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.

Protein Cryoprotectant Ability of the Aqueous Zwitterionic Solution

Protein cryopreservation is important for the long-term storage of unstable proteins. Recently, we found that N-acetylglucosaminyltransferase-V (GnT-V) can be cryopreserved in a deep freezer without temperature control using a dilute binary aqueous solution of 3-(1-(2-(2-methoxyethoxy)ethyl)imidazol-3-io)butane-1-carboxylate (OE2imC3C) [10 wt %, mole fraction of solute (x) = 7.75 × 10-3], an artificial zwitterion. However, it is unclear which solvent properties are required in these media to preserve unstable proteins, such as GnT-V. In this study, we investigated the melting phenomena and solution structure of dilute binary aqueous OE2imC3C solutions [x = 0-2.96 × 10-2 (0-30 wt %)] using differential scanning calorimetry (DSC) and Raman and Fourier transform infrared (FTIR) spectroscopies combined with molecular dynamics (MD) simulation to compare the cryoprotectant ability of OE2imC3C with two general cryoprotectants (CPAs), glycerol and dimethyl sulfoxide. DSC results indicated that aqueous OE2imC3C solutions can be melted at lower temperatures with less energy than the control CPA solution, with increasing x, primarily due to OE2imC3C having a higher content of unfrozen water molecules. Moreover, Raman and FTIR results showed that the high content of unfrozen water molecules in aqueous OE2imC3C solutions was due to the hydration around the ionic parts (the COO- group and imidazolium ring) and the OCH2CH2O segment. In addition, the MD simulation results showed that there were fewer structured water molecules around the OCH2CH2O segment than the hydration water molecules around the ionic parts. These solvent properties suggest that dilute aqueous OE2imC3C solutions are effective in preventing freezing, even in a deep freezer. Therefore, this medium has the potential to act as a novel cryoprotectant for proteins in biotechnology and biomedical fields.

FTIR spectroscopy and molecular level insight of diluted aqueous solutions of acetic acid

Aqueous solutions of acetic acid (AA) have been intensively explored for decades with a particular attention addressed to the hydrogen bond network generated by COOH group at different concentrations. In majority of studies conducted so far the envelope originated from νCO is decomposed into two bands assigned to differently hydrated monomers: the one presumably to AA···H2O, and another one to AA···(H2O)2. In order to examine if species other than the mentioned monomers produce this spectral signature, we performed computational and FTIR spectroscopic study of AA in aqueous solutions. Dilute solutions of deuterated acetic acid (CD3COOD) in D2O and in C2Cl4 as a reference were prepared (c0 = 0.001, 0.01 and 0.1 mol dm-3) as well as of deuterated sodium acetate (CD3COONa) in D2O. CD3COOD in 0.1 mol dm-3 solution in D2O displays a feature that separated in two signals with maxima at 1706 cm-1 and 1687 cm-1. A combined DFT and molecular dynamics study performed in this work showed the assignation of those spectral bands to be a more complex problem than previously thought, with syn-anti isomerism and hydration contributing to the experimentally observed broad νCO envelope.

Ultra-sensitive, rapid detection of dried bloodstains by surface enhanced Raman scattering on Ag substrates

A simple water extraction and transfer procedure is found to result in reproducible and highly sensitive 785 nm excited SERS spectra of 24 h dried bloodstains on Ag nanoparticle substrates. This protocol allows confirmatory detection and identification of dried stains of blood that have been diluted by up to 10⁵ in water on Ag substrates. While previous SERS results demonstrated similar performance on Au substrates when a 50% acetic acid extraction and transfer procedure was used, the water/Ag methodology avoids any potential DNA damage when the sample size is extremely small (≤∼1 μL) due to low pH exposure. The water only procedure is not effective on Au SERS substrates. This metal substrate difference results from the efficient red blood cell lysis and hemoglobin denaturation effects of the Ag nanoparticle surfaces as compare to that of Au nanoparticles. Consequently, the 50% acetic acid exposure is required for the acquisition of 785 nm SERS spectra of dried bloodstains on Au substrates.

