Concentrations Of Alkali Metal Cations Research Articles

Quantitative Analysis and Manipulation of Alkali Metal Cations at the Cathode Surface in Membrane Electrode Assembly Electrolyzers for CO2 Reduction Reactions

The electrochemical CO2 reduction reaction (CO2RR) has garnered significant attention due to its potential for achieving high CO2 conversion rates under ambient temperature and pressure conditions.[1-3] For practical applications of CO2RR, improving energy efficiency, enhancing selectivity toward high-value products, and increasing production rates are critical. In particular, the production of multi-carbon compounds (C2+, such as ethylene, ethanol, acetic acid, and n-propanol), with high current density and low energy consumption, is of paramount importance.Recently, we successfully increased the partial current density for CO2 electroreduction to C2+ products to a record value of 1.7 A cm−2 using gas diffusion electrodes (GDEs) carrying Cu nanoparticles.[4,5]. However, this study employed liquid electrolytes as the catholyte, which led to an ohmic drop in the electrolyte, resulting in decreased energy efficiency, particularly under high-current electrolysis conditions. To address this issue, researchers have begun utilizing membrane electrode assembly (MEA) cells with solid polymer electrolytes for CO2RR.[6] In previous studies, anion-exchange membranes (AEMs), Cu-based catalysts loaded on GDEs, and concentrated alkaline solutions, such as 1 M KOH or 0.1 M KHCO3, were used as electrolytes, cathodes, and anolytes, respectively; these configurations have become standard for MEA-based CO2RR systems. However, alkali metal cations are transported from the anode to the cathode through the AEM, leading to the formation of bicarbonate salts at the cathode, which disrupts stable CO2RR operation. Therefore, to achieve stable CO2 electrolysis in an MEA-based cell, the amount of alkali metal cations must be reduced or eliminated. However, the quantity of alkali metal cations transported from the anode to the cathode has yet to be quantitatively analyzed.In this study, we quantify the transfer of alkali metal cations from the anode to the cathode and investigate their role in C2+ compound formation, determining the optimal concentration of alkali metal cations required for stable operation.[7] While the presence of alkali metal cations at the cathode surface significantly contributes to C2+ production, the transport rate of K+ ions did not correlate with C2+ selectivity, suggesting that a continuous, high supply of K+ to the cathode surface is not essential for C2+ formation (Figure). Based on these findings, we achieved a faradaic efficiency (FE) and partial current density for C2+ products of 77% and 230 mA cm−2, respectively, even after switching the anode solution from 0.1 M KHCO3 to a diluted K+ solution (<7 mM). These results were nearly identical to those obtained with continuous 0.1 M KHCO3 supply. Furthermore, by intermittently supplying concentrated KHCO3, we significantly improved system durability against salt precipitation compared to continuous supply.[1] K. Kamiya et al. Chem. Lett. 2021, 50,166.[2] S. Kato, K. Kamiya et al. Chem. Sci., 2023 , 14, 613[3] P. Su, K. Kamiya et al., Chem. Sci., 2018, 9, 3941[4] A. Inoue, K. Kamiya et al. EES Catal. 2023 , 1, 9.[5] T. Liu, K. Kamiya et al. Small 2022 , 18, 2205323.[6]C. M. Gabardo, et al. , Joule 2019, 3, 2777.[7] S. Kato, K. Kamiya et al. ChemSusChem doi:10.1002/cssc.202401013. Figure 1

A Solid-State Electrolyte Facilitates Acidic CO2 Electrolysis without Alkali Metal Cations by Regulating Proton Transport.

Electrochemical CO2 reduction (CO2R) in acidic media provides a pathway to curtail CO2 losses by suppressing the formation of (bi)carbonates. In such systems, a high concentration of alkali metal cations is necessary for mitigating the proton-rich environment and suppressing the competing hydrogen evolution reaction. However, a high cation concentration also promotes salt precipitation within the gas diffusion layer, resulting in poor system durability. Here, we resolve this conundrum by replacing the liquid catholyte with a solid-state proton conductor to regulate H+ transport. This is postulated to allow for a locally alkaline environment at the cathode, enabling selective CO2R even without alkali metal cations. We show that this strategy is effective over a broad range of catalyst systems. For instance, we achieve an 87% CO faradaic efficiency (FE) at 300 mA/cm2 using a composite nanoporous Au and single-atom Ni catalyst, with 0.25 M H2SO4 as the anolyte. Stable operation over 110 h and a high single-pass carbon efficiency of 82.8% were also successfully demonstrated. Importantly, we find that this solid-state system is also particularly effective at converting dilute feedstock (5% CO2) with a CO FE of 47.7%, a factor of 16.4 times higher than a conventional system. Our results introduce a simple yet effective design approach for developing efficient acidic CO2R electrolyzers.

