Slow Exchange Rates Research Articles

Tris buffer-accelerated ligand exchange rate for instant fluorescence detection of trivalent chromium ion

Functional nucleic acids (FNAs) have attracted a lot of attention for the rapid detection of metal ions. Cr3+ is one of the major heavy metal ions in natural waters. Due to the slow ligand exchange rate of Cr3+, the FNA-based Cr3+ sensors require long assay times, limiting the on-site applications. In this study, we report that the good's buffers containing amino and polyhydroxy groups greatly increase the ligand exchange rate of Cr3+. Using EDTA as a model coordinate ligand, the Tris buffer (100 mM, pH 7.0) showed the best acceleration effect among the eight buffers. It improved the rate constant ∼20-fold, shorten the half-time 19-fold, and lowered the activation energy ∼70% at 40 °C. The Tris buffer was then applied for sensor based on the Cr3+-binding induced fluorescence quenching of fluorescein (FAM)-labeled and single-stranded DNA (ssDNA), which shortened the assay time from 1 h to 1 min. The Tris buffer also ∼100% enhanced the fluorescence intensity of FAM, achieving the 11.4-fold lower limit of detection (LOD = 6.97 nM, S/N = 3). By the combination use of the Tris buffer and ascorbic acid, the strong interference from Cu2+, Pb2+, and Fe3+ suffered in many previous reported Cr3+ sensors was avoided. The practical application of the sensor for the detection of Cr3+ spiked in the real water samples were demonstrated with high recovery percentages. The Tris buffer could be applied for other metal ions with slow ligand exchange rate (such as V2+, Co3+ and Fe2+) to solve diverse issues such as long assay time and low synthesis yield of metal complexes, without the need of heating treatment.

Adjustment of rotation and saturation effects (AROSE) for CEST imaging.

Endogenous CEST signal usually has low specificity due to contaminations from the magnetization transfer contrast (MTC) and other labile protons with overlapping or close Larmor frequencies. We propose to improve CEST signal specificity with adjustment of rotation and saturation effects (AROSE). The AROSE approach measures the difference between CEST signals acquired with the same average irradiation power but largely different duty cycles, for example, a continuous wave or a high duty cycle pulse train versus a low duty cycle pulse train with a flip angle φ. Simulation, phantom, and in vivo rodent studies were performed to evaluate the characteristics of the AROSEφ signal. Simulation and experimental results show that AROSE2π is a low-pass filter that can suppress fast exchanging processes (e.g., >3000 s-1 ), whereas AROSEπ is a band-pass filter suppressing both fast and slow exchange (e.g., <30 s-1 ) rates. For other φ angles, the sensitivity and the exchange-rate filtering effect of AROSEφ falls between AROSEπ and AROSE2π . AROSE can also minimize MTC and improve the Larmor frequency selectivity of the CEST signal. The linewidth of the AROSE1.5π spectrum is about 60% to 65% when compared to the CEST spectrum measured by continuous wave. Depending on the needs of an application, the sensitivity, exchange-rate filtering, and Larmor frequency selectivity can be adjusted by varying the flip angle, duty cycle, and average irradiation power. Compared to conventional CEST signals, AROSE can minimize MTC and improve exchange rate filtering and Larmor frequency specificity.

Effect of treated clinoptilolite zeolite on alkali-silica reaction

The reactive aggregates can cause a volumetric expansion in the concrete leading to cracking and spalling due to alkali-silica reaction (ASR). The use of natural zeolite as the supplementary cementitious material can reduce the expansion due to ASR by providing physical space for the ASR gel along with the adsorption of alkalis through the cation exchange. However, the rate of reduction in expansion may sometimes not be satisfactory due to the slow rate of cation exchange along with other factors. Therefore, the present study investigates three different treatment techniques (calcination, milling, and chemical pre-treatment) on clinoptilolite zeolite to obtain an optimal method for reducing the expansion due to ASR. Calcination was done at 400 °C for 3 h to increase the amorphous contents, while milling was done for 3 h to increase the specific surface area of zeolite particles leading to increasing pozzolanic activities. The chemical pre-treatment was accomplished with a 1 M Ca(OH)2 solution to largely remove any existing sodium or potassium ions via cation exchange. To observe the physical expansion due to ASR, mortar bars were prepared with highly reactive cherty sand and up to 20% of both untreated and treated zeolite with the replacement of the portland cement. Results revealed that calcination slightly increased the amorphous contents while milling substantially increased the specific surface area of zeolite particles. The chemical pre-treatment successfully transformed the zeolite into calcium-based clinoptilolite zeolite. Results also revealed that the application of up to 20% untreated, calcined, milled, and chemically treated zeolite reduced the ASR expansion by up to 60%, 40%, 70%, and 42%, respectively, compared to the control sample at 28 days. Overall, among the three different treatment methods, milling was the most effective method to reduce the ASR expansion, while calcined and chemically treated zeolite was less effective than untreated zeolite.

