paper can be useful in the development of metallacarborane-based antibiotics with potent antibacterial properties and low cytotoxicity.

Solvation of Nanoions in Aqueous Solutions

In recent years it has been increasingly recognized that different classes of large ions with multiple valency have effects conceptually similar to weakly solvated ions in the Hofmeister series, also labeled by the term chaotropic. The term "superchaotropic effect" has been coined because these effects are much more strongly pronounced for nanometer-sized ions, whose adsorption properties often resemble typical surfactants. Despite this growing interest in these nanometer-sized ions, a simple conceptual extension of the Hofmeister series toward nanoions has not been achieved because an extrapolation of the one-dimensional surface charge density scale does not lead to the superchaotropic regime. In this work, we discuss a generic model that is broadly applicable to ions of nearly spherical shape and thus includes polyoxometalates and boron clusters. We present a qualitative classification scheme in which the ion size appears as a second dimension. Ions of different sizes but the same charge density differ in their bulk solvation free energy. As the ions grow bigger at constant surface charge density, they become more stable in solution, but the adsorption behavior is still governed by the surface charge density. A detailed molecular dynamics simulation study of large ions that is based on a shifted Lennard-Jones potential is presented that supports the presented classification scheme.

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.

Single─Not Double─3D-Aromaticity in an Oxidized Closo Icosahedral Dodecaiodo-Dodecaborate Cluster

3D-aromatic molecules with (distorted) tetrahedral, octahedral, or spherical structures are much less common than typical 2D-aromatic species or even 2D-aromatic-in-3D systems. Closo boranes, [BnHn]2- (5 ≤ n ≤ 14) and carboranes are examples of compounds that are singly 3D-aromatic, and we now explore if there are species that are doubly 3D-aromatic. The most widely known example of a species with double 2D-aromaticity is the hexaiodobenzene dication, [C6I6]2+. This species shows π-aromaticity in the benzene ring and σ-aromaticity in the outer ring formed by the iodine substituents. Inspired by the hexaiodobenzene dication example, in this work, we explore the potential for double 3D-aromaticity in [B12I12]0/2+. Our results based on magnetic and electronic descriptors of aromaticity together with 11B{1H} NMR experimental spectra of boron-iodinated o-carboranes suggest that these two oxidized forms of a closo icosahedral dodecaiodo-dodecaborate cluster, [B12I12] and [B12I12]2+, behave as doubly 3D-aromatic compounds. However, an evaluation of the energetic contribution of the potential double 3D-aromaticity through homodesmotic reactions shows that delocalization in the I12 shell, in contrast to the 10σ-electron I62+ ring in the hexaiodobenzene dication, does not contribute to any stabilization of the system. Therefore, the [B12I12]0/2+ species cannot be considered as doubly 3D-aromatic.

Chemistry of Carbon-Substituted Derivatives of Cobalt Bis(dicarbollide)(1-) Ion and Recent Progress in Boron Substitution

The cobalt bis(dicarbollide)(1-) anion (1-), [(1,2-C2B9H11)2-3,3'-Co(III)](1-), plays an increasingly important role in material science and medicine due to its high chemical stability, 3D shape, aromaticity, diamagnetic character, ability to penetrate cells, and low cytotoxicity. A key factor enabling the incorporation of this ion into larger organic molecules, biomolecules, and materials, as well as its capacity for "tuning" interactions with therapeutic targets, is the availability of synthetic routes that enable easy modifications with a wide selection of functional groups. Regarding the modification of the dicarbollide cage, syntheses leading to substitutions on boron atoms are better established. These methods primarily involve ring cleavage of the ether rings in species containing an oxonium oxygen atom connected to the B(8) site. These pathways are accessible with a broad range of nucleophiles. In contrast, the chemistry on carbon vertices has remained less elaborated over the previous decades due to a lack of reliable methods that permit direct and straightforward cage modifications. In this review, we present a survey of methods based on metalation reactions on the acidic C-H vertices, followed by reactions with electrophiles, which have gained importance in only the last decade. These methods now represent the primary trends in the modifications of cage carbon atoms. We discuss the scope of currently available approaches, along with the stereochemistry of reactions, chirality of some products, available types of functional groups, and their applications in designing unconventional drugs. This content is complemented with a report of the progress in physicochemical and biological studies on the parent cobalt bis(dicarbollide) ion and also includes an overview of recent syntheses and emerging applications of boron-substituted compounds.