Activity and Selectivity Modulation of CO2 Electroreductions at Au-Water Interfaces Via Tuning Localcation Concentrations

Electrochemical CO2 reduction reaction (CO2RR) is a promising technique for converting the greenhouse gas CO2 into valuable fuels and chemicals in a clean energy society. To understand the underlying electrocatalytic reaction mechanism from the atomic level, researchers are investigating the role of electrolyte ions, particularly alkali metal cations, in various electrocatalytic reactions including CO2RR. Despite this attention, the impact of alkali metal cations is still a topic of debate, and it remains unclear how cations affect the CO2RR and the competing hydrogen evolution reaction (HER).In this study, explicit cations and water solvents were added to Au-water interfacial models to simulate the pathways of CO2RR and HER. Two cations (K+, Li+, and/or H+) were used in the model system to create similarly charged Au surfaces, and the local concentration of alkali metal cations (AM+) was adjusted by replacing AM+ (K+ and Li+) with H+. The first electron transfer step was considered critical in electrocatalytic reductions, so CO2 activation and water dissociation were evaluated for CO2RR and HER, respectively. Ab initio molecular dynamics (AIMD) simulations with the slow-growth sampling approach (SG-AIMD) were used to simulate the corresponding reaction mechanisms, and kinetic barriers were obtained through thermodynamic integrations. With these Au-water-cations interfacial models, a systematic study was conducted to investigate the mechanism of CO2 activation at Au-water-2AM+, Au-water-1AM+, and Au-water-0AM+ interfaces. These results show that a high concentration of metal cations with 2AM+ promotes CO2 activation through short-range electrostatic interactions between cations and key intermediates.Besides CO2 activation, this study also investigated water dissociation during the competing HER. Contrary to the promotion effect observed in CO2RR, local alkali metal cations were found to suppress water dissociation with a high reaction barrier (Figure 1). This can be attributed to the broken connectivity of the hydrogen bond network at Au-water-2AM+ interfaces. The reaction kinetics can be improved by reducing the metal cation concentration. Notably, K+ was found to have a more pronounced promotion effect than Li+ on CO2 activation, while the opposite suppression effect was observed on HER. By tuning the local alkali metal cation concentration, it is anticipated that the overall performance of CO2RR, including both activity and selectivity, can be engineered. Figure 1

Cathodic corrosion of Au in aqueous methanolic alkali metal hydroxide electrolytes: Notable role of water

AbstractCathodic corrosion is an electrochemical process that induces restructuring, roughening, and etching of metal surfaces at a highly negative surface charge density, yet, details of the reaction mechanism are not fully resolved. An in‐depth fundamental understanding of the processes and parameters underlying cathodic corrosion is crucial for tailoring the surface structure of the metal electrodes and for synthesizing shape‐ and size‐controlled nanoparticles. Here, we investigate the relevance of water and hydrogen evolution in the cathodic corrosion process. To achieve this aim, Au electrodes were polarized at ‐1.6 V versus RHE in KOH and NaOH electrolytes prepared using different water + methanol mixtures. Structural changes of the Au surfaces were studied by cyclic voltammetry and monitored by scanning electron microscopy (SEM). Most importantly, cathodic corrosion does not take place in the absence of water. There is no detectable bubble formation due to the hydrogen evolution reaction on Au in purely methanolic alkali. Furthermore, the electrochemically active surface area, facet distribution, and surface morphology of Au electrodes are significantly altered upon cathodic polarization as a function of the water concentration. Cathodic corrosion features become more and more pronounced with a further increase in water content. In addition, substantial differences in the surface structure of Au are observed as a function of the nature and concentration of alkali metal cations. Overall, this study provides a more detailed understanding of the role of water and the hydrogen evolution reaction in dominating cathodic corrosion, which might advance the understanding of this phenomenon.

