Surface Ion Pairs Research Articles

Surfaces of Atmospheric Droplet Models Probed with Synchrotron XPS on a Liquid Microjet.

ConspectusThe atmosphere is a key part of the earth system comprising myriad chemical species in all basic forms of matter. Ubiquitous nano- and microscopic aerosol particles and cloud droplets suspended in the air play crucial roles in earth's climate and the formation of air pollution. Surfaces are a prominent part of aerosols and droplets, due to the high surface area to bulk volume ratios, but very little is known about their specific properties. Many atmospheric compounds are surface-active, leading to enhanced surface concentrations in aqueous solutions. Their distribution between the surface and bulk may determine heterogeneous chemistry and many other properties of aerosol and cloud droplets, but has not been directly observed.We used X-ray photoelectron spectroscopy (XPS) to obtain direct molecular-level information on the surface composition and structure of aqueous solutions of surface-active organics as model systems for atmospheric aerosol and cloud droplets. XPS is a vacuum-based technique enabled for volatile aqueous organic samples by the application of a high-speed liquid microjet. In combination with brilliant synchrotron X-rays, the chemical specificity of XPS allows distinction between elements in different chemical states and positions within molecular structures. We used core-level C 1s and N 1s signals to identify the alkyl and hydrophilic groups of atmospheric carboxylic acids, alkyl-amines, and their conjugate acids and bases. From this, we infer changes in the orientation of surface-adsorbed species and quantify their relative abundances in the surface. XPS-derived surface enrichments of the organics follow trends expected from their surface activities and we observed a preferential orientation at the surface with the hydrophobic alkyl chains pointing increasingly outward from the solution at higher concentrations. This provides a first direct experimental observation of well-established concepts of surface adsorption and confirms the soundness of the method.We mapped relative abundances of conjugate acid-base pairs in the aqueous solution surfaces from the respective intensities of distinctive XPS signals. For each pair, the protonation equilibrium was significantly shifted toward the neutral form in the surface, compared to the bulk solution, across the full pH range. This represents an apparent shift of the pKa in the surface, which may be toward either higher or lower pH, depending on whether the acid or base form of the pair is the neutral species. The surface shifts are broadly consistent with the relative differences in surface enrichment of the individual acid and base conjugates in binary aqueous solutions, with additional contributions from nonideal interactions in the surface. In aqueous mixtures of surface-active carboxylate anions with ammonium salts at near-neutral pH, we found that the conjugate carboxylic acids were further strongly enhanced. This occurs as the coadsorption of weakly basic carboxylate anions and weakly acidic ammonium cations forms ion-pair surface layers with strongly enhanced local abundances, increasing the probability of net proton transfer according to Le Chatelier's principle. The effect is stronger when the evaporation of ammonia from the surface further contributes to irreversibly perturb the protonation equilibrium, leaving a surplus of carboxylic acid. These surface-specific effects may profoundly influence atmospheric chemistry mediated by aqueous aerosols and cloud droplets but are currently not taken into account in atmospheric models.

Interaction of Nitrite Ions with Hydrated Portlandite Surfaces: Atomistic Computer Simulation Study.

The nitrite admixtures in cement and concrete are used as corrosion inhibitors for steel reinforcement and also as anti-freezing agents. The characterization of the protective properties should account for the decrease in the concentration of free NO2- ions in the pores of cement concretes due to their adsorption. Here we applied the classical molecular dynamics computer simulation approach to quantitatively study the molecular scale mechanisms of nitrite adsorption from NaNO2 aqueous solution on a portlandite surface. We used a new parameterization to model the hydrated NO2- ions in combination with the recently upgraded ClayFF force field (ClayFF-MOH) for the structure of portlandite. The new NO2- parameterization makes it possible to reproduce the properties of hydrated NO2- ions in good agreement with experimental data. In addition, the ClayFF-MOH model improves the description of the portlandite structure by explicitly taking into account the bending of Ca-O-H angles in the crystal and on its surface. The simulations showed that despite the formation of a well-structured water layer on the portlandite (001) crystal surface, NO2- ions can be strongly adsorbed. The nitrite adsorption is primarily due to the formation of hydrogen bonds between the structural hydroxyls on the portlandite surface and both the nitrogen and oxygen atoms of the NO2- ions. Due to that, the ions do not form surface adsorption complexes with a single well-defined structure but can assume various local coordinations. However, in all cases, the adsorbed ions did not show significant surface diffusional mobility. Moreover, we demonstrated that the nitrite ions can be adsorbed both near the previously-adsorbed hydrated Na+ ions as surface ion pairs, but also separately from the cations.

