High Concentrations Of LiCl Research Articles

Novel NhaC Na+/H+ antiporter in cyanobacteria contributes to key molecular processes for salt tolerance.

Genome mining has revealed the halotolerant cyanobacterium Halothece sp. PCC7418 harbors considerable enrichment in the ion transport gene family for putative Na+/H+ antiporters. Here, we compared transcriptomic profiles of these encoding genes under various abiotic stresses and discovered that Halothece NhaC (hnhaC) was one of 24 genes drastically upregulated under salt stress. Critical roles of HnhaC in salt-stress protection and response were identified by a complementation assay using the salt-sensitive mutant Escherichia coli strain TO114. Expression of HnhaC rendered this mutant more tolerant to high concentrations of NaCl and LiCl. Antiporter activity assays showed that HnhaC protein predominantly exhibited Na+/H+ and Li+/H+ antiporter activities under neutral or alkaline pH conditions. Furthermore, expression of HnhaC conferred adaptive benefits onto E. coli by enabling a conditional filamentation phenotype. Dissecting the molecular mechanism of this phenotype revealed that differentially expressed genes were associated with clusters of SOS-cell division inhibitor, SOS response repair, and Z-associated proteins. Together, these results strongly indicate that HnhaC is an Na+/H+ antiporter that contributes to salt tolerance. The ubiquitous existence of several Na+/H+ antiporters represents a complex molecular system in halotolerant cyanobacteria, which can be deployed differently in response to growth and to environmental stresses.

Water-in-Salt: Fast Dynamics, Structure, Thermodynamics, and Bulk Properties.

We present concentration-dependent dynamics of highly concentrated LiBr solutions and LiCl temperature-dependent dynamics for two high concentrations and compare the results to those of prior LiCl concentration-dependent data. The dynamical data are obtained using ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE). The OHD-OKE decays are composed of two pairs of biexponentials, i.e., tetra-exponentials. The fastest decay (t1) is the same as pure water's at all concentrations within error, while the second component (t2) slows slightly with concentration. The slower components (t3 and t4), not present in pure water, slow substantially, and their contributions to the decays increase significantly with increasing concentration, similar to LiCl solutions. Simulations of LiCl solutions from the literature show that the slow components arise from large ion/water clusters, while the fast components are from ion/water structures that are not part of large clusters. Temperature-dependent studies (15-95 °C) of two high LiCl concentrations show that decreasing the temperature is equivalent to increasing the room temperature concentration. The LiBr and LiCl concentration dependences and the two LiCl concentrations' temperature dependences all have bulk viscosities that are linearly dependent on τcslow, the correlation time of the slow dynamics (weighted averages of t3 and t4). Remarkably, all four viscosity vs 1/τCslow plots fall on the same line. Application of transition state theory to the temperature-dependent data yields the activation enthalpies and entropies for the dynamics of the large ion/water clusters, which underpin the bulk viscosity.

Development of Gelatin Methacryloyl/Sodium Alginate Interpenetrating Polymer Network Hydrogels for Bone Regeneration by Activating the Wnt/β-Catenin Signaling Pathway via Lithium Release.

Hydrogels have gained significant attention as biomaterials due to their remarkable properties resembling those of the extracellular matrix (ECM). In the present investigation, we successfully synthesized interpenetrating polymer network (IPN) hydrogels using gelatin methacryloyl (GelMA) and sodium alginate (SA), incorporating various concentrations of lithium chloride (LiCl; 0, 5, and 10 mM), aiming to develop a hydrogel scaffold for bone regeneration. Notably, the compressive modulus of the IPN hydrogels remained largely unaffected upon the inclusion of LiCl. However, the hydrogel with the high concentration of LiCl exhibited reduced fragmentation after compression testing. Intriguingly, we observed a significant improvement in cellular biocompatibility, primarily attributed to activation of the Wnt/β-catenin signaling pathway induced by LiCl. Subsequently, we evaluated the efficacy of the newly developed IPN-Li hydrogels in a rat cranial defect model and found that they substantially enhanced bone regeneration. Nevertheless, it is important to note that the introduction of high concentrations of LiCl did not significantly promote osteogenesis. This outcome can be attributed to the excessive release of Li+ ions into the extracellular matrix, hindering the desired effect. Overall, the IPN-Li hydrogel developed in this study holds great promise as a biodegradable material for bone regeneration applications.