The Interrelated Effect of Cations and Electrolyte pH on the Hydrogen Evolution Reaction on Gold Electrodes in Alkaline Media.

In this work we study the role of alkali metal cation concentration and electrolyte pH in altering the kinetics of the hydrogen evolution reaction (HER) at gold (Au) electrodes. We show that at moderately alkaline pH (pH 11), increasing the cation concentration significantly enhances the HER activity on Au electrodes (with a reaction order ≈0.5). Based on these results we suggest that cations play a central role in stabilizing the transition state of the rate‐determining Volmer step by favorably interacting with the dissociating water molecule (*H–OHδ−–cat+). Moreover, we show that increasing electrolyte pH (pH 10 to pH 13) tunes the local field strength, which in turn indirectly enhances the activity of HER by tuning the near‐surface cation concentration. Interestingly, a too high near‐surface cation concentration (at high pH and high cation concentration) leads to a lowering of the HER activity, which we ascribe to a blockage of the surface by near‐surface cations.

The Interrelated Effect of Cations and Electrolyte pH on the Hydrogen Evolution Reaction on Gold Electrodes in Alkaline Media

AbstractIn this work we study the role of alkali metal cation concentration and electrolyte pH in altering the kinetics of the hydrogen evolution reaction (HER) at gold (Au) electrodes. We show that at moderately alkaline pH (pH 11), increasing the cation concentration significantly enhances the HER activity on Au electrodes (with a reaction order ≈0.5). Based on these results we suggest that cations play a central role in stabilizing the transition state of the rate‐determining Volmer step by favorably interacting with the dissociating water molecule (*H–OHδ−–cat+). Moreover, we show that increasing electrolyte pH (pH 10 to pH 13) tunes the local field strength, which in turn indirectly enhances the activity of HER by tuning the near‐surface cation concentration. Interestingly, a too high near‐surface cation concentration (at high pH and high cation concentration) leads to a lowering of the HER activity, which we ascribe to a blockage of the surface by near‐surface cations.

Understanding the Effects of pH and Alkali Metal Cations on H/OH Adsorption and the Hydrogen Oxidation/Evolution Reaction on Transition Metal Electrodes

The structure and composition of the complex electrode/electrolyte interface have a direct effect on the rate and mechanisms of electrocatalytic reactions. Recent experimental evidence has shown that the rate of the hydrogen oxidation and evolution reactions (HOR/HER) are strongly dependent on electrolyte pH; the rate on the most active catalyst, platinum, is two orders of magnitude slower in an alkaline electrolyte than in an acid electrolyte [1]. This pH dependence has been correlated with an anomalous pH dependence of the sharp, low potential peaks in current measured by cyclic voltammetry on polycrystalline and nanoparticle platinum electrodes, believed to be due to hydrogen adsorption on 110 and 100 step sites [2]. In our prior work, we used density functional theory (DFT) to show that these sharp peaks measured by cyclic voltammetry correspond to competitive hydrogen and hydroxide adsorption onto 110 and 100 step sites and not solely hydrogen adsorption. We further showed that the apparent pH dependence of the location of these step-associated peaks is actually due to the co-adsorption of alkali metal cations, which weaken hydroxide adsorption along the step, shifting these peaks to more positive potentials [3]. We have recently extended this work using both single crystal experiments and DFT to find that the location of the sharp, step-associated peak is independent of pH in the absence of alkali metal cations and is dependent on alkali metal cation concentration and identity, supporting that this effect is mediated by the presence of alkali metal cations along the step [4]. We also find the rate of the hydrogen oxidation reaction on a polycrystalline platinum electrode to be dependent on the identity of the alkali metal cation present in an alkaline electrolyte, and using DFT find that the rate of the HOR is correlated with the alkali cation dependent hydroxide binding strength. Given that we find only a small interaction between the adsorbed cations and adsorbed hydrogen, we support that adsorbed hydroxide may play a role in the HOR mechanism, as has been previously proposed [5]. We show a volcano-type relationship between the experimentally measured hydrogen oxidation rate with the DFT calculated hydroxide adsorption potential across a variety of surfaces. A proposed mechanism will be discussed. [1] W. Sheng, H. A. Gasteiger, Y. Shao-Horn, “Hydrogen oxidation and evolution reaction kinetics on platinum: acid vs alkaline electrolytes,” JECS, vol. 157, pp. B1529-B1536, 2010.[2] W. Sheng, Z. Zhuang, M. Gao, J. Zheng, J. G. Chen, Y. Yan, “Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy,” Nat. Commun., 6:5848, 2015. [3] I. T. McCrum, M. J. Janik, “First principles simulations of cyclic voltammograms on stepped Pt(553) and Pt(533) electrode surfaces,” ChemElectroChem, vol. 3, pp. 1609-1617, 2016.[4] X. Chen, I. T. McCrum, K. A. Schwarz, M. J. Janik, M. T.M. Koper, “Co-adsorption of cations as the cause of the apparent pH dependence of hydrogen adsorption on a stepped platinum single-crystal electrode,” Angew. Chem. Int. Ed. doi:10.1002/anie.201709455[5] D. Strmcnik, M. Uchimura, C. Wang, R. Subbaraman, N. Danilovic, D. van der Vliet, A. P. Paulikas, V. R. Stamenkovic, N. M. Markovic, “Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption,” Nature Chemistry, vol. 5, pp. 300-306.