Theoretical and mechanistic insights into control factor-assisted CO2 mineralization with olivine

We elucidate the CO2 mineralization mechanism with Mg-rich forsterite via theoretical approach including density functional theory (DFT) calculations and molecular dynamics (MD) simulations. Here, the CO2 mineralization follows two steps: the ion dissolution pathway for dissolving Mg ions from the forsterite surface, and the cluster growth pathway, where ion pairs agglomerate into molecular-sized MgCO3 clusters in aqueous solution. Step-by-step reaction mechanism for the dissolution of Mg ion was investigated via DFT calculations, and the formation procedure of solvated magnesite clusters and their structural changes over time were observed through MD simulations. Afterward, the effects of three control factors of pH, temperature, and hetero-metal ions were studied for both pathways. We found that the adjustment of pH contributed to the structural changes of the mineral surface and clustering ion pairs, which in turn affected the kinetics of reaction pathway. Also, high-temperature condition induced positive effects for accelerating both mineralization pathways. Lastly, calcium and ferrous ions showed opposite effects, promoting and hindering the overall processes, respectively. Importantly, our mechanistic study suggests that the pH of the solution changes alternatively between acid and base conditions during each mineralization pathway and pH condition induced by the preceding mineralization pathway facilitates the subsequent one.

Maturation Process and Characterization of a Novel Thermostable and Halotolerant Subtilisin-Like Protease with High Collagenolytic Activity but Low Gelatinolytic Activity.

Enzymatic degradation of collagen is of great industrial and environmental significance; however, little is known about thermophile-derived collagenolytic proteases. Here, we report a novel collagenolytic protease (TSS) from thermophilic Brevibacillus sp. WF146. The TSS precursor comprises a signal peptide, an N-terminal propeptide, a subtilisin-like catalytic domain, a β-jelly roll (βJR) domain, and a prepeptidase C-terminal (PPC) domain. The maturation of TSS involves a stepwise autoprocessing of the N-terminal propeptide and the PPC domain, and the βJR rather than the PPC domain is necessary for correct folding of the enzyme. Purified mature TSS displayed optimal activity at 70°C and pH 9.0, a half-life of 1.5 h at 75°C, and an increased thermostability as the NaCl concentration increased up to 4 M. TSS possesses an increased number of surface acidic residues and ion pairs, as well as four Ca2+-binding sites, which contribute to its high thermostability and halotolerance. At high temperatures, TSS exhibited high activity toward insoluble type I collagen and azocoll but showed a low gelatinolytic activity, with a strong preference for Arg and Gly at the P1 and P1' positions, respectively. Both the βJR and PPC domains could bind but not swell collagen, and thus facilitate TSS-mediated collagenolysis via improving the accessibility of the enzyme to the substrate. Additionally, TSS has the ability to efficiently degrade fish scale collagen at high temperatures. IMPORTANCE Proteolytic degradation of collagen at high temperatures has the advantages of increasing degradation efficiency and minimizing the risk of microbial contamination. Reports on thermostable collagenolytic proteases are limited, and their maturation and catalytic mechanisms remain to be elucidated. Our results demonstrate that the thermophile-derived TSS matures in an autocatalytic manner and represents one of the most thermostable collagenolytic proteases reported so far. At elevated temperatures, TSS prefers hydrolyzing insoluble heat-denatured collagen rather than gelatin, providing new insight into the mechanism of collagen degradation by thermostable collagenolytic proteases. Moreover, TSS has the potential to be used in recycling collagen-rich wastes such as fish scales.