Photochemical processes in surface water bodies and their potential impacts on the chemical composition of water: A review

AbstractThe present study is a review of photochemical processes that occur in surface water bodies and which can affect the chemical composition of water. It is expected that water bodies with a slow rate of water exchange, including lakes, reservoirs, estuaries and ponds with significant areas of shallow water will be most vulnerable to the impacts of these processes. It involves natural organic compounds and xenobiotics of organic origin that enter water bodies from anthropogenic sources or are formed in water bodies because of the development of cyanobacteria and other algae species. Photodegradation of dissolved organic matter (DOM) occurs as a result of both direct photolysis and photosensitized cleavage involving inorganic and organic compounds, which are characterized by chromophore properties, such as humic substances. Photolysis is accompanied by the transformation of high molecular weight organic compounds into lower molecular weight compounds that can become bioavailable to, and promote the development of, microorganisms. Photodegradation results in bioavailable forms of nitrogen and phosphorus being released into the water column. Accordingly, the present study presents the results of analysis of the photochemical destruction of humic substances, such as the most widespread group of the surface water body DOM under the influence of artificial UV irradiation and solar radiation. The results provide data on the products of humic substances photolysis, as well as the influence of the latter on the destruction of a number of organic substances, including xenobiotics (herbicides, pesticides, etc.). Photolysis of algotoxins, which are mainly concentrated in the water of highly trophic reservoirs and lakes during the period of water “blooming,” is among the main concerns being addressed, noting photochemical destruction of these dangerous toxicants can be considered an important means of removing them from drinking water. Also emphasized is that photochemical processes in surface water will likely intensify in the future amid the recent climate changes, also affecting the chemical composition of water.

Synoeca‐MP: New insights into its mechanism of action by using NMR and molecular dynamics simulations approach

AbstractSynoeca‐MP is a 14‐residue amidated peptide, belongs to the mastoparan family and it is found in the venom of the wasp Synoeca surinama and has antibacterial and antifungal activity. The low cytotoxicity of the peptide also makes it an excellent candidate for drug development. To better understand its selectivity and interaction with the membrane, the peptide behavior in membrane‐like environments was studied here and the peptide structure in SDS micelles was determined by NMR spectroscopy. The behavior of the peptide in hydrophobic media and in different pH ranges was studied by CD spectroscopy. The incorporation of residues into the anionic micelles was studied by hydrogen‐deuterium exchange. The peptide stability and insertion in the micelles was studied by molecular dynamics simulations. Synoeca‐MP, bound to SDS micelles, exhibits a partial α‐helix conformation, with the first five residues and the last two unfolded. H/D exchange showed that the peptide has a slow exchange rate. After 164 h, four residues had not yet completed H/D substitution, suggesting parallel alignment of the peptide with the micelle, mainly due to the hydrophobic interface. This may indicate a carpet interaction model of the peptide with micelles. The molecular simulation study of peptide showed that the peptide consists of a well‐folded alpha‐helix core and unfolded extremities, which are responsible for the nature of the peptide interaction. The biophysical analyses can improve the atomic understanding of the mode of action of the peptide and help in future improvements of the peptide for clinical usage.

Stochastic Model of Solvent Exchange in the First Coordination Shell of Aqua Ions.