Involvement of the exomer complex in the polarized transport of Ena1 required for Saccharomyces cerevisiae survival against toxic cations.

Exomer is an adaptor complex required for the direct transport of a selected number of cargoes from the trans-Golgi network (TGN) to the plasma membrane in Saccharomyces cerevisiae However, exomer mutants are highly sensitive to increased concentrations of alkali metal cations, a situation that remains unexplained by the lack of transport of any known cargoes. Here we identify several HAL genes that act as multicopy suppressors of this sensitivity and are connected to the reduced function of the sodium ATPase Ena1. Furthermore, we find that Ena1 is dependent on exomer function. Even though Ena1 can reach the plasma membrane independently of exomer, polarized delivery of Ena1 to the bud requires functional exomer. Moreover, exomer is required for full induction of Ena1 expression after cationic stress by facilitating the plasma membrane recruitment of the molecular machinery involved in Rim101 processing and activation of the RIM101 pathway in response to stress. Both the defective localization and the reduced levels of Ena1 contribute to the sensitivity of exomer mutants to alkali metal cations. Our work thus expands the spectrum of exomer-dependent proteins and provides a link to a more general role of exomer in TGN organization.

Lack of cortical endoplasmic reticulum protein Ist2 alters sodium accumulation in Saccharomyces cerevisiae cells.

The maintenance of intracellular alkali-metal-cation homeostasis is a fundamental property of all living organisms, including the yeast Saccharomyces cerevisiae. Several transport systems are indispensable to ensure proper alkali-metal-cation levels in the yeast cytoplasm and organelles. Ist2 is an endoplasmic reticulum (ER)-resident protein involved, together with other tethering proteins, in the formation of contacts between the plasma and ER membranes. As IST2 gene deletion was shown to influence yeast growth in the presence of sodium, we focused on the roles of Ist2 in the cell response to the presence of various concentrations of alkali metal cations, and its interactions with characterised plasma membrane alkali-metal-cation transporters. Most importantly, we show that, in BY4741 background, the lack of Ist2 results in the accumulation of higher amounts of sodium when the cells are exposed to the presence of this cation, demonstrating the importance of Ist2 for the maintenance of low intracellular levels of toxic sodium. As the function and localisation of alkali-metal-cation exporters is not affected in ist2Δ cells, IST2 deletion results in an increased non-specific uptake of sodium to cells. Moreover, the deletion of IST2 influences relative cell membrane potential, pHin and the growth of cells in the presence of a limiting K+ concentration.