Anchored Ligands Facilitate Efficient B-Site Doping in Metal Halide Perovskites.

Metal halide perovskites exhibit outstanding optoelectronic properties: superior charge carrier mobilities, low densities of deep trap states, high photoluminescence quantum yield, and wide color tunability. The introduction of dopant ions provides pathways to manipulate the electronic and chemical features of perovskites. In metal halide perovskites ABX3, where A is a monovalent cation (e.g., methylammonium (MA+), Cs+), B is the divalent metal ion(s) (e.g., Pb2+, Sn2+), and X is the halide group (e.g., Cl-, Br-, or I-), the isovalent exchange of A- and X-site ions has been widely accomplished; in contrast, strategies to exchange B-site cations are underexamined. The activation energies for vacancy-mediated diffusion of B-site cations are much higher than those for A- and X-sites, leading to slow doping processes and low doping ratios. Herein we demonstrate a new method that exchanges B-site cations in perovskites. We design a series of metal carboxylate solutions that anchor on the perovskite surface, allowing fast and efficient doping of B-sites with both homovalent and heterovalent cations (e.g., Sn2+, Zn2+, Bi3+) at room temperature. The doping process in the reduced-dimensional perovskites is complete within 1 min, whereas a similar reaction only leads to the surface attachment of dopant ions in three-dimensional structures. We offer a model based on ammonium extraction and surface ion-pair substitution.

Shielding effect of ‘surface ion pairs’ on physicochemical and bactericidal properties of N,N,N-trimethyl chitosan salts

Different methodologies were employed in this study to synthesize N,N,N-trimethyl chitosan salts (TMC). TMC free of O-methylation and with partial O-methylation were obtained and characterized through 1H nuclear magnetic resonance, wide angle X-ray scattering, scanning electron microscopy coupled with X-ray energy dispersive spectroscopy, and thermogravimetric analysis. It was verified that the dialysis process allowed the removal of ‘surface ion pairs’ on TMC salt structure, increasing the mobility of TMC chains. The surface ion pairs considerably increased the material crystallinity, this property being independent of the used synthesis methodology. Biological tests showed that after dialysis, TMC salts free of ‘surface ion pairs’ kill Escherichia coli in only 6h of incubation. So, the increase in the mobility of dialyzed TMC chains allowed a strong interaction with the cell envelope and the good bactericidal activity of TMC was enhanced.

Preparation of ZnO Nanowires and Study of Surface‐adsorbate Interaction by Fourier‐Transform Infrared Spectroscopy

AbstractTo study the surface‐adsorbate properties of ZnO nanowires, a hydrothermal method was modified to grow ZnO nanowires directly on ZnSe, which were then characterized by attenuated total reflection infrared (ATR‐IR) spectroscopy. To prepare ZnO nanowires directly on ATR sensing element of ZnSe, ZnO seed layers were first formed by annealing of ZnO seeds on ZnSe surfaces. The ZnO seed layers then were exposed to growth solution, forming ZnO nanowires directly on the ATR crystals. The interaction properties of the resulting surfaces were studied by an ATR‐IR method. The diameter, length and distribution of the ZnO nanowires can be tuned by adjusting the growth conditions, particularly the growing time and the concentrations of reagents. Two surfaces, namely Zn‐rich and Zn‐O ion‐pair surfaces were studied in detail for their adsorption properties toward compounds bearing different functional groups. By examination of several volatile organic compounds (VOCs), it was found that the Zn‐rich surface is less selective and interacts with compounds bearing the functional groups of amino and hydroxyl. The Zn‐O ion‐pair surface is more selective and a much stronger interaction was observed with non‐aromatic amino compounds. These results indicate that the improving of the selectivity of a ZnO‐based sensing device can be achieved by tuning the surface structure of the ZnO nanomaterials.

