Ion hydration: Implications for cellular function, polyelectrolytes, and protein crystallization

Systematic characterization of the homogeneous and heterogeneous hydrogen halide dimers

This study systematically characterizes the four homogeneous and six heterogeneous hydrogen-bonded dimers formed by hydrogen halide pairs (HX/HY where X, Y = F, Cl, Br, and I). The notation HX⋯HY indicates the direction of the hydrogen bond from the HY donor to the HX acceptor. All stationary points reported for these ten dimer systems are fully optimized utilizing the MP2 and CCSD(T) ab initio methods in conjunction with quadruple-ζ correlation-consistent basis sets augmented with diffuse functions, and their nature is verified by harmonic vibrational frequency computations. The electronic dissociation energies (De) for all ten global minima are evaluated near the CCSD(T) complete basis set (CBS) limit via extrapolation schemes. These values are 19.11, 8.32, 7.38, and 6.22 kJ mol-1 for the homogeneous dimers of HF, HCl, HBr, and HI, respectively. For the heterogeneous pairs, the lighter hydrogen halide is consistently the donor in the global minimum configuration, with De ranging from 12.23 kJ mol-1 for HCl⋯HF to 7.22 kJ mol-1 for HI⋯HBr near the CCSD(T) CBS limit. Interestingly, not all heterodimer donor/acceptor permutations correspond to minima. For example, the HCl⋯HBr configuration is identified as a local minimum at all levels of theory employed in this investigation, whereas the in-plane barrier for donor/acceptor exchange vanishes for HCl⋯HI and HBr⋯HI when larger quadruple-ζ basis sets are utilized. For the seven dimer systems containing Br and/or I, the structures, energetics, and vibrational frequencies computed using conventional valence-only electron correlation procedures are similar to those obtained using an expanded valence treatment that includes the (n - 1)d subvalence electrons associated with Br and I.

Brittle-to-Ductile Transitions of Polyelectrolyte Complexes: Humidity, Temperature, and Salt

Polyelectrolyte complexation is an entropically driven, associative phase separation that results in a polymer-rich polyelectrolyte complex (PEC) and a polymer-poor supernatant. PECs show promise as a new class of sustainable materials since they can be processed using aqueous solutions rather than organic solvents. Previous reports have looked at the mechanical properties and glass transitions of PECs as a function of temperature, relative humidity (rH), and salt concentration (CS), but establishing a universal understanding of how these parameters affect PEC mechanics has yet to be achieved. We examined the effects of temperature, rH, and CS on the mechanical properties of PECs formed from poly(methacrylic acid) and poly(trimethyl aminoethyl methacrylate) with a goal of establishing design rules for their mechanical response. Relative humidity was shown to have the most dramatic effect on the mechanical properties, with temperature and salt concentration having far less of an impact. Furthermore, we observed that the glass transition of PECs is tied to both temperature and relative humidity, creating a glass transition rHg/T g line that can be modulated by added salt. Finally, we looked at the thermodynamics behind the glass transition of PECs, which yielded similar energies as the condensation of water. We propose the use of water and/or salt as a low energy and efficient method of processing PECs for various applications.

Electrostatics, Hydration, and Chemical Equilibria at Charged Monolayers on Water

The chemistry and physics of soft matter interfaces, especially aqueous-organic interfaces, are centrally important to many areas of science and technology. Often, the thermodynamics, kinetics, and selectivity of reactions are modified at interfaces. Here, we review the electrostatics and hydration at charged monolayers on water and their influence on interfacial chemical equilibria. First, we provide an understanding of interfaces as a conceptual continuation of the solvation shell of small molecules, along with recent relevant experimental work. Then, we provide a summary of models for describing the electrostatics of aqueous interfaces. While we will discuss a range of new developments, our focus will be on systems where the electrostatics of the surface is controllable by the choice of relatively simple insoluble surfactants. New insights into the molecular structure of the double layer, with particular attention on the knowledge gained from spectroscopy will be reviewed. Our approach is to familiarize the reader with simple models, followed by discussion of models with further complexity for explaining interfacial phenomena. Experiments that test the limits of such models will also be discussed. Finally, we will provide an outlook on engineering the interfacial environment for tailored reactivity, along with the anticipated experimental advancements and potentials impacts.

Selective aqueous anion recognition in an anionic host

Water-soluble Fe4L4 4- cages can be synthesized in a multicomponent self-assembly process exploiting functionalized trigonal ligands, FeII salts, and water-soluble sulfonated formylpyridine components. The cages are soluble in purely aqueous solution and display an overall 4- charge, but are capable of binding suitably sized non-coordinating anions in the host cavity despite their anionic nature. Anions such as PF6 - or AsF6 - occupy the internal cavity, whereas anions that are too small (BF4 -) or too large (NTf2 -) are not encapsulated. The external anionic charge and sterically blocked ligand cores limit the exchange rate of bound anions, as no exchange is seen over a period of weeks with the anion-filled cages, and internalization of added PF6 - by an empty cage takes multiple weeks, despite the strong affinity of the cavity for PF6 - ions. In the future, this recognition mechanism could be used to control release of anions for environmental applications.

Chelation-Induced Zwitterion-like Antifouling Behavior on Anionic Poly(3,4-ethylenedioxythiophene) Surfaces

Antifouling properties are crucial for enhancing the longevity and functionality of biomedical implants, drug delivery systems, and biosensors. Zwitterionic polymers are renowned for their exceptional surface hydration and charge neutrality, which effectively resist biomolecular adsorption and protein attachment. We propose an innovative approach to develop zwitterion-like antifouling surfaces by chelating divalent cations with anionic poly(3,4-ethylenedioxythiophene) (PEDOT) films, specifically PEDOT-PO4 and PEDOT-COOH. The chelation behavior of these films was systematically evaluated using Na+, Mg2+, and Ca2+ ions. Divalent ions, particularly Ca2+ and Mg2+, exhibit a strong affinity for the anionic groups, leading to significant antifouling properties. These modified surfaces effectively repelled both negatively charged bovine serum albumin (BSA) and positively charged lysozyme (LYZ) proteins across various pH environments. This study offers valuable insights into the antifouling characteristics of charged surfaces, enhancing our understanding of how ion-mediated surface modifications influence protein adsorption and interactions.