Ion microsolvation is a basic, yet fundamental, process of ionic solutions underlying many relevant phenomena in either biological or nanotechnological applications, such as solvent reorganization energy, ion transport, catalytic activity, and so on. As a consequence, it is a topic of extensive investigations by various experimental techniques, ranging from X-ray diffraction to NMR relaxation and from calorimetry to vibrational spectroscopy, and theoretical approaches, especially those based on molecular dynamics (MD) simulations. The conventional microscopic view of ion solvation is usually provided by a “static” cluster model representing the first ion–solvent coordination shell. Despite the merits of such a simple model, however, ion coordination in solution should be better regarded as a complex population of dynamically interchanging molecular configurations. Such a more comprehensive view is more subtle to characterize and often elusive to standard approaches. In this work, we report on an effective computational strategy aiming at providing a detailed picture of solvent coordination and exchange around aqua ions, thus including the main structural, thermodynamic, and dynamic properties of ion microsolvation, such as the most probable first-shell complex structures, the corresponding free energies, the interchanging energy barriers, and the solvent-exchange rates. Assuming the solvent coordination number as an effective reaction coordinate and combining MD simulations with enhanced sampling and master-equation approaches, we propose a stochastic model suitable for properly describing, at the same time, the thermodynamics and kinetics of ion–water coordination. The model is successfully tested toward various divalent ions (Ca2+, Zn2+, Hg2+, and Cd2+) in aqueous solution, considering also the case of a high ionic concentration. Results show a very good agreement with those issuing from brute-force MD simulations, when available, and support the reliable prediction of rare ion–water complexes and slow water exchange rates not easily accessible to usual computational methods.

Kinetic characterization of a slow chemical exchange between two sites in N,N-dimethylacetylamide by CEST NMR spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy has provided many powerful tools for the study of dynamic processes. Among the reported methods, chemical exchange saturation transfer (CEST) is more suitable for systems with slow exchange rates, and there will be promising in the detection and dynamic mechanism of metastable substances. It has been widely used in magnetic resonance imaging (MRI), however whether it is applicable in the field of chemical kinetics needs more examples. Here we studied, as a proof of concept, the kinetics of the slow chemical exchange between the two N-methyl protons of N,N-dimethylacetylamide (DMA), exploiting QUantifying Exchange using Z-spectrum (QUEZS) and QUantifying Exchange using Saturation Time (QUEST) methods. It turned out that both of QUEZS and QUEST could give the corresponding exchange rates, showcasing the capability of this method to provide accurate kinetic data under a range of temperatures. Our results clearly demonstrated the reliability of CEST-based techniques as a tool for dynamic kinetics by NMR.

Tafel Slope Analyses for Homogeneous Catalytic Reactions

Tafel analysis of electrocatalysts is essential in their characterization. This paper analyzes the application of Tafel-like analysis to the four-electron nonelectrochemical oxidation of water by the stoichiometric homogeneous 1-electron oxidant [Ru(bpy)3]3+ to dioxygen catalyzed by homogeneous catalysts, [Ru4O4(OH)2(H2O)4(γ-SiW10O36)2]10− (Ru4POM) and [Co4(H2O)2(PW9O34)2]10– (Co4POM). These complexes have slow electron exchange rates with electrodes due to the Frumkin effect, which precludes the use of known electrochemical methods to obtain Tafel plots at ionic strengths lower than 0.5 M. The application of an electron transfer catalyst, [Ru(bpy)3]3+/2+, increases the rates between the Ru4POM and electrode, but a traditional Tafel analysis of such a complex system is precluded due to a lack of appropriate theoretical models for 4-electron processes. Here, we develop a theoretical framework and experimental procedures for a Tafel-like analysis of Ru4POM and Co4POM, using a stoichiometric molecular oxidant [Ru(bpy)3]3+. The dependence of turnover frequency (TOF) as a function of electrochemical solution potential created by the [Ru(bpy)3]3+/[Ru(bpy)3]2+ redox couple (an analog of the Tafel plot) was obtained from kinetics data and interpreted based on the suggested reaction mechanism.

Host–Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR

AbstractDynamics in complexes of porphyrin cage compounds and viologen‐derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host–guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less‐bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host . Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.

Host-Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR.

Dynamics in complexes of porphyrin cage compounds and viologen‐derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host–guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less‐bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host . Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.

Rates and Dynamics of Mercury Isotope Exchange between Dissolved Elemental Hg(0) and Hg(II) Bound to Organic and Inorganic Ligands.