Comprehensive analysis of surface charged residues involved in thermal stability in Alicyclobacillus acidocaldarius esterase 2

Here we report a comprehensive analysis through alanine-scanning mutagenesis of the contribution of surface ion pairs to the thermal stability of Alicyclobacillus acidocaldarius esterase 2 (EST2). We produced 16 single mutants, 4 double mutants corresponding to selected ion pairs R31/E118, E43/K102, R58/D130, D145/R148, 2 double mutants (R63A/R98A and E50A/D94A) involving residues of a large ion network on the protein surface and the double-mutant R98A/R148A meant to disrupt the R98 interactions within the said network and, contextually, the interaction between R148 and D145. The double-mutant E43A/E273K was obtained by chance. All selected residues were replaced with alanine except E91, which was mutated to a glycine and K102, which was changed to a glutamine. All 24 proteins were over-expressed in Escherichia coli, purified and characterized with respect to the main features. Structural stability data were compared with an in silico prediction of ΔΔG values. Our study of the individual factors involved in thermostability and their structural interpretation reveals that the great stability of this thermophilic protein can be explained by the contribution of a few residues at the protein surface.

Structural insight into the molecular basis of polyextremophilicity of short-chain alcohol dehydrogenase from the hyperthermophilic archaeon Thermococcus sibiricus

Biochemical analysis of enantioselective short-chain alcohol dehydrogenase from the hyperthermophilic archaeon Thermococcus sibiricus (TsAdh319) revealed unique polyextremophilic properties of the enzyme - half-life of 1 h at 100 °C, tolerance to high salt (up to 4 M) and organic solvents (50% v/v) concentrations. To elucidate the molecular basis of TsAdh319 polyextremophilicity, we determined the crystal structure of the enzyme in a binary complex with 5-hydroxy-NADP at 1.68 Å resolution. TsAdh319 has a tetrameric structure both in the crystals and in solution with an intersubunit disulfide bond. The substrate-binding pocket is hydrophobic, spacious and open that is consistent with the observed promiscuity in substrate specificity of TsAdh319. The present study revealed an extraordinary number of charged residues on the surface of TsAdh319, 70% of which were involved in ion pair interactions. Further we compared the structure of TsAdh319 with the structures of other homologous short-chain dehydrogenases/reductases (SDRs) from thermophilic and mesophilic organisms. We found that TsAdh319 has the highest arginine and aspartate + glutamate contents compared to the counterparts. The frequency of occurrence of salt bridges on the surface of TsAdh319 is the highest among the SDRs under consideration. No differences in the proline, tryptophan, and phenylalanine contents are observed; the compactness of the protein core of TsAdh319, the monomer and tetramer organization do not differ from that of the counterparts. We suggest that the unique thermostability of TsAdh319 is associated with the rigidity and simultaneous "resilience" of the structure provided by a compact hydrophobic core and a large number of surface ion pairs. An extensive salt bridge network also might maintain the structural integrity of TsAdh319 in high salinity.

Experimental and Theoretical Evaluation of Multisite Cadmium(II) Exchange in Designed Three-Stranded Coiled-Coil Peptides