Pronounced Cold Crystallization and Hydrogen Bonding Distortion of Water Confined in Microphases of Double Zwitterionic Block Copolymer Aqueous Solutions

Advanced materials leveraging water control are garnering considerable interest, with the state of water emerging as a critical aspect of material design. This study explored the impact of microphase separation on water using aqueous solutions of double zwitterionic diblock copolymers, specifically poly(carboxybetaine methacrylate) and poly(sulfobetaine methacrylate) (PCB2-b-PSB4). These copolymers form a mesoscale periodic ordered lattice structure consisting of two distinct aqueous phases. Through differential scanning calorimetry and X-ray emission spectroscopy, it was found that water in these PCB2-b-PSB4 aqueous solutions exhibits pronounced cold crystallization and subtle distortions in hydrogen-bonding configurations.

Water-Induced Dissociative Mechanism of Carboxylate and Divalent Calcium Ions Revealed by IR Laser Spectroscopy

The dissociation of carboxylate and divalent calcium ions is investigated at the molecular level in microsolvation experiments by gradually increasing the number of water molecules around the ions. IR photodissociation (IRPD) laser spectroscopy of H2-tagged (Ca2+, AcO-)(H2O)n=8-21 clusters in the ν(CO2-) spectral range combined with RI-B97-D3-BJ-abc/TZVPPD frequency calculations is used to identify the type of ion pairs involved in this process. These results reveal that the ion dissociation follows a multistep mechanism involving in particular pseudobridged monodentate contact ion pairs (CIPs), which are found to be the first intermediate species formed from bidentate CIPs along the ion dissociation path. Altogether, structural assignments suggest a sequence of simple reactions in the first coordination shell of the carboxylate group, leading us to propose two possible dissociation paths. The appearance threshold of monodentate structures is measured at n = 10, with that of solvent-shared ion pairs (SIPs) being potentially at n = 18. By showing in detail how solvation progressively takes over from the ionic interaction in shaping these supramolecular structures, this study can serve as a reference for solving ion-pairing/dissociation problems.

The contrasting roles of co-solvents in protein formulations and food products

Protein aggregation is a major hurdle in developing biopharmaceuticals, in particular protein formulation area, but plays a pivotal role in food products. Co-solvents are used to suppress protein aggregation in pharmaceutical proteins. On the contrary, aggregation is encouraged in the process of food product making. Thus, it is expected that co-solvents play a contrasting role in biopharmaceutical formulation and food products. Here, we show several examples that utilize co-solvents, e.g., salting-out salts, sugars, polyols and divalent cations in promoting protein-protein interactions. The mechanisms of co-solvent effects on protein aggregation and solubility have been studied on aqueous protein solution and applied to develop pharmaceutical formulation based on the acquired scientific knowledge. On the contrary, co-solvents have been used in food industries based on empirical basis. Here, we will review the mechanisms of co-solvent effects on protein-protein interactions that can be applied to both pharmaceutical and food industries and hope to convey knowledge acquired through research on co-solvent interactions in aqueous protein solution and formulation to those involved in food science and provide those involved in protein solution research with the observations on aggregation behavior of food proteins.

Ion-Specific Effects on Ion and Polyelectrolyte Solvation

Ion-specific effects on aqueous solvation of monovalent counter ions, Na+ ${^+ }$ , K+ ${^+ }$ , Cl- ${^- }$ , and Br- ${^- }$ , and two model polyelectrolytes (PEs), poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA) were here studied with ab initio molecular dynamics (AIMD) and classical molecular dynamics (MD) simulations based on the OPLS-aa force-field which is an empirical fixed point-charge force-field. Ion-specific binding to the PE charge groups was also characterized. Both computational methods predict similar response for the solvation of the PEs but differ notably in description of ion solvation. Notably, AIMD captures the experimentally observed differences in Cl- ${^- }$ and Br- ${^- }$ anion solvation and binding with the PEs, while the classical MD simulations fail to differentiate the ion species response. Furthermore, the findings show that combining AIMD with the computationally less costly classical MD simulations allows benefiting from both the increased accuracy and statistics reach.

Sodium lignosulfonate as an extracting agent of methylene blue dye using a polymer-enhanced ultrafiltration technique

The purpose of this research was to evaluate the efficacy of sodium lignosulfonate (LS) as a dye adsorbent in the removal of methylene blue (MB) from water by polymer-enhanced ultrafiltration. Various parameters were evaluated, such as membrane molecular weight cut-off, pH, LS dose, MB concentration, applied pressure, and the effect of interfering ions. The results showed that the use of LS generated a significant increase in MB removal, reaching an elimination of up to 98.0 % with 50.0 mg LS and 100 mg L-1 MB. The maximum MB removal capacity was 21 g g-1 using the enrichment method. In addition, LS was reusable for up to four consecutive cycles of dye removal-elution. The removal test in a simulated liquid industrial waste from the textile industry was also effective, with a MB removal of 97.2 %. These findings indicate that LS is highly effective in removing high concentrations of MB dye, suggesting new prospects for its application in water treatment processes.

Protein Thermodynamic Properties, Crystallisation, and the Hofmeister Series

The Hofmeister series is a series of ions ordered according to their ability to precipitate proteins. It has also been linked to a host of (bio)chemical phenomena. Several attempts over the years to correlate the series to the varying success of different salts in crystallising proteins have been largely inconclusive. A correlation, based on published data and crystallisation conditions for several proteins, is proposed here between some thermodynamic properties of proteins and the position in the Hofmeister series of the salts from which they preferentially crystallise. Namely, a high ratio between the entropic or enthalpic protein-solvent interactions contribution to thermodynamic stability and the total thermodynamic stability of a given protein, indicate the protein's high propensity to crystallise in solutions of highly kosmotropic salts. Low such ratios on the other hand, indicate that chaotropic salts can be equally successful, i. e. that the protein in question is rather indifferent to the Hofmeister character of the salt. Testing various model proteins for crystallisation against screens containing salts found at different points on the Hofmeister series, as well as further bibliographic analysis, have yielded results that appear to largely corroborate this hypothesis. These conclusions may conceivably be used as a crystallisation predictive tool.

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

Nitrate, a prevalent water pollutant, poses substantial public health concerns and environmental risks. Electrochemical reduction of nitrate (eNO3RR) has emerged as an effective alternative to conventional biological treatments. While extensive lab work has focused on designing efficient electrocatalysts, implementation of eNO3RR in practical wastewater settings requires careful consideration of the effects of various constituents in real wastewater. In this critical review, we examine the interference of ionic species commonly encountered in electrocatalytic systems and universally present in wastewater, such as halogen ions, alkali metal cations, and other divalent/trivalent ions (Ca2+, Mg2+, HCO3-/CO32-, SO42-, and PO43-). Notably, we categorize and discuss the interfering mechanisms into four groups: (1) loss of active catalytic sites caused by competitive adsorption and precipitation, (2) electrostatic interactions in the electric double layer (EDL), including ion pairs and the shielding effect, (3) effects on the selectivity of N intermediates and final products (N2 or NH3), and (4) complications by the hydrogen evolution reaction (HER) and localized pH on the cathode surface. Finally, we summarize the competition among different mechanisms and propose future directions for a deeper mechanistic understanding of ionic impacts on eNO3RR.

