Cation exchange at the mineral-water interface: H3O+/K+ competition at the surface of nano-muscovite

This article describes a (39)K nuclear magnetic resonance (NMR) spectroscopic study of K (+) displacement at the muscovite/water interface as a function of aqueous phase pH. (39)K NMR spectra and T 2 relaxation data for nanocrystalline muscovite wet with a solid/solution weight ratio of 1 at pH 1, 3, and 5.5 show substantial liquid-like K (+) only at pH 1. At pH 3 and 5.5, all K (+) appears to be associated with muscovite as inner- or outer-sphere complexes, indicating that H 3O (+) does not displace basal surface K (+) beyond the (39)K detection limit under these conditions. In our pH 1 mixture, only approximately 1/3 of the initial basal surface K (+) population is located more than 3-4 A from the surface. (29)Si and (27)Al MAS NMR spectra and SEM images show no evidence of dissolution during the (39)K experiments, consistent with the liquid-like (39)K fraction originating from displaced basal surface K (+). Assuming no muscovite dissolution or interlayer exchange, the K (+)/H 3O (+) ratio relevant to the solution/surface exchange equilibrium is controlled by the total amount of K (+) on the surface and H 3O (+) in solution (K (+) surf/H 3O (+) aq). These parameters, in turn, depend on the basal surface area, solution pH, and the solid/solution ratio. The results here are consistent with significant displacement of surface K (+) only under conditions where the initial K (+) surf/H 3O (+) aq ratio is less than approximately 1. Computational molecular models of the muscovite/water interface should account for both K (+) and H 3O (+) in the near-surface region.     

Investigating the potassium interactions with the palytoxin induced channels in Na+/K+ pump

K(+) has been appointed as the main physiological inhibitor of the palytoxin (PTX) effect on the Na(+)/K(+) pump. This toxin acts opening monovalent cationic channels through the Na(+)/K(+) pump. We investigate, by means of computational modeling, the kinetic mechanisms related with K(+) interacting with the complex PTX-Na(+)/K(+) pump. First, a reaction model, with structure similar to Albers-Post model, describing the functional cycle of the pump, was proposed for describing K(+) interference on the complex PTX-Na(+)/K(+) pump in the presence of intracellular ATP. A mathematic model was derived from the reaction model and it was possible to solve numerically the associated differential equations and to simulate experimental maneuvers about the PTX induced currents in the presence of K(+) in the intra- and extracellular space as well as ATP in the intracellular. After the model adjusting to the experimental data, a Monte Carlo method for sensitivity analysis was used to analyze how each reaction parameter acts during each experimental maneuver involving PTX. For ATP and K(+) concentrations conditions, the simulations suggest that the enzyme substate with ATP bound to its high-affinity sites is the main substate for the PTX binding. The activation rate of the induced current is limited by the K(+) deocclusion from the PTX-Na(+)/K(+) pump complex. The K(+) occlusion in the PTX induced channels in the enzymes with ATP bound to its low-affinity sites is the main mechanism responsible for the reduction of the enzyme affinity to PTX.     

Density functional theory investigations on the chemical basis of the selectivity filter in the K+ channel protein

The chemical-physical basis of loading and release of K(+) and Na(+) ions in and out of the selectivity filter of the K(+) channel has been investigated using the B3LYP method of density functional theory. We have shown that the difference between binding free energies of K(+) and Na(+) to the cavity end of the filter is smaller than the difference between the K(+) and Na(+) solvation free energies. Thus, the loading of K(+) ions into the cavity end of the selectivity filter from the solution phase is suggested to be selective prior to the subsequent conduction process. It is shown that the extracellular end of the filter is only optimal for K(+) ions, because K(+) ions prefer the coordination environment of eight carbonyl oxygens. Na(+) ions do not fit into the extracellular end of the filter, since they prefer the coordination environment of six carbonyl oxygens. Overall, the results suggest that the rigid C(4) symmetric selectivity filter is specifically designed for conduction of K(+) ions.     