Atomic Ensemble Effects and Non-Covalent Interactions at the Electrode–Electrolyte Interface

Cyanide-modified Pt(111) electrodes have been recently employed to study atomic ensemble effects in electrocatalysis. This work, which will be briefly reviewed, reveals that the smallest site required for methanol dehydrogenation and formic acid dehydration is composed of three contiguous Pt atoms. By blocking these trigonal sites, the specific adsorption of anions, such as sulfate and phosphate, can be inhibited, thus increasing the rate of oxygen reduction reaction by one order of magnitude or more. Moreover, alkali metal cations affect hydrogen adsorption on cyanide-modified Pt(111). This effect is attributed to the non-covalent interactions at the electrical double layer between specifically adsorbed anions or dipoles and the alkali metal cations. A systematic investigation is conducted on the effect of the concentration of alkali metal cations. Accordingly, a simple model that reproduces the experimental observations accurately and enables the understanding of the trends in the strength of the interaction between M + and CN ad when moving from Li + to Cs + , as well as the deviations from the expected trends, is developed. This simple model can also explain the occurrence of super-Nernstian shifts of the equilibrium potential of interfacial proton-coupled electron transfers. Therefore, the model can be generally applied to explain quantitatively the effect of cations on the properties of the electrical double layer. The recently reported effects of alkali metal cations on several electrocatalytic reactions must be mediated by the interaction between these cations and chemisorbed species. As these interactions seem to be adequately and quantitatively described by our model, we expect the model to also be useful to describe, explain, and potentially exploit these effects.

Transport Systems in Halophilic Fungi.

Fungi that tolerate very high environmental NaCl concentrations are good model systems to study mechanisms that enable them to endure osmotic and salinity stress. The whole genome sequences of six such fungal species have been analysed: Hortaea werneckii, Wallemia ichthyophaga and four Aureobasidium spp.: A. pullulans, A. subglaciale, A. melanogenum and A. namibiae. These fungi show different levels of halotolerance, with the presence of numerous membrane transport systems uncovered here that are believed to maintain physiological intracellular concentrations of alkali metal cations. Despite some differences, the intracellular cation contents of H. werneckii, A. pullulans and W. ichthyophaga remain low even under extreme extracellular salinities, which suggests that these species have efficient cation transport systems. We speculate that cation transporters prevent intracellular accumulation of Na(+), and thus avoid the toxic effects that such Na(+) accumulation would have, while also maintaining the high K(+)/Na(+) ratio that is required for the full functioning of the cell - another crucial task in high-Na(+) environments. This chapter primarily summarises the cation transport systems of these selected fungi, and it also describes other membrane transporters that might be involved in their mechanisms of halotolerance.

Comparison of the influence of small GTPases Arl1 and Ypt6 on yeast cells’ tolerance to various stress factors

The GTPases Arl1 and Ypt6 are involved in the intracellular transport of vesicles and their fusion with the trans-Golgi network. This work is focused on comparing the roles of these GTPases in the tolerance of Saccharomyces cerevisiae cells to an increased concentration of alkali metal cations and other stress factors. We studied the phenotypes of arl1 or ypt6 deletions in combination with the deletions of genes encoding alkali-metal-cation transporters (ena1-4, nha1, nhx1, and kha1). Salt sensitivity of the arl1 and ypt6 mutants was shown to be independent of the tested cation transporters and electrochemical membrane potential. Phenotype manifestations of ypt6 deletion were usually more prominent than those of arl1 (cells were more sensitive to KCl, NaCl, LiCl, hygromycin B, increased temperature, and increased pH). At suboptimal temperature, the growth inhibition of arl1 and ypt6 mutants was approximately the same, and low pH was the only condition where arl1 mutants grew even worse than ypt6 mutants. Overexpression of the ARL1 gene suppressed the phenotypes of ypt6 deletion; however, this did not work vice versa (additional copies of YPT6 could not replace ARL1). Our results suggest partially overlapping functions of the GTPases in resistance to various stress factors, with Ypt6 being more efficient under physiological conditions and Arl1 more versatile when overexpressed.