Probing the Effects of Size and Charge on the Monohydration and Dihydration of SiF5- and SiF62- via Comparisons with BF4- and PF6. J Phys Chem A 2024;128:5637-5645. [PMID: 38976798 DOI: 10.1021/acs.jpca.4c03430] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

This study systematically examines the interactions of the trigonal bipyramidal silicon pentafluoride and octahedral silicon hexafluoride anions with either one or two water molecules, (SiF5-(H2O)n and SiF62-(H2O)n, respectively, where n = 1, 2). Full geometry optimizations and subsequent harmonic vibrational frequency computations are performed using the CCSD(T) ab initio method with a triple-ζ correlation consistent basis set augmented with diffuse functions on all non-hydrogen atoms (cc-pVTZ for H and aug-cc-pVTZ for Si, O, and F; denoted as haTZ). Two monohydrate and six dihydrate minima have been identified for the SiF5-(H2O)n systems, whereas one monohydrate and five dihydrate minima have been identified for the SiF62-(H2O)n systems. Both monohydrated anions have a minimum in which the water molecule adopts a symmetric double ionic hydrogen bond (DIHB) motif with C2v symmetry. However, a second unique monohydrate minimum has been identified for SiF5- in which the water molecule adopts an asymmetric DIHB motif along the edge of the trigonal bipyramidal anion between one axial and one equatorial F atom. This Cs structure is more than 2 kcal mol-1 lower in energy than the C2v local minimum at the CCSD(T)/haTZ level of theory. While the interactions between the solvent and ionic solute are quite strong for the monohydrated anions (electronic dissociation energies of ≈12 and ≈24 kcal mol-1 for the SiF5-(H2O)1 and SiF62-(H2O)1 global minima, respectively), these values are nearly perfectly doubled for the dihydrates, with the lowest-energy SiF5-(H2O)2 and SiF62-(H2O)2 minima exhibiting dissociation energies of ≈24 and ≈47 kcal mol-1, respectively. Structures that form hydrogen bonds between the solvating water molecules also exhibit the largest shifts in the harmonic OH stretching frequencies for the waters of hydration. These shifts can exceed -100 cm-1 for the SiF5-(H2O)2 minimum and -300 cm-1 for the SiF62-(H2O)2 minimum relative to an isolated H2O molecule at the CCSD(T)/haTZ level of theory. This work also corrects the OH stretching frequency shifts for two dihydrate minima of PF6- that were previously erroneously reported ( J. Phys. Chem. A 2020, 124, 8744-8752, DOI: 10.1021/acs.jpca.0c06466).

Antifreezing, Antidrying, and Conductive Hydrogels for Electronic Skin Applications at Ultralow Temperatures

Although conductive hydrogel-based flexible electronic devices have superb flexibility and high conductivities, they tend to malfunction in dry or frigid areas. Herein, an ultralow-temperature tolerant, antidrying, and conductive composite hydrogel is designed for electronic skin applications on the basis of the synergy of double-cross-linked polymer networks, Hofmeister effect, and electrostatic interaction and fabricated by in situ free radical polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid and acrylic acid in the presence of poly(vinyl alcohol) and conductive MXene sheets, followed by impregnation with LiCl. Thanks to the synergy of LiCl and the charged polar terminal groups of the synthesized polymers, the composite hydrogel can not only bear an ultralow temperature of -80 °C without freezing but also maintain its original mass. Meanwhile, the resultant hydrogel possesses satisfactory self-regeneration ability benefiting from the moisturizing effect of LiCl. The conductive network of MXene sheets greatly improves the ionic conductivity of the hydrogel at low temperatures, exhibiting an ionic conductivity of 1.4 S m-1 at -80 °C. Furthermore, the electronic skin assembled by the multifunctional hydrogel is efficient in monitoring human motions at -80 °C. The antifreezing and antidrying features along with favorable ionic conductivity, high tensile strength, and outstanding flexibility make the composite hydrogel promising for applications in frigid and dry regions.

Stepwise hydration of [CH3COOMg]+ studied by cold ion trap infrared spectroscopy: insights into interactions in the magnesium channel selection filters

The magnesium channel controls Mg2+ concentration in the cell and plays an indispensable role in biological functions. The crystal structure of the Magnesium Transport E channel suggested that Mg2+ hydrated by 6 water molecules is transported through a selection filter consisting of COO- groups on two Asp residues. This Mg2+ motion implies successive pairing with -OOC-R and dissociation mediated by water molecules. For another divalent ion, however, it is known that RCOO-⋯Ca2+ cannot be separated even with 12 water molecules. From this discrepancy, we probe the structure of Mg2+(CH3COO-)(H2O)4-17 clusters by measuring the infrared spectra and monitoring the vibrational frequencies of COO- with the help of quantum chemistry calculations. The hydration by (H2O)6 is not enough to induce ion separation, and partially-separated or separated pairs are formed from 10 water molecules at least. These results suggest that the ion separation between Mg2+ and carboxylate ions in the selection-filter of the MgtE channel not only results from water molecules in their first hydration shell, but also from additional factors including water molecules and protein groups in the second solvation shell of Mg2+.