Mercury (Hg) isotope exchange is a common process in biogeochemical transformations of Hg in the environment, but it is unclear whether and at what rates dissolved elemental Hg(0)aq may exchange with divalent Hg(II) bound to various organic and inorganic ligands in water. Using enriched stable isotopes, we investigated the rates and dynamics of isotope exchange between 202Hg(0)aq and 201Hg(II) bound to organic and inorganic ligands with varying chemical structures and binding affinities. Time-dependent exchange reactions were followed by isotope compositional changes using both inductively coupled plasma mass spectrometry and Zeeman cold vapor atomic absorption spectrometry. Rapid, spontaneous isotope exchange (<1 h) was observed between 202Hg(0)aq and 201Hg(II) bound to chloride (Cl-), ethylenediaminetetraacetate (EDTA), and thiols, such as cysteine (CYS), glutathione (GSH), and 2,3-dimercaptopropanesulfonic acid (DMPS) at a thiol ligand-to-Hg(II) molar ratio of 1:1. Without external reductants or oxidants, the exchange resulted in transfer of two electrons and redistribution of Hg isotopes bound to the ligand but no net changes of chemical species in the system. However, an increase in the ligand-to-Hg(II) ratio decreased the exchange rates due to the formation of 2:1 or higher thiol:Hg(II) chelated complexes, but had no effects on exchange rates with 201Hg(II) bound to EDTA or Cl-. The exchange between 202Hg(0)aq and 201Hg(II) bound to dissolved organic matter (DOM) showed an initially rapid followed by a slower exchange rate, likely resulting from Hg(II) complexation with both low- and high-affinity binding functional groups on DOM (e.g., carboxylates vs bidentate thiolates). These results demonstrate that Hg(0)aq readily exchanges with Hg(II) bound to various ligands and highlight the importance of considering exchange reactions in experimental enriched Hg isotope tracer studies or in natural abundance Hg isotope studies in environmental matrices.

Unexpected Trends in the Stability and Dissociation Kinetics of Lanthanide(III) Complexes with Cyclen-Based Ligands across the Lanthanide Series.

We report a detailed study of the thermodynamic stability and dissociation kinetics of lanthanide complexes with two ligands containing a cyclen unit, a methyl group, a picolinate arm, and two acetate pendant arms linked to two nitrogen atoms of the macrocycle in either cis (1,4-H3DO2APA) or trans (1,7-H3DO2APA) positions. The stability constants of the Gd3+ complexes with these two ligands are very similar, with log KGdL values of 16.98 and 16.33 for the complexes of 1,4-H3DO2APA and 1,7-H3DO2APA, respectively. The stability constants of complexes with 1,4-H3DO2APA follow the usual trend, increasing from log KLaL = 15.96 to log KLuL = 19.21. However, the stability of [Ln(1,7-DO2APA)] complexes decreases from log K = 16.33 for Gd3+ to 14.24 for Lu3+. The acid-catalyzed dissociation rates of the Gd3+ complexes differ by a factor of ∼15, with rate constants (k1) of 1.42 and 23.5 M-1 s-1 for [Gd(1,4-DO2APA)] and [Gd(1,7-DO2APA)], respectively. This difference is magnified across the lanthanide series to reach a 5 orders of magnitude higher k1 for [Yb(1,7-DO2APA)] (1475 M-1 s-1) than for [Yb(1,4-DO2APA)] (5.79 × 10-3 M-1 s-1). The acid-catalyzed mechanism involves the protonation of a carboxylate group, followed by a cascade of proton-transfer events that result in the protonation of a nitrogen atom of the cyclen unit. Density functional theory calculations suggest a correlation between the strength of the Ln-Ocarboxylate bonds and the kinetic inertness of the complex, with stronger bonds providing more inert complexes. The 1H NMR resonance of the coordinated water molecule in the [Yb(1,7-DO2APA)] complex at 176 ppm provides a sizable chemical exchange saturation transfer effect thanks to a slow water exchange rate of (15.9 ± 1.6) × 103 s-1.