Cnh1 Na+/H+ antiporter and Ena1 Na+-ATPase play different roles in cation homeostasis and cell physiology of Candida glabrata

Yeasts tightly regulate their intracellular concentrations of alkali metal cations. In Saccharomyces cerevisiae, the Nha1 Na(+) /H(+) -antiporter and Ena1 Na(+) -ATPase, mediate the efflux of toxic sodium and surplus potassium. We report the characterization of Candida glabrata CgCnh1 and CgEna1 homologues. Their substrate specificity and transport properties were compared upon expression in S. cerevisiae, and their function characterized directly in C. glabrata. The CgCnh1 antiporter and the CgEna1 ATPase transport both potassium and sodium when expressed in S. cerevisiae. CgEna1p fully complements the lack of S. cerevisiae own Na(+) -ATPases but the activity of the CgCnh1 antiporter is lower than that of ScNha1p. Candida glabrata deletion mutants and analyses of their phenotypes revealed that though both transporters have a broad substrate specificity, their function in C. glabrata cells is not the same. Their differing physiological roles are also reflected in their regulation of expression, CgENA1 is highly upregulated by an increased osmotic pressure or sodium concentration, whereas CgCNH1 is expressed constitutively. The Cnh1 antiporter is involved in the regulation of potassium content and the Ena1 ATPase in sodium detoxification of C. glabrata cells. This situation differs from S. cerevisiae, where the Nha1 antiporter and Ena ATPases both participate together in Na(+) detoxification and in the regulation of K(+) homeostasis.

Lack of main K+ uptake systems in Saccharomyces cerevisiae cells affects yeast performance in both potassium-sufficient and potassium-limiting conditions

A new YNB medium containing very low concentrations of alkali metal cations has been developed to carry out experiments to study potassium homoeostasis. Physiological characterization of Saccharomyces cerevisiae BY4741 strain and the corresponding mutant lacking the main potassium uptake systems (trk1 trk2) under potassium nonlimiting and limiting concentrations was performed, and novel important differences between both strains were found. At nonlimiting concentrations of KCl, the two strains had a comparable cell size and potassium content. Nevertheless, mutants were hyperpolarized, had lower pH and extruded fewer protons compared with the BY4741 strain. Upon transfer to K(+)-limiting conditions, cells of both strains became hyperpolarized and their cell volume and K(+) content diminished; however, the decrease was more relevant in BY4741. In low potassium, trk1 trk2 cells were not able to accomplish the cell cycle to the same extent as in BY4741. Moreover, K(+) limitation triggered a high-affinity K(+)/Rb(+) uptake process only in BY4741, with the highest affinity being reached as soon as 30 min after transfer to potassium-limiting conditions. By establishing basic cellular parameters under standard growth conditions, this work aims to establish a basis for the investigation of potassium homoeostasis at the system level.

Four Pathogenic Candida Species Differ in Salt Tolerance

The virulence of Candida species depends on many environmental conditions, including extracellular pH and concentration of alkali metal cations. Tests of the tolerance/sensitivity of four pathogenic Candida species (C. albicans, C. dubliniensis, C. glabrata, and C. parapsilosis) to alkali metal cations under various growth conditions revealed significant differences among these species. Though all of them can be classified as rather osmotolerant yeast species, they exhibit different levels of tolerance to different salts. C. parapsilosis and C. albicans are the most salt-tolerant in general; C. dubliniensis is the least tolerant on rich YPD media and C. glabrata on acidic (pH 3.5) minimal YNB medium. C. dubliniensis is relatively salt-sensitive in spite of its ability to maintain as high intracellular K(+)/Na(+) ratio as its highly salt-tolerant relative C. albicans. On the other hand, C. parapsilosis can grow in the presence of very high external NaCl concentrations in spite of its high intracellular Na(+) concentrations (and thus lower K(+)/Na(+) ratio) and thus resembles salt-tolerant (halophilic) Debaryomyces hansenii.

Alkali Metal Cation Transport and Homeostasis in Yeasts

The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K(+) and Na(+), is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K(+) transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na(+) can be tolerated due to the existence of an Na(+), K(+)-ATPase and an Na(+), K(+)/H(+)-antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

N, N, N, N′, N′, N′-hexamethylpentanediammonium-MWW layered precursor: A reaction intermediate in the synthesis of zeolites TNU-9 and EU-1

Abstract It is found that the layered precursor of a medium-pore zeolite MCM-22, frequently referred as MCM-22(P), which was initially formed in the presence of a flexible, linear diquaternary cation, N,N,N,N′,N′,N′-hexamethylpentanediammonium (Me6-diquat-5), and Na+ is a reaction intermediate in the synthesis of another two medium-pore zeolites TNU-9 and EU-1. The overall results of this study reveal that the crystallization of the latter two zeolites under hydrothermal conditions at the expense of the Me6-diquat-5/MCM-22(P) composite material depends not only on the concentration of alkali metal cations in synthesis mixtures, but also on their type.