Impact of supporting electrolyte on electrochemical performance of borophene-functionalized graphene sponge anode and degradation of per- and polyfluoroalkyl substances (PFAS)

Graphene sponge anode functionalized with two-dimensional (2D) boron, i.e., borophene, was applied for electrochemical oxidation of C4-C8 per- and polyfluoroalkyl substances (PFASs). Borophene-doped graphene sponge outperformed boron-doped graphene sponge anode in terms of PFASs removal efficiencies and their electrochemical degradation; whereas at the boron-doped graphene sponge anode up to 35% of the removed PFASs was recovered after the current was switched off, the switch to a 2D boron enabled further degradation of the electrosorbed PFASs. Borophene-doped graphene sponge anode achieved 32-77% removal of C4-C8 PFASs in one-pass flow-through mode from a 10 mM phosphate buffer at 230 A m-2 of anodic current density. Higher molarity phosphate buffer (100 mM) resulted in lower PFASs removal efficiencies (11-60%) due to the higher resistance of the graphene sponge electrode in the presence of phosphate ions, as demonstrated by the electrochemical impedance spectroscopy (EIS) analyses. Electro-oxidation of PFASs was more efficient in landfill leachate despite its high organic loading, with up to 95% and 75% removal obtained for perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), versus 77% and 57% removal in the 10 mM phosphate buffer, respectively. Defluorination efficiencies as determined relative to the electrooxidized fraction of PFASs indicated up to 69% and 82% of defluorination of PFOS and PFOA in 10 mM phosphate buffer, which was decreased to 16 and 29% defluorination, respectively, for higher buffer molarity (100 mM) due to the worsened electrochemical performance of the sponge. In landfill leachate, relative defluorination efficiencies of PFOS and PFOA were 33% and 45%, respectively, indicating the inhibiting effect of complex organic and inorganic matrix of landfill leachate on the C-F bond breakage. This study demonstrates that electrochemical degradation of PFASs is possible to achieve in complex and brackish streams using a low-cost graphene sponge anode, without forming toxic chlorinated byproducts even in the presence of >7 g L-1 of chloride.

Light-driven textile sensors with potential application of UV detection

Smart textiles based on monitoring systems of health conditions, structural behaviour, and external environmental conditions have been presented as elegant solutions for the increasing demands of health care. In this study, cotton fabrics (CFs) were modified by a common strategy with a dipping-padding procedure using reduced graphene oxide (RGO) and a photosensitive dye, spiropyran (SP), which can detect environmental UV light. The morphology of the CF is observed by scanning electron microscopy (SEM) measurements showing that the topography structure of coatings is related to the SP content. The resistance of the textile sensors decreases after UV radiation, which may be attributed to the easier electron transmission on the coatings of the CF. With the increase of SP content, the introduction of a large amount of SP within the composites could cause discontinuous distributions of RGO in the fiber surfaces, preventing electron transmission within the coatings of the RGO. The surface wettability of the coatings and the sweat sensitivity are also studied before and after UV radiation.

Tailored Polyelectrolyte Multilayer Systems by Variation of Polyelectrolyte Composition and EDC/NHS Cross-Linking: Physicochemical Characterization and In Vitro Evaluation

The layer-by-layer (LbL) self-assembly technique is an effective method to immobilize components of the extracellular matrix (ECM) such as collagen and heparin onto, e.g., implant surfaces/medical devices with the aim of forming polyelectrolyte multilayers (PEMs). Increasing evidence even suggests that cross-linking influences the physicochemical character of PEM films since mechanical cues inherent to the substrate may be as important as its chemical nature to influence the cellular behavior. In this study, for the first-time different collagen/heparin films have been prepared and cross-linked with EDC/NHS chemistry. Quartz crystal microbalance, zeta potential analyzer, diffuse reflectance Fourier transform infrared spectroscopy, atomic force microscopy and ellipsometry were used to characterize film growth, stiffness, and topography of different film systems. The analysis of all data proves a nearly linear film growth for all PEM systems, the efficacy of cross-linking and the corresponding changes in the film rigidity after cross-linking and an appropriate surface topography. Furthermore, preliminary cell culture experiments illustrated those cellular processes correlate roughly with the quantity of newly created covalent amide bonds. This allows a precise adjustment of the physicochemical properties of the selected film architecture regarding the desired application and target cells. It could be shown that collagen improves the biocompatibility of heparin containing PEMs and due to their ECM-analogue nature both molecules are ideal candidates intended to be used for any biomedical application with a certain preference to improve the performance of bone implants or bone augmentation strategies.

Stepwise dissociation of ion pairs by water molecules: cation-dependent separation mechanisms between carboxylate and alkali-earth metal ions

Microhydrated H₂-tagged ion pairs (Ca²⁺, AcO^-)(H₂O)_n=0-8 and (Ba²⁺, AcO^-)(H₂O)_n=0-5 are investigated by IR photodissociation laser spectroscopy and DFT-D frequency calculations. The detailed picture of the first steps of ion dissociation reveals two mechanisms, where water molecules promote dissociation either directly or indirectly depending on the nature of the cation.

The Ionic Composition and Chemistry of Nanopore-Confined Solutions

There is increasing interest in understanding the properties of solutions confined within nanotubes and synthetic or biological nanopores. How the ionic content of a nanopore-confined solution differs from that of a contacting bulk salt solution is of particular importance, for example, to water desalinization, industrial electrolysis, and all living systems. This paper explores ionic content, ionic interactions, and ion-transport properties of solutions confined within the 10 nm diameter pores of a synthetic polymer membrane. The membrane has a fixed negative pore-wall and surface charge due to ionizable carbonate groups. As a result, under some conditions, the nanopore-confined solution contains only cations and no anions or salt present in a contacting solution, ideal cation permselectivity. This anion- and salt-rejecting ability varies greatly with the cation of the salt, a result that is in contradiction to the prevailing model for permselectivity in nanopores. The extant model fails because it does not account for specific chemical interactions between the cation and the carbonate groups. The nature of these ion-selective interactions is discussed here.

Recognition competes with hydration in anion-triggered monolayer formation of cyanostar supra-amphiphiles at aqueous interfaces

The triggered self-assembly of surfactants into organized layers at aqueous interfaces is important for creating adaptive nanosystems and understanding selective ion extraction. While these transformations require molecular recognition, the underlying driving forces are modified by the local environment in ways that are not well understood. Herein, we investigate the role of ion binding and ion hydration using cyanosurf, which is composed of the cyanostar macrocycle, and its binding to anions that are either size-matched or mis-matched and either weakly or highly hydrated. We utilize the supra-amphiphile concept where anion binding converts cyanosurf into a charged and amphiphilic complex triggering its self-organization into monolayers at the air-water interface. Initially, cyanosurf forms aggregates at the surface of a pure water solution. When the weakly hydrated and size-matched hexafluorophosphate (PF6 -) and perchlorate (ClO4 -) anions are added, the macrocycles form distinct monolayer architectures. Surface-pressure isotherms reveal significant reorganization of the surface-active molecules upon anion binding while infrared reflection absorption spectroscopy show the ion-bound complexes are well ordered at the interface. Vibrational sum frequency generation spectroscopy shows the water molecules in the interfacial region are highly ordered in response to the charged monolayer of cyanosurf complexes. Consistent with the importance of recognition, we find the smaller mis-matched chloride does not trigger the transformation. However, the size-matched phosphate (H2PO4 -) also does not trigger monolayer formation indicating hydration inhibits its interfacial binding. These studies reveal how anion-selective recognition and hydration both control the binding and thus the switching of a responsive molecular interface.