Complementary Operando Electrochemical Quartz Crystal Microbalance and UV/Vis Spectroscopic Studies: Acetate Effects on Zinc-Manganese Batteries

Zinc-ion batteries, in which zinc ions and protons do intercalation and de-intercalation during battery cycling with various proposed mechanisms under debate, have been studied. Recently, electrolytic zinc-manganese batteries, exhibiting the pure dissolution-deposition behavior with a large charge capacity, have been accomplished through using electrolytes with Lewis acid. However, the complicated chemical environment and mixed products hinder the investigation though it is crucial to understand the detailed mechanism. Here, cyclic voltammetry coupled electrochemical quartz crystal microbalance (EQCM) and ultraviolet-visible spectrophotometry (UV-Vis) are respectively, for the very first time, used to study the transition from zinc-ion batteries to zinc electrolytic batteries by the continuous addition of acetate ions. These complementary techniques operando trace the mass and the composition evolution. The observed formation and dissolution of zinc hydroxide sulfate (ZHS) and manganese oxides evince the effect of acetate ions on zinc-manganese batteries from an alternative perspective. Both the amount of acetate and the pH value have large impacts on the capacity and Coulombic efficiency of the MnO₂ electrode, and thus they should be optimized when constructing a full zinc-manganese battery with high rate capability and reversibility.

Unraveling the Interphasial Chemistry for Highly Reversible Aqueous Zn Ion Batteries

A robust solid electrolyte interface (SEI) is crucial to widen the electrochemical stability window of the electrolyte and enable sustainably stable electrode reactions in aqueous Zn ion batteries. Different from the SEI in nonaqueous electrolytes, it is of great importance to form a functional and stable SEI due to parasitic reactions with water in aqueous Zn ion batteries. However, the concrete SEI formation in aqueous electrolytes has been elusive so far. Here, we regulate and unravel the decomposition mechanisms of organic Zn salts at the Zn anode-electrolyte interface in the widely studied zinc triflate-based aqueous electrolytes. By introducing a buffering adsorption layer with an optimal concentration of acetate anions, the uncontrollable decomposition of organic zinc triflate salt is greatly inhibited on Zn anodes, resulting in a stable interface. The average Coulombic efficiency of the Zn anode thus can reach as high as 99.95% and stable cycling for 4200 h. With the cooperation of buffering adsorption layers, the tetraethyl ammonium trifluoromethanesulfonate additive as the decomposition promoter could further regulate the decomposition of triflate anions for the formation of robust SEI layers for Zn anodes in electrolytes with a dilute salt concentration. Zn-polyaniline (PANI) full cells demonstrate stable cycling with controlled N/P ratios in such electrolytes. This work proposes an insightful perspective on rational regulation of the decomposition pathway of electrolyte components by forming a stable electrode-electrolyte interface for improved electrochemical performance of aqueous Zn ion batteries.

Exploring the Carbon/Electrolyte Interface in Supercapacitors Operating in Highly Concentrated Aqueous Electrolytes

The development of superconcentrated or water-in-salt electrolytes (WISEs) has paved a new way toward realizing environmentally friendly, nonflammable batteries and supercapacitors based on aqueous electrolytes. The development of new electrolytes, such as WISEs, needs to be accompanied by further studies of the charging mechanism. This is essential to guide the choice of the electrode/electrolyte pairs for optimizing the performance of WISE-based supercapacitors. Therefore, to optimize the performance of carbon/carbon supercapacitors when using new, superconcentrated electrolytes, we present a detailed investigation of the carbon/electrolyte interface by combining electrochemical measurements with Raman and NMR spectroscopy and mass spectrometry. In particular, NMR provides crucial information about the local environment of electrolyte ions inside the carbon pores of the electrode. The results show that the structure of the electrolyte strongly depends on the concentration of the electrolyte and affects the mechanism of charge storage at the positive and negative electrodes.