Functional comparison of plasma-membrane Na+/H+ antiporters from two pathogenic Candida species

BackgroundThe virulence of Candida species depends on many environmental conditions. Extracellular pH and concentration of alkali metal cations belong among important factors. Nevertheless, the contribution of transporters mediating the exchange of alkali metal cations for protons across the plasma membrane to the cell salt tolerance and other physiological properties of various Candida species has not been studied so far.ResultsThe tolerance/sensitivity of four pathogenic Candida species to alkali metal cations was tested and the role of one of the cation transporters in that tolerance (presumed to be the plasma-membrane Na+/H+ antiporter) was studied. The genes encoding these antiporters in the most and least salt sensitive species, C. dubliniensis and C. parapsilosis respectively, were identified, cloned and functionally expressed in the plasma membranes of Saccharomyces cerevisiae cells lacking their own cation exporters. Both CpCnh1 and CdCnh1 antiporters had broad substrate specificity and transported Na+, K+, Li+, and Rb+. Their activity in S. cerevisiae cells differed; CpCnh1p provided cells with a much higher salt tolerance than the CdCnh1 antiporter. The observed difference in activity was confirmed by direct measurements of sodium and potassium efflux mediated by these antiporters.ConclusionWe have cloned two genes encoding putative Na+/H+ antiporters in C. parapsilosis and C. dubliniensis, and characterized the transport properties of encoded proteins. Our results show that the activity of plasma-membrane Na+/H+ antiporters is one of the factors determining the tolerance of pathogenic Candida species to high external concentrations of alkali metal cations.

Application of a Zeolite-Poly(vinyl chloride) Electrode to Potentiometric Studies of Alkali Metal Ions

Potentiometric electrodes based on the incorporation of zeolite particles into poly(vinyl chloride) (PVC) membranes are described. The electrode characteristics are evaluated regarding the response towards alkali ions. PVC membranes plasticized with dibutyl phthalate and without lipophilic additives (co-exchanger) are used throughout this study. The electrode exhibits a Nernstian response over the alkali metal cation concentration range of 1.0 × 10−4 to 1.0 × 10−1 M with a slope of 57.0 ± 0.9 mV per decade of concentration with a working pH range 3.0–9.0 and a fast response time of ≤15 s. The selectivity coefficients for cesium ion as test species with respect to alkaline earth, ammonium, and some heavy metal ions are determined. The proposed zeolite-PVC electrode is applied to the determination of ionic surfactant.

Translocation of alkali metal cations by lipophilic cyclodextrin derivatives through black lipid membranes

Lipophilic cyclodextrin (CD) derivatives, synthetic ionophores, were prepared to transport alkali metal cations across a black lipid membrane (BLM). The purpose of this study is to develop a new class of an artificial transportation system of alkali metal cations via bilayer lipid membranes, by using CD derivatives as a cation carrier. A lipophilic CD derivative incorporated into a BLM forms a complex with an alkali metal cation at one surface of the membrane. This charged complex migrates to the opposite side of the membrane and then releases the cation into the subphase. CD derivatives have various types of acyl groups as a complexing site and formed a 1:1 complex with the alkali metal cation. The complex formation was interpreted by an induced-fit mechanism. It is found that the ability of CD derivative for forming a complex and/or transporting cations across the BLM depends on the bulkiness of acyl groups. The conductivities of heptakis (2,6-di-O-propyl-3-O-propionyl)-beta-CD were higher than those of valinomycin regardless of sizes of cations. The order of the conductivity in all derivatives is Li+ < Na+ < K+ approximately = Rb+ approximately = Cs+, regardless of the types of acyl groups in the derivatives. The effects of alkali metal cation concentration in the aqueous phase and CD concentration in the membrane on the translocation are also discussed.