Formation and physicochemical properties of glycogen phosphorylase in complex with a cationic polyelectrolyte

The accumulation of rabbit muscle glycogen phosphorylase b (RMGPb) in electrostatic complexes with the cationic polyelectrolyte poly 2-(dimethylamino) ethyl methacrylate in its quenched form (QPDMAEMA) was studied in two buffer solutions. In the N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES) buffer, large complexes of RMGPb-QPDMAEMA were formed which adopted smaller sizes as QPDMAEMA concentration increased. However, in N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) (HEPES) buffer, the hydrodynamic radius of the formed complexes gradually increased as the polymer concentration increased. Zeta potential measurements (ζ_p) showed that RMGPb significantly changed the ζ_p of the QPDMAEMA aggregates. Fluorescence studies showed that the interaction between RMGPb and QPDMAEAMA was enhanced as polymer concentration increased. Specifically, 8-anilinonaphthalene-1-sulfonic acid (ANS) fluorescence indicated that in the BES buffer the aggregates became denser as more QPDMAEMA was added, while in the HEPES buffer the density of the formed structures decreased. RMGPb's secondary structure was examined by Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) and Circular Dichroism (CD) showing that QPDMAEMA interaction with RMGPb does not induce any changes to the secondary structure of the enzyme. These observations suggest that cationic polyelectrolytes may be utilized for the formulation of RMGPb in multifunctional nanostructures and be further exploited in innovative biotechnology applications and bioinspired materials development.

Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing

Vibrational sum frequency spectroscopy (VSFS) and pressure-area Langmuir trough measurements were used to investigate the binding of alkali metal cations to eicosyl sulfate (ESO₄) surfactants in monolayers at the air/water interface. The number density of sulfate groups could be tuned by mixing the anionic surfactant with eicosanol. The equilibrium dissociation constant for K⁺ to the fatty sulfate interface showed 10 times greater affinity than for Li⁺ and approximately 3 times greater than for Na⁺. All three cations formed solvent shared ion pairs when the mole fraction of ESO₄ was 0.33 or lower. Above this threshold charge density, Li⁺ formed contact ion pairs with the sulfate headgroups, presumably via bridging structures. By contrast, K⁺ only bound to the sulfate moieties in solvent shared ion pairing configurations. The behavior for Na⁺ was intermediate. These results demonstrate that there is not necessarily a correlation between contact ion pair formation and stronger binding affinity.

Ion Pair Supramolecular Structure Identified by ATR-FTIR Spectroscopy and Simulations in Explicit Solvent*

The present work uses ATR-FTIR spectroscopy assisted by simulations in explicit solvent and frequency calculations to investigate the supramolecular structure of carboxylate alkali-metal ion pairs in aqueous solutions. ATR-FTIR spectra in the 0.25-4.0 M concentration range displayed cation-specific behaviors, which enabled the measurement of the appearance concentration thresholds of contact ion pairs between 1.9 and 2.6 M depending on the cation. Conformational explorations performed using a non-local optimization method associated to a polarizable force-field (AMOEBA), followed by high quantum chemistry level (RI-B97-D3/dhf-TZVPP) optimizations, mode-dependent scaled harmonic frequency calculations and electron density analyses, were used to identify the main supramolecular structures contributing to the experimental spectra. A thorough analysis enables us to reveal the mechanisms responsible for the spectroscopic sensitivity of the carboxylate group and the respective role played by the cation and the water molecules, highlighting the necessity of combining advanced experimental and theoretical techniques to provide a fair and accurate description of ion pairing.

Cyclosporin Structure and Permeability: From A to Z and Beyond

Cyclosporins are natural or synthetic undecapeptides with a wide range of actual and potential pharmaceutical applications. Several members of the cyclosporin compound family have remarkably high passive membrane permeabilities that are not well-described by simple structural metrics. Here we review experimental studies of cyclosporin structure and permeability, including cyclosporin-metal complexes. We also discuss models for the conformation-dependent permeability of cyclosporins and similar compounds. Finally, we identify current knowledge gaps in the literature and provide recommendations regarding future avenues of exploration.

Long Tracts of Guanines Drive Aggregation of RNA G-Quadruplexes in the Presence of Spermine

G-Quadruplexes (GQs) are compact, stable structures in DNA and RNA comprised of two or more tiers of quartets whose G-rich motif of tracts of two or more G's occurs commonly within genomes and transcriptomes. While thermodynamically stable in vitro, these structures remain difficult to study in vivo. One approach to understanding GQ in vivo behavior is to test whether conditions and molecules found in cells facilitate their folding. Polyamines are biogenic polycations that interact with RNA. Among common polyamines, spermine contains the highest charge and is found in eukaryotes, making it a good candidate for association with high-charge density nucleic acid structures like GQs. Using a variety of techniques, including ultraviolet-detected thermal denaturation, circular dichroism, size exclusion chromatography, and confocal microscopy, on an array of quadruplex sequence variants, we find that eukaryotic biological concentrations of spermine induce microaggregation of three-tiered G-rich sequences, but not of purely two-tiered structures, although higher spermine concentrations induce aggregation of even these. The formation of microaggregates can also be induced by addition of as little as a single G to a two-tiered structure; moreover, they form at biological temperatures, are sensitive to salt, and can form in the presence of at least some flanking sequence. Notably, GQ aggregation is not observed under prokaryotic-like conditions of no spermine and higher NaCl concentrations. The sequence, polyamine, and salt specificity of microaggregation reported herein have implications for the formation and stability of G-rich nucleic acid aggregates in vivo and for functional roles for understudied GQ sequences with only two quadruplex tiers.

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.