One-pot synthesis of anti-freezing carrageenan/polyacrylamide double-network hydrogel electrolyte for low-temperature flexible supercapacitors

Stretchable carrageenan (CG) based double-network (DN) hydrogel has great potential in flexible energy storage devices. To achieve high conductivity at sub-zero temperatures, one-pot synthesis method is adopted for the fabrication of CG/polyacrylamide DN hydrogel with high LiCl concentration (termed CG/PAAm-xLi). In this method, a high temperature above 90 °C is required to fully melt CG network in the presence of high-concentration LiCl and to add the monomers of the second polymer network. Furthermore, by incorporation of small amounts of KCl, the obtained CG/PAAm-7Li/K DN hydrogel shows strong tensile properties with tensile elongation of 567.7%, and fracture energy of 967.3 kJ/m2. In addition, the conductivity of the CG/PAAm-7Li/K DN hydrogel achieves 1.9 S/m at − 40 °C. The flexible supercapacitor prepared by using CG/PAAm-7Li/K hydrogel electrolyte exhibits excellent low-temperature electrochemical properties (specific capacitance of 73.4 F/g, and capacitance retention of 95.6% after 20,000 cycles at − 40 °C). This high-temperature strategy may be extended to construct other ionic polysaccharide-based hydrogel electrolytes with high mechanical property and excellent low-temperature electrochemical performance, showing a broad prospect in the field of flexible electronics.

Application and Analysis of Bipolar Membrane Electrodialysis for LiOH Production at High Electrolyte Concentrations: Current Scope and Challenges.

The objective of this work was to evaluate obtaining LiOH directly from brines with high LiCl concentrations using bipolar membrane electrodialysis by the analysis of Li+ ion transport phenomena. For this purpose, Neosepta BP and Fumasep FBM bipolar membranes were characterized by linear sweep voltammetry, and the Li+ transport number in cation-exchange membranes was determined. In addition, a laboratory-scale reactor was designed, constructed, and tested to develop experimental LiOH production tests. The selected LiCl concentration range, based on productive process concentrations for Salar de Atacama (Chile), was between 14 and 34 wt%. Concentration and current density effects on LiOH production, current efficiency, and specific electricity consumption were evaluated. The highest current efficiency obtained was 0.77 at initial concentrations of LiOH 0.5 wt% and LiCl 14 wt%. On the other hand, a concentrated LiOH solution (between 3.34 wt% and 4.35 wt%, with a solution purity between 96.0% and 95.4%, respectively) was obtained. The results of this work show the feasibility of LiOH production from concentrated brines by means of bipolar membrane electrodialysis, bringing the implementation of this technology closer to LiOH production on a larger scale. Moreover, being an electrochemical process, this could be driven by Solar PV, taking advantage of the high solar radiation conditions in the Atacama Desert in Chile.

Chitin Hydrogels Prepared at Various Lithium Chloride/N,N-Dimethylacetamide Solutions by Water Vapor-Induced Phase Inversion