Exploring the impact of sodium salts on hydrotropic solubilization

Some ionic liquids (ILs) were shown to display a strong ability to enhance the solubility of phenolic compounds through hydrotropy. However, evidence shows that salt ions in hydrotropic aqueous solutions may change the behavior of molecules by promoting possible interactions between the components of the system, thus causing changes in solubility. Herein, we study the impact of sodium salt anions on the hydrotropic dissolution of syringic acid using 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) as a hydrotrope, with a focus on dicyanamide Na[N(CN)2] and thiocyanate Na[SCN] salts. Dynamic light scattering, Raman spectroscopy, and nuclear magnetic resonance spectroscopy were used to investigate how the mixture of IL-salts affects the solvation. The results obtained show that [C4mim]Cl is able to increase the solubility of syringic acid 80-fold. Despite their structural similarities, the presence of Na[N(CN)2] or Na[SCN] in an aqueous solution of [C4mim]Cl induced opposite solubility trends. The addition of Na[N(CN)2] promotes a higher ability to solubilize syringic acid than in the corresponding IL system due to a pH buffering effect, resulting in the deprotonation of the solute. The addition of Na[SCN], on the other hand, induces a relative decrease in syringic acid solubilization at higher concentrations of ILs due to the negative contribution of the NaCl formed by anion-exchange. These results emphasise the often overlooked pH contribution provided by ILs for biomolecule solubilisation whilst providing experimental insights into the structure of aqueous solutions of ionic liquids and the role it plays in the formation of IL-salt aggregates.

NMR spectroscopy to study cyclodextrin-based host-guest assemblies with polynuclear clusters

Natural cyclodextrin (CD) macrocycles are known to form diverse inclusion complexes with a wide variety of organic molecules, but recent work has revealed that inorganic clusters also form multicomponent supramolecular complexes and edifices. Such molecular assemblies exhibit a high degree of organization in solution governed by various chemical processes including molecular recognition, host-guest attraction, hydrophobic repulsion, or chaotropic effect. Nuclear magnetic resonance (NMR) spectroscopy is one of the most efficient and practical analytical techniques to characterize the nature, the strength and the mechanism of these interactions in solution. This review provides a brief overview on recent examples of the contribution of NMR to the characterization of hybrid systems in solution based on CD with polynuclear clusters, including polyoxometalates (POMs), metallic clusters and hydroborate clusters. The focus will be first on using 1H (and 13C) NMR of the host, i.e., CD, to identify the nature of the interactions and measure their strength. Then, 2D NMR methods will be illustrated by DOSY as a means of highlighting the clustering phenomena, and by NOESY/ROESY to evidence the spatial proximity and contact within the supramolecular assemblies. Finally, other NMR nuclei will be selected to probe the inorganic part as a guest molecule. Attention will be paid to classical host-guest complexes Cluster@CD, but also to hierarchical multi-scale, multi-component assemblies such as Cluster@CD@Cluster.

Boron Clusters Alter the Membrane Permeability of Dicationic Fluorescent DNA-Staining Dyes

Membrane-permeable fluorescent dyes that stain DNA are useful reagents for microscopic imaging, as they can be introduced into living cells to label DNA. However, the number of these dyes, such as Hoechst 33342, is limited. Here, we show that the icosahedral dodecaborate B12Br122-, a superchaotropic carrier that delivers different types of molecules into cells, functions as an excellent carrier for membrane-impermeable fluorescent dyes. Propidium iodide (PI) and 4',6-diamidino-2-phenylindole (DAPI), dicationic membrane-impermeable fluorescent dyes that stain DNA, can permeate cell membranes in the presence of boron clusters. Methyl green (MG), a dicationic dye used in the histological and fluorescent staining of DNA, permeated cell membranes in the presence of boron clusters. In contrast, monocationic membrane-permeable fluorescent dyes, such as acridine orange and pyronin Y, exhibited reduced fluorescence in cells in the presence of boron clusters. Boron clusters do not quench dicationic fluorescent dyes in water in vitro but have quenching effects on monocationic fluorescent dyes. We have demonstrated that the addition of B12Br122- to impermeable dicationic fluorescent DNA-staining dyes, such as DAPI, PI, and MG, which have been widely used for numerous years, imparts membrane permeability to introduce these dyes into living cells.

Incorporating Chaotropic/Kosmotropic Chemistries onto Plasmonic Nanoheater to Boost Steam Generation Beyond its Photothermal Property

Photothermal steam generation promises decentralized water purification, but current methods suffer from slow water evaporation even at high photothermal efficiency of ≈98%. This drawback arises from the high latent heat of vaporization that is required to overcome the strong and extensive hydrogen bonding network in water for steam generation. Here, light-to-vapor conversion is boosted by incorporating chaotropic/kosmotropic chemistries onto plasmonic nanoheater to manipulate water intermolecular network at the point-of-heating. The chaotropic-plasmonic nanoheater affords rapid light-to-vapor conversion (2.79 kg m-2  h-1  kW-1 ) at ≈83% efficiency, with the steam generation rate up to 6-fold better than kosmotropic platforms or emerging photothermal designs. Notably, the chaotropic-plasmonic nanoheater also lowers the enthalpy of water vaporization by 1.6-fold when compared to bulk water, signifying that a correspondingly higher amount of steam can be generated with the same energy input. Simulation studies unveil chaotropic surface chemistry is crucial to disrupt water hydrogen bonding network and suppress the energy barrier for water evaporation. Using the chaotropic-plasmonic nanoheater, organic-polluted water is purified at ≈100% efficiency, a feat otherwise challenging in conventional treatments. This study offers a unique chemistry approach to boost light-driven steam generation beyond a material photothermal property.

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.