Use of Ionic Liquids in Protein and DNA Chemistry

Ionic liquids (ILs) have been receiving much attention as solvents in various areas of biochemistry because of their various beneficial properties over the volatile solvents and ILs availability in myriad variants (perhaps as many as 108) owing to the possibility of paring one cation with several anions and vice-versa as well as formulations as zwitterions. Their potential as solvents lies in their tendency to offer both directional and non-directional forces toward a solute molecule. Because of these forces, ionic liquids easily undergo intermolecular interactions with a range of polar/non-polar solutes, including biomolecules such as proteins and DNA. The interaction of genomic species in aqueous/non-aqueous states assists in unraveling their structure and functioning, which have implications in various biomedical applications. The charge density of ionic liquids renders them hydrophilic and hydrophobic, which retain intact over long-range of temperatures. Their ability in stabilizing or destabilizing the 3D-structure of a protein or the double-helical structure of DNA has been assessed superior to the water and volatile organic solvents. The aptitude of an ion in influencing the structure and stability of a native protein depends on their ranking in the Hofmeister series. However, at several instances, a reverse Hofmeister ordering of ions and specific ion-solute interaction has been observed. The capability of an ionic liquid in terms of the tendency to promote the coiling/uncoiling of DNA structure is noted to rely on the basicity, electrostatic interaction, and hydrophobicity of the ionic liquid in question. Any change in the DNA's double-helical structure reflects a change in its melting temperature (T m), compared to a standard buffer solution. These changes in DNA structure have implications in biosensor design and targeted drug-delivery in biomedical applications. In the current review, we have attempted to highlight various aspects of ionic liquids that influence the structure and properties of proteins and DNA. In short, the review will address the issues related to the origin and strength of intermolecular interactions, the effect of structural components, their nature, and the influence of temperature, pH, and additives on them.

Restructuring of Hydration Shell Water due to Solvent-Shared Ion Pairing (SSIP): A Case Study of Aqueous MgCl2 and LaCl3 Solutions

Hydration of ions plays a crucial role in interionic interactions and associated processes in aqueous media, but selective probing of the hydration shell water is nontrivial. Here, we introduce Raman difference with simultaneous curve fitting (RD-SCF) analysis to extract the OH-stretch spectrum of hydration shell water, not only for the fully hydrated ions (Mg²⁺, La³⁺, and Cl^-) but also for the ion pairs. RD-SCF analyses of diluted MgCl₂ (0.18 M) and LaCl₃ (0.12 M) solutions relative to aqueous NaCl of equivalent Cl^- concentrations provide the OH-stretch spectra of water in the hydration shells of fully hydrated Mg²⁺ and La³⁺ cations relative to that of Na⁺. Integrated intensities of the hydration shell spectra of Mg²⁺ and La³⁺ ions increase linearly with the salt concentration (up to 2.0 M MgCl₂ and 1.3 M LaCl₃), which suggests no contact ion pair (CIP) formation in the MgCl₂ and LaCl₃ solutions. Nevertheless, the band shapes of the cation hydration shell spectra show a growing signature of Cl^--associated water with the rising salt concentration, which is a manifestation of the formation of a solvent-shared ion pair (SSIP). The OH-stretch spectrum of the shared/intervening water in the SSIP, retrieved by second-round RD-SCF analysis (2RD-SCF), shows that the average H-bonding of the shared water is weaker than that of the hydration water of the fully hydrated cation (Mg²⁺ or La³⁺) but stronger than that of the anion (Cl^-). The shared water displays an overall second-order dependence on the concentration of the interacting ions, unveiling 1:1 stoichiometry of the SSIP formed between Mg²⁺ and Cl^- as well as La³⁺ and Cl^-.

Cation Specific Effects on the Domain-Domain Interaction of Heterogeneous Dimeric Protein Revealed by FRET Analysis

Specific monovalent cation effects on the domain-domain interaction of heterogeneous dimeric protein were investigated using green fluorescent protein (GFP)-glutathione-s-transferase (GST) fusion protein as a model protein. Conjugating N-terminal of GST domain with a fluorescence probe Cyanine3, complementary increase and decrease of fluorescence intensities of Cyanine3 and GFP were recognized on the exclusive excitation of GFP and further the fluorescence decay of GFP was remarkably accelerated to show that an excellent Förster type of resonance excitation energy transfer (FRET) pair was constructed between GFP- and GST-domain. The spectral overlap integral and critical distance of the FRET pair were estimated to be 5.96×10¹³ M^-1cm³ and 62.5 Å, respectively. The FRET rate and efficiency evaluated by fluorescence lifetime of the energy donor, GFP, were influenced by the monovalent cations included in the buffer solution to suggest that the domain-domain interactions of GFP-GST fusion protein would be susceptible to cation species and their concentrations. The order affecting the domain-domain interaction was estimated to be Li⁺>NH₄⁺ >Na⁺>K⁺>Cs⁺, almost corresponding to the reverse Hofmeister series.

Multivalent ions and biomolecules: Attempting a comprehensive perspective

Ions are ubiquitous in nature. They play a key role for many biological processes on the molecular scale, from molecular interactions, to mechanical properties, to folding, to self-organisation and assembly, to reaction equilibria, to signalling, to energy and material transport, to recognition etc. Going beyond monovalent ions to multivalent ions, the effects of the ions are frequently not only stronger (due to the obviously higher charge), but qualitatively different. A typical example is the process of binding of multivalent ions, such as Ca2+ , to a macromolecule and the consequences of this ion binding such as compaction, collapse, potential charge inversion and precipitation of the macromolecule. Here we review these effects and phenomena induced by multivalent ions for biological (macro)molecules, from the "atomistic/molecular" local picture of (potentially specific) interactions to the more global picture of phase behaviour including, e. g., crystallisation, phase separation, oligomerisation etc. Rather than attempting an encyclopedic list of systems, we rather aim for an embracing discussion using typical case studies. We try to cover predominantly three main classes: proteins, nucleic acids, and amphiphilic molecules including interface effects. We do not cover in detail, but make some comparisons to, ion channels, colloidal systems, and synthetic polymers. While there are obvious differences in the behaviour of, and the relevance of multivalent ions for, the three main classes of systems, we also point out analogies. Our attempt of a comprehensive discussion is guided by the idea that there are not only important differences and specific phenomena with regard to the effects of multivalent ions on the main systems, but also important similarities. We hope to bridge physico-chemical mechanisms, concepts of soft matter, and biological observations and connect the different communities further.