An important factor that defines the toxicity of elements such as cadmium(II), mercury(II), and lead(II) with biological macromolecules is metal ion exchange dynamics. Intriguingly, little is known about the fundamental rates and mechanisms of metal ion exchange into proteins, especially helical bundles. Herein, we investigate the exchange kinetics of Cd(II) using de novo designed three-stranded coiled-coil peptides that contain metal complexing cysteine thiolates as a model for the incorporation of this ion into trimeric, parallel coiled coils. Peptides were designed containing both a single Cd(II) binding site, GrandL12AL16C [Grand = AcG-(LKALEEK)(5)-GNH(2)], GrandL26AL30C, and GrandL26AE28QL30C, as well as GrandL12AL16CL26AL30C with two Cd(II) binding sites. The binding of Cd(II) to any of these sites is of high affinity (K(A) > 3 × 10(7) M(-1)). Using (113)Cd NMR spectroscopy, Cd(II) binding to these designed peptides was monitored. While the Cd(II) binding is in extreme slow exchange regime without showing any chemical shift changes, incremental line broadening for the bound (113)Cd(II) signal is observed when excess (113)Cd(II) is titrated into the peptides. Most dramatically, for one site, L26AL30C, all (113)Cd(II) NMR signals disappear once a 1.7:1 ratio of Cd(II)/(peptide)(3) is reached. The observed processes are not compatible with a simple "free-bound" two-site exchange kinetics at any time regime. The experimental results can, however, be simulated in detail with a multisite binding model, which features additional Cd(II) binding site(s) which, once occupied, perturb the primary binding site. This model is expanded into differential equations for five-site NMR chemical exchange. The numerical integration of these equations exhibits progressive loss of the primary site NMR signal without a chemical shift change and with limited line broadening, in good agreement with the observed experimental data. The mathematical model is interpreted in molecular terms as representing binding of excess Cd(II) to surface Glu residues located at the helical interfaces. In the absence of Cd(II), the Glu residues stabilize the three-helical structure though salt bridge interactions with surface Lys residues. We hypothesize that Cd(II) interferes with these surface ion pairs, destabilizing the helical structure, and perturbing the primary Cd(II) binding site. This hypothesis is supported by the observation that the Cd(II)-excess line broadening is attenuated in GrandL26AE28QL30C, where a surface Glu(28), close to the metal binding site, was changed to Gln. The external binding site may function as an entry pathway for Cd(II) to find its internal binding site following a molecular rearrangement which may serve as a basis for our understanding of metal complexation, transport, and exchange in complex native systems containing α-helical bundles.

Environment effect on the redox properties of Self-Assembled Monolayers: a theoretical investigation of the nature of the supporting electrolyte

Gibbs free energy calculations have been performed to calculate the change in the simulated redox properties of FcC6S–/C12S–Au and FcC6S–/C4S–Au in three different electrolytes (NaClO4, Na2SO4 and NaPF6). The effect of surface ion association between the ferricinium cation and the electrolyte anions is investigated on the redox properties of SAMs. The microscopic description of the binary SAMs before and after oxidation indicates that the formation of surface ion pairs is strongly correlated with the hydration energy of the corresponding anion. The correlation between the desolvation of the anions and the ion-pairs formation is established. We have shown from the energy contributions that a general understanding of the thermodynamic behavior of these systems should be analyzed in the context of the enthalpy–entropy compensation effect.

Thermodynamic evidence for K+–SO42− ion pair formation on Pt(111). New insight into cation specific adsorption

This work contributes to the understanding of cation specific effects on platinum electrochemistry by means of a thorough thermodynamic analysis of potassium adsorption on Pt(111) in sulfuric acid solutions. It is concluded that potassium specific adsorption is better described as the adsorption of the K(+)-SO ion pair. From the evaluation of the potassium sulfate concentration, it is found that potassium specific adsorption only takes place in the presence of coadsorbed sulfate species. Within the main sulfate adsorption state, for ∼0.3 V < E < ∼0.4 V (vs. SHE), the extent of potassium specific adsorption is small, reaching ∼0.1 × 10(14) species per cm(2) for c(K(+)) > 0.1 M. Then, at higher potentials, E > 0.55 V (vs. SHE), a second potassium adsorption process takes place, concomitant with the second sulfate adsorption process (associated to the small voltammetric feature called "the hump"). This last process involves the adsorption of an equal amount of potassium and sulfate species, leading to the adsorption of ∼0.5 × 10(14) ion pair species per cm(2) (∼0.03 ion pair species per platinum surface atom). Furthermore, the results of the formal partial charge numbers corroborate that potassium adsorption involves sulfate cooperative coadsorption, in such a way that the effective adsorbing species is anionic, rather than cationic. In conclusion, this work evidences that cation specific effects may originate from the formation of surface ion pairs, which is probably related to the presence of ion pairs in solution.