Raman Spectroscopic Studies on Aqueous Sodium Formate Solutions and DFT Calculations

NaHCOO(aq) and NaDCOO(aq) solutions were measured using Raman spectroscopy from dilute to concentrated solutions at 23 °C in water and heavy water from 50 to 4300 cm−1. A concentrated NaHCOO solution in heavy water was also measured. The Raman band parameters of HCOO−(aq) and DCOO−(aq) such as peak position, full width at half maximum (fwhm), integrated intensities, and depolarization values were determined. From the Raman spectroscopic data, it was concluded that the HCOO−(aq) and DCOO−(aq) symmetry is lower than C2v and probably as low as C1. In contrast to the solution state, $$\tt \normalsize \tt \normalsize {\text{HCO}}_{2}^{ - }$$ HCO 2 - ($$\tt \normalsize\tt\normalsize {\text{DCO}}_{2}^{ - }$$ DCO 2 - ) possess C2v symmetry in the gas phase and the DFT frequencies are given. DFT frequencies on a cluster of HCOO−/DCOO− with five implicit water molecules in the first sphere and placed in a polarizable continuum deviate not more than 1–2% from the measured ones. In the Raman spectrum in NaHCOO(aq), a band doublet at 2730 cm−1 and 2820 cm−1 occurs instead of a single band. The band doublet is due to Fermi resonance and results from the interaction of the overtone of the bending C–H mode, 2ν6 at 1382 cm−1 and ν1. The undisturbed C–H stretching mode, ν1 amounts to 2785 cm−1. In DCOO−(aq), a Fermi doublet was also observed at 2030.5 and 2116.5 cm−1, and the undisturbed wavenumber position amounts to 2101 cm−1. Furthermore, a solution of HCOO− in D2O showed slightly changed frequencies compared with the ones in water caused by the solvent isotope effect. Ion pairing between Na+ and HCOO− characterizes the Raman spectrum at high solute concentrations which are melt-like enabling direct contact between the ions. A NaHCOO solution with high amounts of LiCl added showed large perturbations of the HCOO− bands especially νsCOO− and δ COO− of HCOO−and revealed a stronger affinity of Li+ toward HCOO−. The ion pairs formed are most likely contact ion pairs between Li+ and HCOO− which have different stoichiometry of Li+: HCOO− such as 1:1 and 2:1.

Graphical Abstract

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.

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.

In Situ Differentiation of Multiplex Noncovalent Interactions Using SERS and Chemometrics

Probing changes of noncovalent interactions is crucial to study the binding efficiencies and strengths of (bio)molecular complexes. While surface-enhanced Raman scattering (SERS) offers unique molecular fingerprints to examine such interactions in situ, current platforms are only able to recognize hydrogen bonds because of their reliance on manual spectral identification. Here, we differentiate multiple intermolecular interactions between two interacting species by synergizing plasmonic liquid marble-based SERS platforms, chemometrics, and density functional theory. We demonstrate that characteristic 3-mercaptobenzoic acid (probe) Raman signals have distinct peak shifts upon hydrogen bonding and ionic interactions with tert-butylamine, a model interacting species. Notably, we further quantify the contributions from each noncovalent interaction coexisting in different proportions. As a proof-of-concept, we detect and categorize biologically important nucleotide bases based on molecule-specific interactions. This will potentially be useful to study how subtle changes in biomolecular interactions affect their structural and binding properties.

Dimerization of Acetic Acid in the Gas Phase-NMR Experiments and Quantum-Chemical Calculations

Due to the nature of the carboxylic group, acetic acid can serve as both a donor and acceptor of a hydrogen bond. Gaseous acetic acid is known to form cyclic dimers with two strong hydrogen bonds. However, trimeric and various oligomeric structures have also been hypothesized to exist in both the gas and liquid phases of acetic acid. In this work, a combination of gas-phase NMR experiments and advanced computational approaches were employed in order to validate the basic dimerization model of gaseous acetic acid. The gas-phase experiments performed in a glass tube revealed interactions of acetic acid with the glass surface. On the other hand, variable-temperature and variable-pressure NMR parameters obtained for acetic acid in a polymer insert provided thermodynamic parameters that were in excellent agreement with the MP2 (the second order Møller–Plesset perturbation theory) and CCSD(T) (coupled cluster with single, double and perturbative triple excitation) calculations based on the basic dimerization model. A slight disparity between the theoretical dimerization model and the experimental data was revealed only at low temperatures. This observation might indicate the presence of other, entropically disfavored, supramolecular structures at low temperatures.