Chitin was chemically extracted from crab shells and then dissolved in N,N-dimethylacetamide (DMAc) solvent with lithium chloride (LiCl) at 3, 5, 7, and 10%. The concentrated chitin-DMAc/LiCl solutions were used for the preparation of chitin hydrogels by water vapor-induced phase inversion at 20°C. The coagulation process was investigated while altering the concentration of LiCl in the DMAc solution. The shear viscosity of the chitin solution increased with higher LiCl amounts and decreased when the concentration of LiCl was reduced by adding water to the chitin solution, implying high LiCl concentration delayed the coagulation of chitin solution in the presence of water. The viscoelasticity of the chitin solutions indicated the gel formation intensification was dependent on the dose of LiCl and chitin in the DMAc solution. After the chitin solution was coagulated, the resultant hydrogels had water contents of 387–461% and the tensile strength varied from 285 to 400 kPa when the concentration of LiCl in the hydrogel was adjusted to 3% and 7%, respectively. As for viscoelasticity, the complex modulus of the chitin hydrogels indicated that the increment of the LiCl concentration up to 7% formed the tight hydrogels. Atomic force microscopic (AFM) image revealed the formation of the entanglement network and larger domains of the aggregated chitin segments. However, the hydrogel prepared at 10% LiCl in DMAc solution exhibited weak mechanical properties due to the loose hydrogel networking caused by the strong aggregation of the chitin segments.

Electrochemical behavior of protons and cupric ions in water in salt electrolytes with alkaline metal chloride

The proton reduction and copper deposition in water-in-salt electrolytes with LiCl were studied. Cyclic voltammetry on stationary electrode with various scan rates and linear sweep voltammetry on disk electrodes with different rotation rates were used to determine not only the diffusion coefficients but also the effective concentrations of protons and cupric cations. The suppression of hydrogen evolution limiting current observed in water-in-salt electrolyte was determined to result from the inhibition of proton diffusion. This diffusion inhibition was mainly related to the structural diffusion across hydrogen bond network and is not observed for cupric cations. The effects of alkaline metal cations on the acidity of electrolytes, the diffusivity of protons as well as the copper deposition rate were also studied using various super high concentrations of LiCl, NaCl and KCl.

Water-in-Salt Electrolytes for the Electrodeposition of Ruthenium for Interconnect Applications

As interconnect trench dimensions continue to scale down, the use of electrodeposited copper faces challenges due to its exponentially increasing resistance as trench widths approach and become shorter than the electron mean free path. This limitation has resulted in the desire to develop electroplating processes suitable for metallization with alternate elements with lower electron mean free paths. Namely, Co and Ru are particularly well-suited to interconnect applications, both of which being less prone to scaling issues than Cu[1]. Whereas much effort has been devoted to developing Co damascene processes[2], electrodeposition processes for Ru are much less developed due to its slow deposition kinetics, which generally entails deposition occurring in the presence of significant hydrogen formation[3]. Many studies of electrodeposited Ru have been performed using various non-aqueous solvents to widen the solvent electrochemical window and deposit Ru in the absence of hydrogen formation[4], however the use of non-aqueous solvents has significant obstacles, such as high impurity incorporation, atmospheric instability, toxicity, and expense, all of which must be overcome before industrial use becomes practical. A super-high concentration of LiTFSI salt has recently been implemented in Li-ion batteries to expand the electrochemical window of the electrolyte[5]. This effect is partially due to a lack of free water molecules, as they are nearly all within the solvation sheaths of the Li salt and are therefore less electrochemically active. A similar approach can be implemented to reduce the activity of water molecules during electrodeposition, in this case by using a super-high concentration of LiCl salt. So-called water-in-salt electrolytes enable reduced hydrogen evolution in high-overpotential electrodeposition while retaining compatibility with current industrial processes designed for aqueous electrolytes. Water-in-salt electrolytes have been used to permit the electrodeposition of Ru at high cathodic potentials with limited hydrogen evolution. Electrochemical studies show that high concentrations of LiCl suppress hydrogen evolution, inducing a mass-transfer limit of hydrogen evolution at significantly reduced current densities. The significance of limiting hydrogen evolution during Ru deposition is compounded by the observation that fast hydrogen evolution acts to suppress the Ru plating current, a phenomenon that has also been observed in other metals, including Co and Mn. In addition to electrochemical studies, characterizations of Ru films deposited from water-in-salt electrolytes at various conditions will be presented. Focus will be on the crystallization behavior and corresponding resistivity change of films deposited from water-in-salt electrolytes, which has been previously observed to vary significantly from metal films deposited from aqueous electrolytes[6], and is crucially important for interconnect applications.