Effects of Halide Anions on the Solution Behavior of Double Hydrophilic Carboxy-Sulfobetaine Block Copolymers

The solution behavior of the double polybetaine block copolymer poly(2-((2-(methacryloyloxy)ethyl)dimethylammonio)acetate)-block-poly(3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate (PGLBT-b-PSPE) in sodium halide aqueous solutions was investigated. In the presence of salt ions, the unimer-to-micelle transition of PGLBT-b-PSPE that originated by Coulombic attraction between PSPE motifs was suppressed and shifted to much lower temperatures. The transition was hindered more by increases in the salt concentration because of additional counterion binding on the ionized site of PGLBT-b-PSPE chains, which screens the dipole-dipole attractions. The specific ion effect was investigated on four different halides, Cl^-, Br^-, I^-, and F^-. Cl^- and two chaotropes (Br^- and I^-) apparently prevented micelle formation, and the hindering effectiveness on the PSPE pairing followed the general Hofmeister series of anions: I^- > Br^- > Cl^-. More chaotropic anions strongly maintained the polymer chains in a fully hydrated state when the same amount of salts was incorporated. However, F^-, which is classified as a kosmotrope, only made a small contribution to lowering the transition point and led to abrupt transition without showing a gradual phase change prior to the transition. The variations of hydrodynamic radius and zeta potentials of unimers and micelles gave hints of the solvation state of salt-incorporated PGLBT-b-PSPEs in each state. These results suggest that chaotropic halides tend to exist in the vicinity of the diblock polybetaine chain surface and thus prominently influenced the thermoresponsive solution behavior, whereas kosmotropic F^- prefers water molecules and causes minor changes in the PGLBT-b-PSPE aqueous solution.

Census of halide-binding sites in protein structures

Motivation

Halides are negatively charged ions of halogens, forming fluorides (F⁻), chlorides (Cl⁻), bromides (Br⁻) and iodides (I⁻). These anions are quite reactive and interact both specifically and non-specifically with proteins. Despite their ubiquitous presence and important roles in protein function, little is known about the preferences of halides binding to proteins. To address this problem, we performed the analysis of halide–protein interactions, based on the entries in the Protein Data Bank.

Results

We have compiled a pipeline for the quick analysis of halide-binding sites in proteins using the available software. Our analysis revealed that all of halides are strongly attracted by the guanidinium moiety of arginine side chains, however, there are also certain preferences among halides for other partners. Furthermore, there is a certain preference for coordination numbers in the binding sites, with a correlation between coordination numbers and amino acid composition. This pipeline can be used as a tool for the analysis of specific halide–protein interactions and assist phasing experiments relying on halides as anomalous scatters.

Availability and implementation

All data described in this article can be reproduced via complied pipeline published at https://github.com/rostkick/Halide_sites/blob/master/README.md.

Supplementary information

Supplementary data are available at Bioinformatics online.

Quantifying Fluorogenic Dye Hydration in an Epoxy Resin by Noncontact Microwave Dielectric Spectroscopy

We investigated a chemically modified rhodamine B dye as a sensor of local water content in dye-modified epoxy resins, where these measurements were combined with dielectric measurements to estimate the dye-water association ratio in the material. In particular, the water-sensitive fluorogenic dye was covalently attached to the epoxy resin backbone. This dye becomes fluorescent only upon photoactivation by ultraviolet light and its protonation in the presence of water. High-resolution noncontact microwave cavity dielectric measurements on these materials indicate a decrease of the dielectric permittivity upon photoactivation. We utilize this effect to determine the average extent of hydration of the activated dye molecules. Our results suggest that fluorogenic dyes are promising for the quantification of the local water content in polymer materials, such as the technologically important problem of interfacial water in epoxy materials.

Recent progress in the science of complex coacervation

Complex coacervation is an associative, liquid-liquid phase separation that can occur in solutions of oppositely-charged macromolecular species, such as proteins, polymers, and colloids. This process results in a coacervate phase, which is a dense mix of the oppositely-charged components, and a supernatant phase, which is primarily devoid of these same species. First observed almost a century ago, coacervates have since found relevance in a wide range of applications; they are used in personal care and food products, cutting edge biotechnology, and as a motif for materials design and self-assembly. There has recently been a renaissance in our understanding of this important class of material phenomena, bringing the science of coacervation to the forefront of polymer and colloid science, biophysics, and industrial materials design. In this review, we describe the emergence of a number of these new research directions, specifically in the context of polymer-polymer complex coacervates, which are inspired by a number of key physical and chemical insights and driven by a diverse range of experimental, theoretical, and computational approaches.

Characterization of Specific Ion Effects on PI(4,5)P2 Clustering: Molecular Dynamics Simulations and Graph-Theoretic Analysis

Numerous cellular functions mediated by phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2; PIP2) involve clustering of the lipid as well as colocalization with other lipids. Although the cation-mediated electrostatic interaction is regarded as the primary clustering mechanism, the ion-specific nature of the intermolecular network formation makes it challenging to characterize the clusters. Here we use all-atom molecular dynamics (MD) simulations of PIP2 monolayers and graph-theoretic analysis to gain insight into the phenomenon. MD simulations reveal that the intermolecular interactions preferentially occur between specific cations and phosphate groups (P1, P4, and P5) of the inositol headgroup with better-matched kosmotropic/chaotropic characters consistent with the law of matching water affinities (LMWA). Ca2+ is strongly attracted to P4/P5, while K+ preferentially binds to P1; Na+ interacts with both P4/P5 and P1. These specific interactions lead to the characteristic clustering patterns. Specificially, the size distributions and structures of PIP2 clusters generated by kosmotropic cations Ca2+ and Na+ are bimodal, with a combination of small and large clusters, while there is little clustering in the presence of only chaotropic K+; the largest clusters are obtained in systems with all three cations. The small-world network (a model with both local and long-range connections) best characterizes the clusters, followed by the random and the scale-free networks. More generally, the present results interpreted within the LMWA are consistent with the relative eukaryotic intracellular concentrations Ca2+ ≪ Na+ < Mg2+ < K+; that is, concentrations of Ca2+ and Na+ must be low to prevent damaging aggregation of lipids, DNA, RNA and phosphate-containing proteins.

Influence of Sodium Salts on the Swelling and Rheology of Hydrophobically Cross-linked Hydrogels Determined by QCM-D

Hydrophobically modified copolymers provide a versatile platform of hydrogel materials for diverse applications, but the influence of salts on the swelling and material properties of this class of hydrogels has not been extensively studied. Here, we investigate model hydrogels with three different sodium salts with anions chosen from the classic Hofmeister series to determine how these counterions influence the swelling and mechanical properties of neutral hydrogels. The gel chosen was based on a statistical copolymer of dimethylacrylamide and 2-(N-ethylperfluorooctane sulfonamido) ethyl acrylate (FOSA). Our measurements utilize a quartz crystal microbalance with dissipation (QCM-D) to quantify both swelling and rheological properties of these gels. We find that a 1 mol/L solution of Na₂SO₄, corresponding to a kosmotropic anion, leads to nearly a 2.6-fold gel deswelling and correspondingly, the complex modulus increases by an order of magnitude under these solution conditions. In contrast, an initial increase in swelling and then a swelling maximum is observed for a 0.02 mol/L concentration in the case of a chaotropic anion, NaClO₄, but the changes in the degree of gel swelling in this system are not directly correlated with changes in the gel shear modulus. The addition of NaBr, an anion salt closer to the middle of the chaotropic to kosmotropic range, leads to hydrogel deswelling where the degree of deswelling and the shear modulus are both nearly independent of salt concentration. Overall, the observed trends are broadly consistent with more kosmotropic ions causing diminished solubility ("salting out") and strongly chaotropic ions causing improved solubility ("salting in"), a trend characteristic of the Hoffmeister series governing the solubility of many proteins and synthetic water-soluble polymers, but trends in the shear stiffness with gel swelling are clearly different from those normally observed in chemically cross-linked gels and are correspondingly difficult to interpret. The salt specificity of swelling and mechanical properties of nonionic hydrogels is important for any potential application in which a wide range of salt concentrations and types are encountered.