New uranium(vi) and isothiouronium complexes: synthesis, crystal structure, spectroscopic characterization and a DFT study

U(vi) and isothiouronium salts create a strong charge-assisted network of hydrogen bonds and ionic interactions.

Identification of ion pairs in solution by IR spectroscopy: crucial contributions of gas phase data and simulations

In a context where structure elucidation of ion pairs in solution remains a contemporary challenge, this work explores an original approach where accurate gas phase spectroscopic data are used to refine high level quantum chemistry calculations of ion pairs in solution, resulting in an unprecedented level of accuracy in vibrational frequency prediction. First, gas phase studies focus on a series of isolated contact ion pairs (M⁺, Ph-CH₂-COO^-, with M = Li, Na, K, Rb, Cs) for which conformer-selective IR spectra in the CO₂^- stretch region are recorded. These experiments reveal the interactions at play in isolated contact ion pairs, and provide vibrational frequencies enabling us to assess the accuracy of the theoretical approach used, i.e., mode-dependent scaled harmonic frequency calculations at the RI-B97-D3/dhf-TZVPP level. This level of calculation is then employed on large water clusters embedding either a free acetate ion or its contact or solvent-shared pairs with a sodium cation in order to simulate the individual vibrational spectra of these species in solution. This study shows that the stretching modes of carboxylate are sensitive to both solvent-shared and contact ion pair formation. FTIR spectra of solutions of increasing concentrations indeed reveal several spectral changes consistent with the presence of specific types of solvent-shared and contact ion pairs. By providing relevant guidelines for the interpretation of solution phase IR spectra, this work illustrates the potential of the approach for the elucidation of supramolecular structures in electrolyte solutions.

Cation-specific interactions of protein surface charges in dilute aqueous salt solutions: a combined study using dielectric relaxation spectroscopy and Raman spectroscopy

We exploited glycine as a zwitterionic model system to experimentally probe the cation specific interaction of protein surface charges in dilute (≤0.25 mol L-1) aqueous solutions of four biologically relevant inorganic salts, NaCl, KCl, MgCl2 and CaCl2, via dielectric relaxation spectroscopy (DRS) and Raman spectroscopy. Glycine is the simplest building block of proteins and it exposes the same charged groups (carboxylate and ammonium) to the solvent that dominate the protein-water interface. As a counter ion, we selected Cl- due to its biological importance. For all systems, we performed simultaneous fitting of the real (ε') and imaginary (ε″) parts of the dielectric functions, assuming a multimodal relaxation model, obtained from concentration dependent dielectric measurements at ∼293 K. We observe a reduction of the dielectric amplitude for the glycine relaxation while the corresponding time constant shows only small (<7%) deviations compared to aqueous glycine solutions. We propose that the observed reduction in dielectric amplitude is due to a reduction of the effective dipole moment (µeff) of zwitterionic glycine caused by the interaction of glycine with the ion even at very low (0.05 M) salt concentrations. The interaction between divalent metal ions and zwitterionic glycine is increased compared to the monovalent cation-zwitterion interaction; a finding that is also supported by Raman spectroscopy. Our combined dielectric relaxation and Raman spectroscopic study indicates that ion-glycine interactions are weak and mediated by the solvent. Cation-specificity of protein surface charges is also observed in dilute salt solutions (≤0.25 mol L-1), where electrostatic interactions dominate.