Millisecond association kinetics of K+ with triazacryptand-based K+ indicators measured by fluorescence correlation spectroscopy

We recently introduced a water-soluble, long-wavelength K(+)-sensing indicator, TAC-Red, consisting of a triazacryptand K(+)-selective ionophore coupled to a xanthylium chromophore (Nat. Methods 2005, 2, 825-827). Stopped-flow kinetic analysis indicated that in response to changes in K(+) concentration TAC-Red fluorescence enhancement occurs in milliseconds or less. Here, we use fluorescence correlation spectroscopy to quantify the binding kinetics of K(+) with TAC-Red and a new, longer-wavelength sensor, TAC-Crimson. Autocorrelation functions, G(tau), were similar at 0 and high (150 mM) K(+) concentrations, with the appearance of a prominent kinetic process with a correlation time in the millisecond range for K(+) concentrations between approximately 20 and 60 mM. Control experiments with increased illumination volume and solution viscosity indicated that the millisecond component represented K(+)/TAC-Red association. K(+)-dependent G(tau) data, modeled using a global regression to a binding/diffusion model, gave association and dissociation rate constants of 0.0020 +/- 0.0003 mM(-1) ms(-1) and 0.12 +/- 0.02 ms(-1), respectively, for TAC-Red. Similar results were obtained for TAC-Crimson. The rapid K(+) binding kinetics with triazacryptand-based sensors support their utility for measuring changes in K(+) concentrations during rapid neural signaling and ion channel gating.     

Water molecule adsorption on short alanine peptides: how short is the shortest gas-phase alanine-based helix?

Water adsorption measurements have been performed under equilibrium conditions for unsolvated Ac-A(n)K+H(+) and Ac-KA(n)+H(+) peptides with n = 4 - 10. Previous work on larger alanine peptides has shown that two dominant conformations (helices and globules) are present for these peptides and that water adsorbs much more strongly to the globules than to the helices. All the Ac-KA(n)+H(+) peptides studied here (which are expected to be globular) adsorb water strongly, and so do the Ac-A(n)K+H(+) peptides with n < 8. However, for Ac-A(n)K+H(+) with n = 8-10 there is a substantial drop in the propensity to adsorb water. This result suggests that Ac-A(8)K+H(+) is the smallest Ac-A(n)K+H(+) peptide to have a significant helical content in the gas phase. Water adsorption measurements for Ac-V(n)K+H(+) and Ac-L(n)K+H(+) with n = 5-10 suggest that the helix emerges at n = 8 for these peptides as well.     

Competition between pi and non-pi cation-binding sites in aromatic amino acids: a theoretical study of alkali metal cation (Li+, Na+, K+)-phenylalanine complexes

To understand the cation-pi interaction in aromatic amino acids and peptides, the binding of M(+) (where M(+) = Li(+), Na(+), and K(+)) to phenylalanine (Phe) is studied at the best level of density functional theory reported so far. The different modes of M(+) binding show the same order of binding affinity (Li(+)>Na(+)>K(+)), in the approximate ratio of 2.2:1.5:1.0. The most stable binding mode is one in which the M(+) is stabilized by a tridentate interaction between the cation and the carbonyl oxygen (O[double bond]C), amino nitrogen (--NH(2)), and aromatic pi ring; the absolute Li(+), Na(+), and K(+) affinities are estimated theoretically to be 275, 201, and 141 kJ mol(-1), respectively. Factors affecting the relative stabilities of various M(+)-Phe binding modes and conformers have been identified, with ion-dipole interaction playing an important role. We found that the trend of pi and non-pi cation bonding distances (Na(+)-pi>Na(+)-N>Na(+)-O and K(+)-pi>K(+)-N>K(+)-O) in our theoretical Na(+)/K(+)-Phe structures are in agreement with the reported X-ray crystal structures of model synthetic receptors (sodium and potassium bound lariat ether complexes), even though the average alkali metal cation-pi distance found in the crystal structures is longer. This difference between the solid and the gas-phase structures can be reconciled by taking the higher coordination number of the cations in the lariat ether complexes into account.     

Structure-based discovery of potassium channel blockers from natural products: virtual screening and electrophysiological assay testing

Potassium ion (K(+)) channels are attractive targets for rational drug design. Based upon a three-dimensional model of the eukaryotic K(+) channels, the docking virtual screening approach was employed to search the China Natural Products Database. Compounds were ranked according to the relative binding energy, favorable shape complementarity, and potential of forming hydrogen bonds with the K(+) channel. Four candidate compounds found by virtual screening were investigated by using the whole-cell voltage-clamp recording in rat dissociated hippocampal neurons. When applied extracellularly, compound 1 markedly depressed the delayed rectifier K(+) current (I(K)) and fast transient K(+) current (I(A)), whereas compounds 2, 3, and 4 exerted a more potent and selective inhibitory effect on I(K). Intracellular application of the four compounds had no effect on both the K(+) currents.     