The behavior of ions in water is controlled by their water affinity

The strong, long-range electrostatic forces described by Coulomb's law disappear for ions in water, and the behavior of these ions is instead controlled by their water affinity – a weak, short-range force which arises from their charge density. This was established experimentally in the mid-1980s by size-exclusion chromatography on carefully calibrated Sephadex®G-10 (which measures the effective volume and thus the water affinity of an ion) and by neutron diffraction with isotopic substitution (which measures the density and orientation of water molecules near the diffracting ion and thus its water affinity). These conclusions have been confirmed more recently by molecular dynamics simulations, which explicitly model each individual water molecule. This surprising change in force regime occurs because the oppositely charged ions in aqueous salt solutions exist functionally as ion pairs (separated by 0, 1 or 2 water molecules) as has now been shown by dielectric relaxation spectroscopy; this cancels out the strong long-range electrostatic forces and allows the weak, short-range water affinity effects to come to the fore. This microscopic structure of aqueous salt solutions is not captured by models utilizing a macroscopic dielectric constant. Additionally, the Law of Matching Water Affinity, first described in 1997 and 2004, establishes that contact ion pair formation is controlled by water affinity and is a major determinant of the solubility of charged species since only a net neutral species can change phases.

Ion speciation: a key for the understanding of the solution properties of ionic liquid mixtures

Recently, combinations of two (or more) ionic liquids, known as ionic liquid mixtures, have become popular and have a broad range of applications. However, the fundamental knowledge on the molecular interactions that exist in ionic liquid mixtures is far from being understood. In this work, the experimental measurement of the water activity coefficient and computational modelling using Conductor-like Screening Model for Real Solvent (COSMO-RS) were carried out to get an insight into the molecular interactions that are present in ionic liquid mixtures in aqueous solution. The results show that the combination of two ionic liquids of different basicity in aqueous solution allows fine tuning of the water activities, covering a wide range of values that could replace several pure fluids. This is an important feature resulting from the unexpected ion speciation of the ionic liquid mixtures in aqueous solution.

Specific Ion Effects on the Enzymatic Degradation of Polymeric Marine Antibiofouling Materials

It is expected that the widely dispersed ions in seawater would have strong influence on the performance of polymeric marine antibiofouling materials through the modulation of enzymatic degradation of the materials. In this work, poly(ε-caprolactone)-based polyurethane and poly(triisopropylsilyl methacrylate-co-2-methylene-1,3-dioxepane) have been employed as model systems to explore the specific ion effects on the enzymatic degradation of polymeric marine antibiofouling materials. Our study demonstrates that the specific ion effects on the enzymatic degradation of the polymer films are closely correlated with the ion-specific enzymatic hydrolysis of the ester. In the presence of different cations, the effectiveness of the enzyme to degrade the polymer films is dominated by the direct specific interactions between the cations and the negatively charged enzyme molecules. In the presence of different anions, the kosmotropic anions give rise to a high enzyme activity in the degradation of polymer films induced by the salting-out effect, whereas the chaotropic anions lead to a low enzyme activity in the degradation of the polymer films owing to the salting-in effect. This work highlights the opportunities available for the use of specific ion effects to modulate the enzymatic degradation of polymeric antibiofouling materials in the marine environment.

Development of Nonbonded Models for Metal Cations Using the Electronic Continuum Correction

The parametrization of classical nonbonded models of metal ions has been widely addressed in the recent years. Despite the continuous development of novel and more physically inspired functional forms, the 12-6 Lennard-Jones plus Coulomb potential is still the most adopted force field in molecular dynamics (MD) codes, owing to its simple form and easy implementation. However, due to the integer formal charge, unpolarizable force fields of ions may suffer from overestimated interatomic electrostatic interactions, leading to nonphysical clustering or repulsion between such full charges. The electronic continuum correction (ECC) can fix this problem through a simple inclusion of solvent polarization effects via ionic charge rescaling. In this work, the development of novel nonbonded models for mono, divalent, and highly charged metal ions is presented. For each metal species, the ionic charge has been scaled, according to the ECC. Lennard-Jones parameters have been optimized using experimental structural and thermodynamic properties as target quantities. Performances of the proposed models are discussed and compared with the literature data, while transferability attitudes among different and well-known water models are evaluated. © 2019 Wiley Periodicals, Inc.

Unraveling the water states of waste-activated sludge through transverse spin-spin relaxation time of low-field NMR

The physical states of water are crucial for the dewatering efficiency of waste-activated sludge (WAS). However, to date, there is still lack of promising methods for the distinct differentiation of water states in colloidal microbial aggregates. This study proposed that the transverse spin-spin relaxation time (T₂) distribution of low-field nuclear magnetic resonance (NMR) could be a useful tool to unravel the occurrence state of water in WAS. Due to the different interaction strengths of protons with the surrounding environment, the three water states with different T₂ ranges were identified. The water strongly trapped on the surface of solid phase through hydrogen bound could be classified as vicinal water; interstitial water refers to the water physically trapped in bio-floc by steric hindrance or adsorption; and the water that is least affected by solid compositions is categorized as moderately mobile water. The potential ways of typical conditioning approaches for shifting water states were also investigated. The removal of hydrophilic compounds in extracellular polymeric substances (EPS) and surface charge neutralization were both found to be possible ways to influence the percentage of vicinal water (Pearson correlation coefficient R_p > 0.950, p-value ≤ 0.05). The disintegration or compaction of colloidal microbial aggregates could induce the transformation of interstitial water into moderately mobile water. All the above results are believed to deepen the mechanism insights into the differentiation and interactive transformation of water states in bio-floc of WAS.