Preparation of a Well-Defined Silica-Supported Nickel-Diimine Alkyl Complex − Application for the Gas-Phase Polymerization of Ethylene

The interaction of [(α-diimine)Ni(CH2SiMe3)2] (1) (α-diimine = 2,3-dimethyl-1,4-[(2′,6′)-diisopropylphenyl]-N,N′-diazadiene) with the silanol ≡Si−OH groups of Aerosil SiO2-(700) leads to the formation of the surface complex [≡SiO−Ni(α-diimine)(CH2SiMe3)] (2) with the concomitant emission of 1 mol equiv of SiMe4 (TMS). The surface complex characterized by IR, chemical and mass balance analyses, solid-state NMR, and EXAFS has been activated by gaseous BF3 to give an active gas-phase ethylene polymerization catalyst. The polymers thus obtained present a highly branched structure. The activity is attributed to the formation of the putative surface ion pair [≡SiOBF3]−[Ni(α-diimine)(CH2SiMe3)]+.

Magnetically driven plasmid DNA delivery with biodegradable polymeric nanoparticles

Targeting gene therapy remains a challenge. The use of magnetic force to achieve this was investigated in the present study. It was hypothesized that nanoparticles with both controllable particle size and magnetic properties would enable magnetically driven gene delivery. We investigated this hypothesis by creating a family of novel biodegradable polymeric superparamagnetic nanoparticle (MNP) formulations. Polylactide MNP were formulated using a modified emulsification-solvent evaporation methodology with both the incorporation of oleate-coated iron oxide and a polyethylenimine (PEI) oleate ion-pair surface modification for DNA binding. MNP size could be controlled by varying the proportion of the tetrahydrofuran cosolvent. Magnetically driven MNP-mediated gene transfer was studied using a green fluorescent protein reporter plasmid in cultured arterial smooth muscle cells and endothelial cells. MNP-DNA internalization and trafficking were examined by confocal microscopy. Cell growth inhibition after MNP-mediated adiponectin plasmid transfection was studied as an example of a therapeutic end point. MNP-DNA complexes protected DNA from degradation and efficiently transfected quiescent cells under both low and high serum conditions after a 15 min exposure to a magnetic field (500 G). There was negligible transfection with MNP in the absence of a magnetic field. Larger sized MNP (375 nm diameter) exhibited higher transfection rates compared with 185 nm- and 240 nm-sized MNP. Internalized larger sized MNP escaped lysosomal localization and released DNA in the perinuclear zone. Adiponectin plasmid DNA delivery using MNP resulted in a dose-dependent growth inhibition of cultured arterial smooth muscle cells. It is concluded that magnetically driven plasmid DNA delivery can be achieved using biodegradable MNP containing oleate-coated magnetite and surface modified with PEI oleate ion-pair complexes that enable DNA binding.

Electrostatic Interactions Involving Lysine Make Major Contributions to Collagen Triple-Helix Stability

Important stabilizing features for the collagen triple helix include the presence of Gly as every third residue, a high content of imino acids, and interchain hydrogen bonds. Host-guest peptides have been used previously to characterize triple-helix propensities of individual residues and Gly-X-Y triplets. Here, comparison of the thermal stabilities of host-guest peptides of the form (Gly-Pro-Hyp)3-Gly-X-Y-Gly-X'-Y'-(Gly-Pro-Hyp)3 extends the study to adjacent tripeptide sequences, to encompass the major classes of potential direct intramolecular interactions. Favorable hydrophobic interactions were observed, as well as stabilizing intrachain interactions between residues of opposite charge in the i and i + 3 positions. However, the greatest gain in triple-helix stability was achieved in the presence of Gly-Pro-Lys-Gly-Asp/Glu-Hyp sequences, leading to a T(m) value equal to that seen for a Gly-Pro-Hyp-Gly-Pro-Hyp sequence. This stabilization is seen for Lys but not for Arg and can be assigned to interchain ion pairs, as shown by molecular modeling. Computational analysis shows that Lys-Gly-Asp/Glu sequences are present at a frequency much greater than expected in collagen, suggesting this interaction is biologically important. These results add significantly to the understanding of which surface ion pairs can contribute to protein stability.