Electrostatic, steric, and hydration interactions favor Na(+) condensation around DNA compared with K(+)

Condensation of monovalent counterions around DNA influences polymer properties of the DNA chain. For example, the Na(+) ions show markedly stronger propensity to induce multiple DNA chains to assemble into compact structures compared with the K(+) ions. To investigate the similarities and differences in the sodium and potassium ion condensation around DNA, we carried out a number of extensive all-atom molecular dynamics simulations of a DNA oligomer consisting of 16 base pairs, [d(CGAGGTTTAAACCTCG)](2), in explicit water. We found that the Na(+) ions penetrate the DNA interior and condense around the DNA exterior to a significantly larger degree compared with the K(+) ions. We have provided a microscopic explanation for the larger Na(+) affinity toward DNA that is based on a combination of steric, electrostatic, and hydration effects. Unexpectedly, we found that the Cl(-) co-ions provide more efficient electrostatic screening for the K(+) ions than for the Na(+) ions, contributing to the larger Na(+) condensation around DNA. To examine the importance of the discrete nature of water and ions, we also computed the counterion distributions from the mean-field electrostatic theory, demonstrating significant disagreements with the all-atom simulations. Prior experimental results on the relative extent of the Na(+) and K(+) condensation around DNA were somewhat contradictory. Recent DNA compaction experiments may be interpreted to suggest stronger Na(+) condensation around DNA compared to K(+), which is consistent with our simulations. We also provide a simple interpretation for the experimentally observed increase in DNA electrophoretic mobility in the alkali metal series, Li(+) < Na(+) < K(+) < Rb(+). We compare the DNA segment conformational preferences in various buffers with the proposed NMR models.     

Spectroscopy of K+ x Rg and transport coefficients of K+ in Rg (Rg=He-Rn)

Ab initio calculations employing the coupled-cluster method, with single and double substitutions and accounting for triple excitations noniteratively [CCSD(T)], are used to obtain accurate potential energy curves for the K(+)He, K(+)Ne, K(+)Ar, K(+)Kr, K(+)Xe, and K(+)Rn cationic complexes. From these potentials, rovibrational energy levels and spectroscopic parameters are calculated. In addition, mobilities and diffusion coefficients for K(+) cations moving through the six rare gases are calculated, under conditions that match previous experimental determinations. A detailed statistical comparison of the present and previous potentials is made with available experimental data, and critical conclusions are drawn as to the reliability of each set of data. It is concluded that the present ab initio potentials match the accuracy of the best model potentials and the most reliable experimental data.     

Insights into the interactions between a drug and a membrane protein target by fluorine cross-polarization magic angle spinning NMR

The fluorinated anti-psychotic drug trifluoperazine (TFP) has been shown to be a K(+)-competitive inhibitor of gastric H(+)/K(+)-ATPase, a membrane-embedded therapeutic target for peptic ulcer disease. This paper describes how variable contact time (19)F cross-polarization magic angle spinning (VCT-CP/MAS) NMR has been used to probe the inhibitory interactions between TFP and H(+)/K(+)-ATPase in native gastric membranes. The (19)F CP/MAS spectra for TFP in H(+)/K(+)-ATPase enriched (GI) gastric membranes and in control membranes containing less than 5 nmol of the protein indicated that the drug associates with the membranes independently of the presence of H(+)/K(+)-ATPase. The (19)F peak intensities in the VCT-CP/MAS experiment confirmed that TFP undergoes slow dissociation (k(off) < 100 s(-1)) from binding sites in GI membranes, and more rapid dissociation (k(off) < 100 s(-1)) from control membranes. The spectra showed that up to 40% of bound TFP was displaced from GI membranes by 100 mM K(+) and by the K(+)-competitive inhibitor TMPIP, but TFP was not displaced from the control membranes. Hence the spectra of TFP in GI membranes represent the drug bound to the K(+)-competitive inhibitory site of H(+)/K(+)-ATPase and to other non-specific sites. The affinity of TFP for the K(+)-competitive site (K(D) = 4 mM) was determined from a binding curve of (19)F peak intensity versus TFP concentration after correction for non-specific binding. The K(D) was much higher than the IC(50) for ATPase inhibition (8 microM), which suggests that the substantial non-specific binding of TFP to the membranes contributes to ATPase inhibition. This novel approach to probing ligand binding can be applied to a wide range of membrane-embedded pharmaceutical targets, such as G-protein coupled receptors and ion channels, regardless of the size of the protein or strength of binding.     