Topotactic conversion of layered silicate RUB-15 to silica sodalite through interlayer condensation in N-methylformamide

Layered silicate RUB-15 was topotactically converted to silica sodalite through interlayer condensation by refluxing in N-methylformamide.

A correlative spatiotemporal microscale study of calcium phosphate formation and transformation within an alginate hydrogel matrix

The modification of soft hydrogels with hard inorganic components is a method used to form composite materials with application in non-load-bearing bone tissue engineering. The inclusion of an inorganic component may provide mechanical enhancement, introduce osteoconductive or osteoinductive properties, or change other aspects of interactions between native or implanted cells and the material. A thorough understanding of the interactions between such components is needed to improve the rational design of such biomaterials. To achieve this goal, model systems which could allow study of the formation and transformation of mineral phases within a hydrogel network with a range of experimental methods and high spatial and time resolution are needed. Here, we report a detailed investigation of the formation and transformation process of calcium phosphate mineral within an alginate hydrogel matrix. A combination of optical microscopy, confocal Raman microspectroscopy and electron microscopy was used to investigate the spatial distribution, morphology and crystal phase of the calcium phosphate mineral, as well as to study transformation of the mineral phases during the hydrogel mineralization process and upon incubation in a simulated body fluid. It was found, that under the conditions used in this work, mineral initially formed as a metastable amorphous calcium phosphate phase (ACP). The ACP particles had a distinctive spherical morphology and transformed within minutes into brushite in the presence of brushite seed crystals or into octacalcium phosphate, when no seeds were present in the hydrogel matrix. Incubation of brushite-alginate composites in simulated body fluid resulted in formation of hydroxyapatite. The characterization strategy presented here allows for non-destructive, in situ observation of mineralization processes in optically transparent hydrogels with little to no sample preparation.

STATEMENT OF SIGNIFICANCE

The precipitation and transformations of calcium phosphates (CaP) is a complex process, where both formation kinetics and the stability of different mineral phases control the outcome. This situation is even more complex if CaP is precipitated in a hydrogel matrix, where one can expect the organic matrix to modulate crystallization by introducing supersaturation gradients or changing the nucleation and growth kinetics of crystals. In this study we apply a range of characterization techniques to study the mineral formation and transformations of CaP within an alginate matrix with spatiotemporal resolution. It demonstrates how a detailed investigation of the mineral precipitation and transformations can aid in the future rational design of hydrogel-based materials for bone tissue engineering and studies of biomineralization processes.

Gas-Phase Spectroscopic Signatures of Carboxylate-Li(+) Contact Ion Pairs: New Benchmarks For Characterizing Ion Pairing in Solution

The coexistence of several types of ion pairs in solution together with their elusive nature hampers their experimental characterization, which relies in practice on theoretical models resorting to numerous approximations. In this context, a series of isolated contact ion pairs between a lithium cation and phenyl-tagged carboxylate anions of various lengths (Ph-(CH2)n-COO(-), n = 1-3) has been investigated in a conformer-selective manner by IR and UV laser spectroscopy, in conjunction with quantum chemistry calculations. The typical gas-phase IR signature of the bidentate structure formed between the carboxylate moiety and Li(+) has thus been obtained in the CO2(-) stretch region. In addition to the cation-anion interaction, a cation-π interaction occurs simultaneously in the largest system investigated (n = 3). The resulting distorted ion pair structure has been evidenced from both the IR signature of the CO2(-) stretches and the unique vibrationally resolved UV spectroscopy of a phenyl ring interacting with a cation. Such specific spectroscopic signatures of contact ion pairs provide experimental benchmarks, alternative to theoretical predictions, that can assist the assignment of vibrational spectra in solution.

Raman spectroscopic studies and DFT calculations on NaCH3CO2 and NaCD3CO2 solutions in water and heavy water

Sodium acetate and acetate-d₃ solutions in water and heavy water were studied using Raman spectroscopy over a wide concentration range from 40–4200 cm⁻¹ and DFT calculations were performed on acetate–water clusters.