Hofmeister Effects in the Restabilization of IgG−Latex Particles: Testing Ruckenstein's Theory

The hydration interaction is responsible for the colloidal stability observed in protein-coated particles at high ionic strengths. The origin of this non-DLVO interaction is related not only to the local structure of the water molecules located at the surface but also to the structure of those molecules involved in the hydration of the ions that surround the colloidal particles. Ruckenstein and co-workers have recently developed a new theory based on the coupling of double-layer and hydration interactions. Its validity was contrasted by their fitting of experimental data obtained with IgG-latex particles restabilized at high salt concentration. The theory details the important role played by the counterions in the stability at high salt concentrations by proposing an ion pair reaction forming surface dipoles. These surface dipoles are responsible of repulsive interactions between two approaching surfaces. This paper checks the theory with recent data where some ions associated with the Hofmeister series (NO(3)(-), SCN(-) and Ca(2+)) restabilize the same kind of IgG-latex systems by means of hydration forces. Surprisingly, these ions induce stability acting even as co-ions, likely by modifying the water structure at the surface, but not forming surface ion pairs. Therefore, this experimental evidence would question Ruckenstein's theory based on the surface dipole formation for explaining the observed restabilization phenomena.

Sorption kinetics of strontium in porous hydrous ferric oxide aggregates II. Comparison of experimental results and model predictions

In a previous paper, we introduced the Donnan diffusion model to describe cation diffusion into microporous solids with variably charged surfaces, such as hydrous ferric oxides (HFO). Here, we present experiments investigating slow diffusion and sorption of strontium by HFO aggregates with well-characterized porosity. Adsorption of protons and strontium at the HFO surface was evaluated by acid–base titration and batch adsorption experiments with dispersed HFO. The experimental data were fitted with a 1-p K basic Stern model including surface ion pair formation of Na + and NO 3 − and charge distribution for Sr surface complexes. Sorption–diffusion experiments were conducted in flow-through columns at controlled flow rates and at two different pH values, pH 4 and 7. Wet HFO aggregates, which were synthesized using a freezing and thawing method, were packed into chromatographic columns, pre-equilibrated to reach a constant pH, and then Sr breakthrough curves for adsorption and desorption of Sr were recorded. Strong retardation of Sr indicated that diffusion was sufficiently fast in a fraction of pores, so that sorption sites in these pores were rapidly accessible. Based on the analysis of NaNO 3 breakthrough curves, this rapidly accessible pore fraction was estimated to be 37% of the total aggregate pore volume at pH 4.0 and 72% at pH 7.0, respectively. Taking this into account, the Donnan diffusion model gave a good description of the experimental Sr breakthrough curves. Cation exclusion was correctly predicted at pH 4.0. At pH 7, the strong tailing of Sr breakthrough curves due to Sr diffusion into the smallest pores was very well simulated. The Donnan diffusion model proved adequate for pore sizes between approximately 2 and 5 nm, depending on pH and ionic strength. This category of pores was dominant in the HFO aggregates used in this work.