Studies on exchange equilibria of cations between cation-exchange membranes and electrolytic solutions

The variations of the selectivity coefficient K(A)(B) between Na(+)-H(+), Na(+)-K(+), and Na(+)-Cu(2+) systems and the separation factor alpha(A)(B) between Na(+)-Cu(2+) and K(+)-Cu(2+) systems in cation-exchange membranes as functions of loading and particle size of resin have been measured. The exchange affinities of all the membranes increase as H(+)相似文献   

Polyionic charge density plays a key role in differential recognition of mobile ions by biopolymers

We address the question of what are the molecular mechanisms providing discrimination between seemingly similar counterions binding to various biomolecular surfaces. In the case of protein association with Na (+) and K (+) ions, recent works proposed that specificity of carboxylate functional groups interacting with these mobile ions rationalizes the observed ionic discrimination. We probe in this work whether similar arguments may be used to explain higher propensity of Na (+) ions to associate with DNA compared with K (+) ions, which was suggested by our simulations and some experiments. By comparing our extensive molecular dynamics simulations of Na (+) and K (+) distributions around a 16-base-pair DNA oligomer, [(CGAGGTTTAAACCTCG)] 2, with additional simulations where DNA is replaced by a "soup" of monomers (dimethylphosphate anion), we conclude that DNA specificity toward Na (+)/K (+) is not determined by the underlying functional group specificity. Instead, the collective effect of DNA charges drives larger Na (+) association. To gain additional microscopic insights into the mechanisms of specificity on ionic associations in these systems, we carried out energetic analysis of the association between Na (+) and K (+) with chloride and dimethylphosphate anions. The insights gained from our computational work shed light on a number of experiments on electrolyte solutions of monovalent salts and DNA.     

Model and simulation of Na+/K+ pump phosphorylation in the presence of palytoxin

The ATP hydrolysis reactions responsible for the Na(+)/K(+)-ATPase phosphorylation, according to recent experimental evidences, also occur for the PTX-Na(+)/K(+) pump complex. Moreover, it has been demonstrated that PTX interferes with the enzymes phosphorylation status. However, the reactions involved in the PTX-Na(+)/K(+) pump complex phosphorylation are not very well established yet. This work aims at proposing a reaction model for PTX-Na(+)/K(+) pump complex, with similar structure to the Albers-Post model, to contribute to elucidate the PTX effect over Na(+)/K(+)-ATPase phosphorylation and dephosphorylation. Computational simulations with the proposed model support several hypotheses and also suggest: (i) phosphorylation promotes an increase of the open probability of induced channels; (ii) PTX reduces the Na(+)/K(+) pump phosphorylation rate; (iii) PTX may cause conformational changes to substates where the Na(+)/K(+)-ATPase may not be phosphorylated; (iv) PTX can bind to substates of the two principal states E1 and E2, with highest affinity to phosphorylated enzymes and with ATP bound to its low-affinity sites. The proposed model also allows previewing the behavior of the PTX-pump complex substates for different levels of intracellular ATP concentrations.     

High Na+ and K+ -induced fluorescence enhancement of a π-conjugated phenylaza-18-crown-6-triazol-substituted coumarin fluoroionophore

The new π-conjugated 1,2,3-triazol-1,4-diyl fluoroionophore 1 generated via Cu(I) catalyzed [3 + 2] cycloaddition shows high fluorescence enhancement factors (FEF) in the presence of Na(+) (FEF=58) and K(+) (FEF=27) in MeCN and high selectivity towards K(+) under simulated physiological conditions (160 mM K(+) or Na(+), respectively) with a FEF of 2.5 for K(+).     