Stable and Inert Mn(II)-Based and pH-Responsive Contrast Agents

Smart/intelligent contrast agent candidates for MRI based on Mn(II) ion are rare, as it usually forms labile complexes with polyaminocarboxylate-type ligands. Here, we report the first example of a Mn(II) complex that can be activated by changing the pH of its local environment. The PC2A-EA ligand with an ethylamine pendant arm was found to form a thermodynamically stable (log KMnL = 19.01, pMn = 9.27) and kinetically inert complex with Mn(II) with respect to trans-chelation with a metal ion such as Cu(II). The [MnH(PCA2-EA)] complex displays a relatively slow water exchange rate ((4.0 ± 0.2) × 107 s-1), but the pH-dependent coordination of the ethylamine moiety occurs in the pH range of 6-8 (log KMnLH = 6.88), enabling the complex to exhibit pH-sensitive relaxivity in the biologically relevant pH range.

Structures of 1:1 and 2:1 complexes of BMVC and MYC promoter G-quadruplex reveal a mechanism of ligand conformation adjustment for G4-recognition.

BMVC is the first fluorescent probe designed to detect G-quadruplexes (G4s) in vivo. The MYC oncogene promoter forms a G4 (MycG4) which acts as a transcription silencer. Here, we report the high-affinity and specific binding of BMVC to MycG4 with unusual slow-exchange rates on the NMR timescale. We also show that BMVC represses MYC in cancer cells. We determined the solution structures of the 1:1 and 2:1 BMVC–MycG4 complexes. BMVC first binds the 5′-end of MycG4 to form a 1:1 complex with a well-defined structure. At higher ratio, BMVC also binds the 3′-end to form a second complex. In both complexes, the crescent-shaped BMVC recruits a flanking DNA residue to form a BMVC-base plane stacking over the external G-tetrad. Remarkably, BMVC adjusts its conformation to a contracted form to match the G-tetrad for an optimal stacking interaction. This is the first structural example showing the importance of ligand conformational adjustment in G4 recognition. BMVC binds the more accessible 5′-end with higher affinity, whereas sequence specificity is present at the weaker-binding 3′-site. Our structures provide insights into specific recognition of MycG4 by BMVC and useful information for design of G4-targeted anticancer drugs and fluorescent probes.

Investigation of the Amide Proton Solvent Exchange Properties of Glycosaminoglycan Oligosaccharides.

One-dimensional 1H NMR experiments were conducted for aqueous solutions of glycosaminoglycan oligosaccharides to measure the amide proton temperature coefficients and activation energy barriers for solvent exchange and evaluate the effect of pH on the solvent exchange properties. A library of mono- and oligosaccharides was prepared by enzymatic depolymerization of amide-containing polysaccharides and by chemical modification of heparin and heparan sulfate saccharides including members that contain a 3- O-sulfated glucosamine residue. The systematic evaluation of this saccharide library facilitated assessment of the effects of structural characteristics, such as size, sulfation number and site, and glycosidic linkage, on amide proton solvent exchange rates. Charge repulsion by neighboring negatively charged sulfate and carboxylate groups was found to have a significant impact on the catalysis of amide proton solvent exchange by hydroxide. This observation leads to the conclusion that solvent exchange rates must be interpreted within the context of a given chemical environment. On their own, slow exchange rates do not conclusively establish the involvement of a labile proton in a hydrogen bond, and additional supporting experimental evidence such as reduced temperature coefficients is required.

The macroeconomic effects of trade tariffs: Revisiting the Lerner symmetry result

We study the robustness of the Lerner symmetry result in an open economy New Keynesian model with price rigidities. While the Lerner symmetry result, i.e. the absence of allocative and trade-flow effects of an equally-sized change in import tariff and export subsidy, holds up approximately for a number of alternative assumptions, we obtain quantitatively important long-term deviations under complete international asset markets. Direct pass-through of tariffs and subsidies to prices and slow exchange rate adjustment can also generate significant short-term deviations from Lerner. Deviations from symmetry, however, do not necessarily imply an impact on global output and are often limited to a redistribution of production and consumption across countries. Finally, we quantify the macroeconomic costs of a trade war and find that they can be substantial, with permanently lower income and trade volumes. However, a fully symmetric retaliation to an unilaterally imposed border adjustment tax can prevent any sizable adverse real or nominal effects.

Green solvent assisted synthesis of cesium bismuth halide perovskite nanocrystals and the influences of slow and fast anion exchange rates.