An SFG investigation of Au(1 1 1) and Au(2 1 0) electrodes in aqueous solutions containing KCN and cetylpyridinium chloride

Cetylpyridinium chloride (CPC) is known to affect electrocrystallisation processes in the presence of cyanide; this behaviour is believed to be due to surface-specific differences in coadsorption properties. In this paper, the behaviour of Au(1 1 1) and Au(2 1 0) electrodes in contact with neutral aqueous solutions of KCN in the absence and in the presence of CPC has been studied as a function of potential by means of sum frequency generation (SFG) spectroscopy. The potential-dependent CN − stretching band position has been followed. Spectroelectrochemical results were complemented with cyclic voltammetric measurements. The nature and potential dependence of the CN − adsorbed onto Au(2 1 0) are not measurably affected by the presence of CPC in the electrolyte. On the contrary, with the Au(1 1 1) surface, two CN −-related bands appear, due to adsorbed cyanide and ionic cyanide present at the surface, respectively. This phenomenon might be linked to the formation of surface ion pairs occupying a fraction of the electrode surface. These SFG results have thus disclosed that the more strongly bound CN − onto the more open Au(2 1 0) surface is not appreciably influenced by the presence of cetylpyridinium, while the more loosely bound species present on Au(1 1 1) are affected by coadsorption.

Anion effect on mediated electron transfer through ferrocene-terminated self-assembled monolayers.

Inorganic anions strongly influence the electron transfer rate from the ascorbate to the ferrocene-terminated self-assembled monolayer (SAM) composed of 9-mercaptononyl-5'-ferrocenylpentanoate (Fc(CH2)4COO(CH2)9SH, MNFcP). At the 1 M concentration level of the supporting anion (sodium salt electrolyte), a more than 10-fold increase in the electrocatalytic oxidation rate constant of the ascorbate is observed in the following sequence: PF6-, ClO4-, BF4-, NO3-, Cl-, SO4(2-), NH2SO3- (sulfamate), and F-. The sequence corresponds to the direction of increasing hydration energy of the corresponding anion, suggesting that highly hydrated ions promote electrocatalytic electron transfer to the ferrocene-terminated SAMs, while poorly hydrated ions inhibit it. Fourier transform surface-enhanced Raman spectroscopy (FT-SERS), in combination with cyclic voltammetry, indicates the formation of surface ion pairs between the ferricinium cation (Fc+) and low hydration energy anions, while, on the contrary, no ion pairs were observed in the electrolytes dominated by the high hydration energy anions. Though it is evident that the ion-pairing ability of hydrophobic anions is directly responsible for the electrocatalytic electron transfer inhibition, an estimate of the free, ion-unpaired Fc+ surface concentration shows that it cannot be directly related to the electron transfer rate. This suggests that the principal reason of the anion-induced electron transfer rate modulation might be related to the molecular level changes of the physical and chemical properties as well as the structure of the self-assembled monolayer.

Hyperthermophilic and salt-dependent formyltransferase from Methanopyrus kandleri.

Methanopyrus kandleri is a hyperthermophilic methanogenic archaeon, which grows on H(2) and CO(2) as its sole energy source. Its growth temperature optimum is 98 degrees C. One of the interesting characteristics of this archaeon is its high intracellular salt content. The organism has been reported to contain the trianionic cDPG (cyclic 2,3-diphosphoglycerate) and K+ at concentrations of 1.1 and 3 M, respectively. Reflecting the high cellular salt concentration, the enzymes in this organism are adapted not only to high temperature but also to high salt concentrations. The formyltransferase from M. kandleri was characterized extensively with respect to thermo- and halophilicity. The crystal structure of the formyltransferase at 1.73 A shows the enzyme to be composed of four identical subunits of molecular mass 32 kDa. The formyltransferase is thermostable and active only at relatively high concentrations of potassium phosphate (1 M) or other salts with strongly hydrated anions (strong salting-out salts). Potassium phosphate and potassium cDPG were found to be equivalent in activating and stabilizing the enzyme. At low concentrations of these salts, the enzyme is inactive and thermolabile. It was shown by equilibrium sedimentation analysis that the enzyme is in a monomer/dimer/tetramer equilibrium, the equilibrium constant being dependent on the concentration of salts: the higher oligomeric species increase with increasing salt concentrations. Evidence was provided that the monomer is both inactive and thermolabile. Experiments using a mutation which is directed to break surface ion pairs between two dimers indicated that dimerization is required for activity and tetramerization leads to thermostability.