Experimental validation of theoretical potassium and sodium cation affinities of amides by mass spectrometric kinetic method measurements

In this study the theoretical Gaussian-2 K(+)/Na(+) binding affinities (enthalpies) at 0 K (in kJ mol(-1)) for six amides in the order: formamide (109.2/138.5) < N-methylformamide (117.7/148.6) < acetamide (118.7/149.5) < N,N-dimethylformamide (123.9/156.4) < N-methylacetamide (125.6/157.7) < N,N-dimethylacetamide (129.2/162.6), reported previously (Siu et al., J. Chem. Phys. 2001; 114: 7045-7051), were validated experimentally by mass spectrometric kinetic method measurements. By monitoring the collision-induced dissociation (CID) of K(+)/Na(+)-bound heterodimers of the amides, the relative affinities were shown to be accurate to within +/-2 kJ mol(-1). With these six theoretical K(+)/Na(+) binding affinities as reference values, the absolute K(+)/Na(+) affinities of imidazole, 1-methylimidazole, pyridazine and 1,2-dimethoxyethane were determined by the extended kinetic method, and found to be consistent (to within +/-9 kJ mol(-1)) with literature experimental values obtained by threshold-CID, equilibrium high-pressure mass spectrometry, and Fourier transform ion cyclotron resonance/ligand-exchange equilibrium methods. A self-consistent resolution is proposed for the inconsistencies in the relative order of K(+)/Na(+) affinities of amides reported in the literature. These two sets of validated K(+) and Na(+) affinity values are useful as reference values in kinetic method measurements of K(+)/Na(+) affinity of model biological ligands, such as the K(+) affinities of aliphatic amino acids.     

Solvent effects on the conformation of DNA dodecamer segment: a simulation study

Different solvent temperatures with five kinds of counterions are used to investigate solvent effects on the DNA microscopic structure. The dodecamer d (CGCGAATTCGCG) DNA segment is merged into the solvents and its conformation transition is studied with the molecular dynamics simulations in detail. For the simple point charge model of water molecule with Na(+) counterions, as temperature increases from 200 K to 343 K, the duplex DNA changes from stiff B form to a state between A form and B form, which we define as mixed (A-B) structure, with a double helix unwinding. To study the counterions effects, other four alkali cations, Li(+), K(+), Rb(+), or Cs(+) ions, are substituted for Na(+) ions at 298 K and 343 K, respectively. For the cases of Li(+), Rb(+), and Cs(+) ions, the duplex DNA becomes more flexible with sugar configuration changing form C2'-endo to C1'-endo type and the width and depth of minor groove at CpG and GpC steps moving towards A values, as the mass of the counterions decreasing. For the case of K(+) ions, DNA-K(+) interaction widens the width of minor and major grooves at ApA steps and TpT steps, respectively. It seems that the light ions (Li(+) or Na(+)) prefer to interact with the free phosphate oxygen atoms while the heavier ions (Rb(+) and Cs(+)) strongly interact with the base pairs.     

Extraction of sodium and potassium perchlorates with benzo-18-crown-6 into various organic solvents. Quantitative elucidation of anion effects on the extraction-ability and -selectivity for Na(+) and K(+)

To investigate quantitatively the anion effect on the extraction-ability and -selectivity of benzo-18-crown-6 (B18C6) for alkali metal ions, the constants for overall extraction into various diluents having low dielectric constants (K(ex)) and aqueous ion-pair formation (K(MLA)) of B18C6-sodium and potassium perchlorate 1:1:1 complexes (MLA) were determined at 25 degrees C. The K(ex) value was analyzed by the four fundamental equilibrium constants. The K(MLA) values were determined by applying our established method to this perchlorate extraction system. The K(M(B18C6)A) value of the perchlorate is much larger for K(+) than for Na(+), and is much smaller than that of the picrate. The K(M(B18C6)A) value makes a minor contribution to the magnitude of K(ex) for the perchlorate system, but a major contribution to that for the picrate one. The distribution behavior of the B18C6 1:1:1 complexes with the alkali metal perchlorates follows the regular solution theory. For the diluent with a high dipole moment, however, the 1:1:1 complexes somewhat undergo the dipole-dipole interaction. B18C6 always shows very high extraction selectivity for KClO(4) over NaClO(4), which is determined mostly by the much greater log/(log K(MLA)) value for K(+) than for Na(+). The extraction-ability and -selectivity of B18C6 for Na(+) and K(+) ions with a perchlorate ion were compared with those with a picrate ion in terms of the fundamental equilibrium constants. The K(+) extraction-selectivity of B18C6 over Na(+) for the perchlorate system is superior to that for the picrate one, which is caused largely by the greater log/(log K(K(B18C6)A))-log/(log K(Na(B18C6)A)) value for the perchlorate than for the picrate. The perchlorate system is recommended for extraction separation of K(+) from Na(+).     