Overexpression of the DEAD-Box RNA Helicase Gene AtRH17 Confers Tolerance to Salt Stress in Arabidopsis.

Plants adapt to abiotic stresses by complex mechanisms involving various stress-responsive genes. Here, we identified a DEAD-box RNA helicase (RH) gene, AtRH17, in Arabidopsis, involved in salt-stress responses using activation tagging, a useful technique for isolating novel stress-responsive genes. AT895, an activation tagging line, was more tolerant than wild type (WT) under NaCl treatment during germination and seedling development, and AtRH17 was activated in AT895. AtRH17 possesses nine well-conserved motifs of DEAD-box RHs, consisting of motifs Q, I, Ia, Ib, and II-VI. Although at least 12 orthologs of AtRH17 have been found in various plant species, no paralog occurs in Arabidopsis. AtRH17 protein is subcellularily localized in the nucleus. AtRH17-overexpressing transgenic plants (OXs) were more tolerant to high concentrations of NaCl and LiCl compared with WT, but no differences from WT were detected among seedlings exposed to mannitol and freezing treatments. Moreover, in the mature plant stage, AtRH17 OXs were also more tolerant to NaCl than WT, but not to drought, suggesting that AtRH17 is involved specifically in the salt-stress response. Notably, transcriptions of well-known abscisic acid (ABA)-dependent and ABA-independent stress-response genes were similar or lower in AtRH17 OXs than WT under salt-stress treatments. Taken together, our findings suggest that AtRH17, a nuclear DEAD-box RH protein, is involved in salt-stress tolerance, and that its overexpression confers salt-stress tolerance via a pathway other than the well-known ABA-dependent and ABA-independent pathways.

Prokaryotic diversity and community composition in the Salar de Uyuni, a large scale, chaotropic salt flat.

Salar de Uyuni (SdU), with a geological history that reflects 50000 years of climate change, is the largest hypersaline salt flat on Earth and is estimated to be the biggest lithium reservoir in the world. Its salinity reaches saturation levels for NaCl, a kosmotropic salt, and high concentrations of MgCL2 and LiCl, both salts considered important chaotrophic stressors. In addition, extreme temperatures, anoxic conditions, high UV irradiance, high albedo and extremely low concentrations of phosphorous, make SdU a unique natural extreme environment in which to contrast hypotheses about limiting factors of life diversification. Geophysical studies of brines from different sampling stations show that water activity is rather constant along SdU. Geochemical measurements show significant differences in magnesium concentration, ranging from 0.2 to 2M. This work analyses the prokaryotic diversity and community structure at four SdU sampling stations, selected according to their location and ionic composition. Prokaryotic communities were composed of both Archaea (with members of the classes Halobacteria, Thermoplasmata and Nanohaloarchaea, from the Euryarchaeota and Nanohaloarcheota phyla respectively) and Bacteria (mainly belonging to Bacteroidetes and Proteobacteria phyla). The important differences in composition of microbial communities inversely correlate with Mg2+ concentration, suggesting that prokaryotic diversity at SdU is chaotropic dependent.

Lithium chloride has a biphasic effect on prostate cancer stem cells and a proportional effect on midkine levels.