Replacing lead in halide perovskites to address the concerns of their toxicity and stability has driven a recent surge in research toward alternative lead-free perovskite materials. Lead-free all inorganic cesium bismuth halide (Cs3Bi2X9) perovskite nanocrystals have attracted attention in recent years due to the air-stability and non-toxic nature of bismuth. Herein, we demonstrate a facile sonication-assisted approach for the preparation of all-inorganic cesium bismuth iodide (Cs3Bi2I9) perovskite nanocrystals (NCs) using propylene carbonate as a green, alternative solvent. The photoluminescence (PL) spectra of the Cs3Bi2X9 NCs have a peak emission that can be tuned from 410 to 550 nm by controlling the composition of the NCs through an anion exchange reaction using tetraalkylammonium halides as a source of halide ions. The rate of this anion exchange reaction is demonstrated to have a significant influence on the dimensions of the NCs obtained from the parent Cs3Bi2I9 NCs. The PL emission of these nanocrystals is predominately due to exciton recombination processes. The NCs also exhibit air-stability for at least 150 days.

Fractionation of Styrene and Maleic Acid Copolymers: The “Holy Grail” of Membrane Solubilization

Styrene-and-maleic acid copolymers (SMA) are gaining interest in membrane protein research due to their ability of solubilizing lipid membranes into nanodiscs. Using different commercially available SMA preparations it was found that the average length and hydrophobicity are important parameters in determining solubilization efficiency and properties of the resulting nanodiscs. However, our understanding of the underlying SMA-lipid interactions is hampered by the fact that SMA preparations are very polydisperse. Here, we fractionated widely used SMA preparations (Mw ∼10kD) into 4 fractions of polymers with different average length, based on their solubility in hexane/acetone mixtures and we investigated their interaction with lipid self-assemblies. We found that fractions with shorter SMA polymers solubilize phosphatidylcholine vesicles more rapidly than fractions with longer polymers. On the other hand, nanodiscs bounded by long SMA polymers were notably more stable than those bounded by shorter polymers, as indicated by a better retention of the native lipid thermotropic properties and by slower exchange rates between lipids in nanodiscs. When the SMA fractions were mixed with E.coli membranes, a somewhat different behavior was observed, probably due to the complexity of these membranes. Here the fractions with relatively long polymers were found to be most efficient in solubilizing the tetrameric KcsA channel. Competitive solubilization experiments showed that short polymers even hinder solubilization by long polymers. In conclusion, we here present a simple method to purify SMA fractions from commercial SMA in order to improve solubilization of biomembranes as well as stability of the resulting nanodiscs.

Yb(AAZTA)(H2O)]-: an unconventional ParaCEST MRI probe.

An unexpectedly slow exchange rate has been observed for the coordinated water molecule of [Yb(AAZTA)(H2O)]- using 1H-NMR and CEST-MRI studies. This made it possible for the first time to exploit the bound water molecule of a LnIII complex with carboxylic donor groups for CEST imaging. This result envisages the development of a new family of thermodynamically stable ParaCEST probes based on anionic chelates.

Electrochemical Investigation of the Hydrogen Evolution Reaction on Electrodeposited Films of Cr(OH)3 and Cr2O3 in Mild Alkaline Solutions

The hydrogen evolution reaction (HER) from water reduction is the main cathodic reaction in the sodium chlorate process. The reaction typically takes place on electrodes covered with a Cr(III) oxide-like film formed in situ by reduction of sodium dichromate in order to avoid reduction of hypochlorite and thereby increase the selectivity for the HER. However, the chemical structure of the Cr(III) oxide-like film is still under debate. In the present work, the kinetics of the HER were studied using titanium electrodes covered with electrodeposited Cr(OH)3 or Cr2O3, which were characterized by means of scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. A clear difference in the morphology of the deposited surfaces was obtained, and the structure could be revealed with Raman spectroscopy. The kinetics for the HER were investigated using potentiodynamic and potentiostatic techniques. The results show that the first electron transfer is rate limiting and that the activity decreases in the order Cr2O3@Ti > bare Ti > Cr(OH)3@Ti. The low activity obtained for Cr(OH)3@Ti is discussed in terms of the involvement of structural water in the HER and the slow ligand exchange rate for water in Cr(III) complexes, while the high activity obtained for Cr2O3@Ti is rationalized by a surface area effect in combination with reduction of surface water and water in solution.Graphical ᅟ