A theoretical study of potassium cation binding to glycylglycine (GG) and alanylalanine (AA) dipeptides

By combining Monte Carlo conformational search technique with high-level density functional calculations, the geometry and energetics of K(+) interaction with glycylglycine (GG) and alanylalanine (AA) were obtained for the first time. The most stable K(+)-GG and K(+)-AA complexes are in the charge-solvated (CS) form with K(+) bound to the carbonyl oxygens of the peptide backbone, and the estimated 0 K binding affinities (DeltaH(0)) are 152 and 157 kJ mol(-1), respectively. The K(+) ion is in close alignment with the molecular dipole moment vector of the bound ligand, that is, electrostatic ion-dipole interaction is the key stabilizing factor in these complexes. Furthermore, the strong ion-dipole interaction between K(+) and the amide carbonyl oxygen atom of the peptide bond is important in determining the relative stabilities of different CS binding modes. The most stable zwitterionic (ZW) complex involves protonation at the amide carbonyl oxygen atom and is approximately 48 kJ mol(-1) less stable than the most stable CS form. The usefulness of proton affinity (PA) as a criterion for estimating the relative stability of ZW versus CS binding modes is examined. The effect of chain length and the nature of metal cations on cation-dipeptide interactions are discussed. Based on results of this study, the interaction of K(+) with longer peptides consisting of aliphatic amino acids are rationalized.     

ICP-MS法测定山绿茶降压胶囊无机元素及其质量评价

目的建立了电感耦合等离子体质谱法(ICP-MS)测定不同批次山绿茶降压胶囊内容物中无机元素的含量,从无机元素角度探讨山绿茶降压胶囊的质量控制方法。方法采用微波消解处理山绿茶降压胶囊内容物样品,用ICP-MS对样品中多种无机元素进行全定量测定。结果测定10个不同批次山绿茶降压胶囊内容物的22种无机元素,其中K、Mg、Ca、Zn等元素含量较高,且与药效相关性较强,Cu、As、Hg、Pb、Cd全部符合标准要求。结论本法灵敏度高,专属性好,适用于山绿茶降压胶囊内容物中无机元素测定,为山绿茶降压胶囊内容物重金属含有量及用药提供依据。     

UV and IR spectroscopy of cold 1,2-dimethoxybenzene complexes with alkali metal ions

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of 1,2-dimethoxybenzene (DMB) complexes with alkali metal ions, M(+)·DMB (M = Li, Na, K, Rb, and Cs), in a cold, 22-pole ion trap. The UVPD spectrum of the Li(+) complex shows a strong origin band. For the K(+)·DMB, Rb(+)·DMB, and Cs(+)·DMB complexes, the origin band is very weak and low-frequency progressions are much more extensive than that of the Li(+) ion. In the case of the Na(+)·DMB complex, spectral features are similar to those of the K(+), Rb(+), and Cs(+) complexes, but vibronic bands are not resolved. Geometry optimization with density functional theory indicates that the metal ions are bonded to the oxygen atoms in all the M(+)·DMB complexes. For the Li(+) complex in the S(0) state, the Li(+) ion is located in the same plane as the benzene ring, while the Na(+), K(+), Rb(+), and Cs(+) ions are located off the plane. In the S(1) state, the Li(+) complex has a structure similar to that in the S(0) state, providing the strong origin band in the UV spectrum. In contrast, the other complexes show a large structural change in the out-of-plane direction upon S(1)-S(0) excitation, which results in the extensive low-frequency progressions in the UVPD spectra. For the Na(+)·DMB complex, fast charge transfer occurs from Na(+) to DMB after the UV excitation, making the bandwidth of the UVPD spectrum much broader than that of the other complexes and producing the photofragment DMB(+) ion.