Prostate cancer (PCa) is the second most frequent type of cancer in men worldwide and the levels of differentiation growth factor midkine (MK) are increased in PCa. Cancer and/or the treatment process itself may lead to psychiatric disorders. Lithium chloride (LiCl) has anti-manic properties and has been used in cancer therapy; however, it has a queried safety profile. In addition, cancer stem cells are responsible for the heterogeneous phenotype of tumor cells; they are involved in progression, metastasis, recurrence and therapy resistance in various cancer types. The aims of the present study were to investigate the effect of different concentrations of LiCl on PCa stem cells (whether a shift from tumorigenic to non-tumorigenic cells occurs) and to determine if these results can be explained through changes in MK levels. Monolayer and spheroid cultures of human prostate stem cells and non-stem cells were incubated with low (1, 10 µM) and high (100, 500 µM) concentrations of LiCl for 72 h. Cell proliferation, apoptotic indices, MK levels and ultrastructure were evaluated. Cells stimulated with low concentrations showed high proliferation, low apoptotic indices, high MK levels and more healthy ultrastructure. Opposite results were obtained at high concentrations. Furthermore, stem cells were more sensitive to stimulation and more resistant to inhibition than non-stem cells. LiCl exhibited concentration-dependent effects on stem cell and non-stem cell groups. MK levels were not involved in the biphasic effect of LiCl; however, they were proportionally affected. To the best of our knowledge, the present study was the first to show the effect of LiCl on PCa stem cells through MK.

The Enhancement of the Formation of Wells–Dawson-Type Polyoxometalates by the Addition of High Concentrations of LiCl

Wells–Dawson-type polyoxometalates are formed by heating an acidic Mo(VI) or W(VI) solution containing significantly higher concentrations of hetero ions, P(V) or As(V), than the ideal molar ratio [M]/[X] = 1/9 (M = Mo, W; X = P, As). Most of the phosphate and arsenate remains unreacted in the reaction mixture. These reaction conditions are unusual, even though more reagents than the stoichiometric amounts are frequently needed to obtain complete reactions. In the present study, the replacement of a large amount of unreacted hetero ions containing salts by normal ions (salts) was investigated. The formation of Wells–Dawson-type polyoxometalates, [X2M18O62]6− (X = P, As; M = Mo, W), in the presence of LiCl and NaCl was investigated by Raman spectroscopy and 31P NMR in aqueous solution. The addition of LiCl enhanced the formation of Wells–Dawson-type polyoxometalates, which formed under solution conditions close to the ideal molar ratio [M]/[X] = 1/9.

Arabidopsis Qc-SNARE gene AtSFT12 is involved in salt and osmotic stress responses and Na(+) accumulation in vacuoles.

AtSFT12, an Arabidopsis Qc-SNARE protein, is localized to Golgi organelles and is involved in salt and osmotic stress responses via accumulation of Na (+) in vacuoles. To reduce the detrimental effects of environmental stresses, plants have evolved many defense mechanisms. Here, we identified an Arabidopsis Qc-SNARE gene, AtSFT12, involved in salt and osmotic stress responses using an activation-tagging method. Both activation-tagged plants and overexpressing transgenic plants (OXs) of the AtSFT12 gene were tolerant to high concentrations of NaCl, LiCl, and mannitol, whereas loss-of-function mutants were sensitive to NaCl, LiCl, and mannitol. AtSFT12 transcription increased under NaCl, ABA, cold, and mannitol stresses but not MV treatment. GFP-fusion AtSFT12 protein was juxtaposed with Golgi marker, implying that its function is associated with Golgi-mediated transport. Quantitative measurement of Na(+) using induced coupled plasma atomic emission spectroscopy revealed that AtSFT12 OXs accumulated significantly more Na(+) than WT plants. In addition, Na(+)-dependent fluorescence analysis of Sodium Green showed comparatively higher Na(+) accumulation in vacuoles of AtSFT12 OX cells than in those of WT plant cells after salt treatments. Taken together, our findings suggest that AtSTF12, a Golgi Qc-SNARE protein, plays an important role in salt and osmotic stress responses and functions in the salt stress response via sequestration of Na(+) in vacuoles.

The transcriptional activator Pog1 controls cell cycle, and its phosphorylated form is downregulated by the ubiquitin ligase Dma2 inSaccharomyces cerevisiae

The POG1 gene in Saccharomyces cerevisiae is suggested to encode the transcriptional activator that promotes growth in the presence of a mating pheromone. We previously showed that the overexpression of POG1 conferred tolerance to high concentrations of LiCl and sugar on laboratory and baker's yeast strains, respectively. Here, the overexpression of POG1 was shown to induce cell cycle delay at the G1 phase and morphological abnormality. In addition, by yeast two-hybrid screening, the really interesting new gene (RING)-type ubiquitin ligase Dma2, which is involved in cell cycle regulation, was identified as the protein interacting with Pog1. The gene mutation and deletion analysis revealed that the interaction between Pog1 and Dma2 requires the phosphorylation of Thr253 in Pog1 and the forkhead-associated domain in Dma2. The phosphorylation status of Pog1 changed along with progression of the cell cycle. Interestingly, our results showed that Pog1 might be ubiquitinated by Dma2, but a dephosphorylation-mimic mutation in POG1 increased the cellular Pog1 level possibly due to the failure of ubiquitination. Furthermore, growth of the dma1/2-disrupted strain was greatly inhibited by the overexpression of POG1. These results suggest that Pog1 controls the cell cycle and its phosphorylated form is downregulated by Dma2.

Swelling dynamics of zwitterionic copolymers: The effects of concentration and type of anion and cation

The effect of different salts and their concentration on the swelling of zwitterionic copolymers has been investigated for bulk polymer samples as well as for thin films. Relatively low ratios of the zwitterionic monomer already radically change the swelling properties of the copolymer. Increasing the NaCl concentration results in a significant increase of the swelling, which is indicative of the so-called anti-polyelectrolyte effect. The dynamic swelling experiments on the thin film show that only at high NaCl concentrations, the swelling is limited by solute diffusion. These swelling characteristics are distinctly affected by the type of ions present in the aqueous solution. The changes in maximum swelling degree are most sensitive to the type of anion, where the order of the extent of the effect of the anions follows the Hofmeister lyotropic series. For different cations, smaller changes in maximum swelling degree are found that only partially comply with the Hofmeister series. In most cases similar semi-Fickian swelling behavior is found, except in the case of LiCl concentrations above 0.8M, where anomalous sigmoidal swelling curves are obtained. This behavior in high LiCl concentrations can be explained by the loss of interchain molecular interactions during swelling. This work provides meaningful insights on the behavior of zwitterionic copolymer films for applications such as membrane filtration that utilize different ions and ionic strengths.

Variable mechanisms of action of lithium during generating of membrane potential oscilations across the excitable membrane of the Nitela cell

This study presents results on variable mechanisms of lithium transport processes during generating of membrane potential oscillations across the very excitable membrane of the Nittela cell. Generating of several classes of oscillations, single and local impulses of the membrane potential, were presented in dependence on effects of a high LiCl concentration (10 mM), with which the cell membrane is very excited. Results on membrane potential oscillations are presented, and then some of oscillogram parameters were displayed. The assertion is that oscillations of the membrane potential are caused by total oscillatory transport processes: Li+, K+, Na+ and Cl-across the very excitable cell membrane. The paper presents the hypothesis on mechanisms of oscillatory transport processes of ions (Li, Na, K and Cl) expressed over different classes of oscillations, single and local impulses of the membrane potential across the excitable membrane of the Nittela cell.

Use of ribozyme cleavage kinetics to measure salt-induced changes in solution pH

Several ribozymes are active in molar concentrations of monovalent salts, and pH rate curves under such conditions are consistent with mechanisms involving general acid–base catalysis. Interpreting the apparent p K a values requires an accurate estimation of solution pH, which can be difficult to obtain using a typical glass pH electrode in the presence of high salt concentrations. In the current work we have used the VS ribozyme as a “biological pH meter” to evaluate the effects of molar concentrations of monovalent salts on changes in solution pH. We find that almost all of the measured change in pH observed in high concentrations of LiCl is due to a real change in solution pH. In contrast, high concentrations of NaCl cause errors in pH measurement in addition to changes in actual pH. Different buffer systems affect both the direction and the magnitude of pH changes: we observed changes in measured pH of up to 1 pH unit, which require proper interpretation to avoid adverse effects on the conclusions drawn from pH rate and other experiments that utilize very high salt concentrations.

Biochemical characterization of two novel β-glucosidase genes by metagenome expression cloning

Two novel β-glucosidase genes designated as bgl1D and bgl1E, which encode 172- and 151-aa peptides, respectively, were cloned by function-based screening of a metagenomic library from uncultured soil microorganisms. Sequence analyses indicated that Bgl1D and Bgl1E exhibited lower similarities with some putative β-glucosidases. Functional characterization through high-performance liquid chromatography demonstrated that purified recombinant Bgl1D and Bgl1E proteins hydrolyzed D-glucosyl-β-(1-4)-D-glucose to glucose. Using p-nitrophenyl-β-D-glucoside as substrate, K(m) was 0.54 and 2.11 mM, and k(cat)/K(m) was 1489 and 787 mM(-1) min(-1) for Bgl1D and Bgl1E, respectively. The optimum pH and temperature for Bgl1D was pH 10.0 and 30°C, while the optimum values for Bgl1E were pH 10.0 and 25°C. Bgl1D exhibited habitat-specific characteristics, including higher activity in lower temperature and at high concentrations of AlCl(3) and LiCl. Bgl1D also displayed remarkable activity across a broad pH range (5.5-10.5), making it a potential candidate for industrial applications.

The Influence of Monovalent Cation Size on the Stability of RNA Tertiary Structures

Many RNA tertiary structures are stable in the presence of monovalent ions alone. To evaluate the degree to which ions at or near the surfaces of such RNAs contribute to stability, the salt-dependent stability of a variety of RNA structures was measured with each of the five group I cations. The stability of hairpin secondary structures and a pseudoknot tertiary structure are insensitive to the ion identity, but the tertiary structures of two other RNAs, an adenine riboswitch and a kissing loop complex, become more stable by 2–3 kcal/mol as ion size decreases. This "default" trend is attributed to the ability of smaller ions to approach the RNA surface more closely. The degree of cation accumulation around the kissing loop complex was also inversely proportional to ion radius, perhaps because of the presence of sterically restricted pockets that can be accessed only by smaller ions. An RNA containing the tetraloop-receptor motif shows a strong (up to ∼ 3 kcal/mol) preference for Na + or K + over other group I ions, consistent with the chelation of K + by this motif in some crystal structures. This RNA reverts to the default dependence on ion size when a base forming part of the chelation site is mutated. Lastly, an RNA aptamer for cobinamide, which was originally selected in the presence of high concentrations of LiCl, binds ligand more strongly in the presence of Li + than other monovalent ions. On the basis of these trends in RNA stability with group I ion size, it is argued that two features of RNA tertiary structures may promote strong interactions with ions at or near the RNA surface: negative charge densities that are higher than that in secondary structures, and the occasional presence of chelation sites, which are electronegative pockets that selectively bind ions of an optimum size.

Tuning block copolymer structural information by adjusting salt concentration in liquid chromatography at critical conditions coupled with electrospray tandem mass spectrometry

Different cationic adducts of poly(ethylene oxide)/polystyrene block co-oligomers could be produced by adjusting the salt concentration in the mobile phase using a coupling between liquid chromatography at critical conditions and electrospray ionization mass spectrometry. Formation of doubly lithiated adducts was observed at high LiCl concentration (1 mM) while lowering the salt concentration down to 0.1 mM allowed co-oligomers to be ionized with both a proton and a lithium. The fragmentation pathways observed to occur upon collision-induced dissociation of ionized copolymers were shown to be highly dependent on the nature of the cationic adducts. As a result, complementary structural information could be reached by performing MS/MS experiments on different ionic forms of the same co-oligomer molecule. On one hand, release of the nitroxide end-group as a radical from [M+2Li](2+) was followed by a complete depolymerization of the polystyrene block, allowing both this end-group and the polystyrene segment size to be determined. On the other hand, [M+H+Li](2+) precursor ions mainly dissociated via reactions involving bond cleavages within the nitroxide moiety, yielding useful structural information on this end-group.