Electrostatic effects on the stability of condensed DNA in the presence of divalent cations.

Cylindrical cell model Poisson-Boltzmann (P-B) calculations are used to evaluate the electrostatic contributions to the relative stability of various DNA conformations (A, B, C, Z, and single-stranded (ss) with charge spacings of 3.38 and 4.2 A) as a function of interhelix distance in a concentrated solution of divalent cations. The divalent ion concentration was set at 100 mM, to compare with our earlier reports of spectroscopic and calorimetric experiments, which demonstrate substantial disruption of B-DNA geometry. Monovalent cations neutralize the DNA phosphates in two ways, corresponding to different experimental situations: 1) There is no significant contribution to the ionic strength from the neutralizing cations, corresponding to DNA condensation from dilute solution and to osmotic stress experiments in which DNA segments are brought into close proximity to each other in the presence of a large excess of buffer. 2) The solution is uniformly concentrated in DNA, so that the neutralizing cations add significantly to those in the buffer at close DNA packing. In case 1), conformations with lower charge density (Z and ssDNA) have markedly lower electrostatic free energies than B-DNA as the DNA molecules approach closely, due largely to ionic entropy. If the divalent cations bind preferentially to single-stranded DNA or a distorted form of B-DNA, as is the case with transition metals, the base pairing and stacking free energies that stabilize the double helix against electrostatic denaturation may be overcome. Strong binding to the bases is favored by the high concentration of divalent cations at the DNA surface arising from the large negative surface potential; the surface concentration increases sharply as the interhelical distance decreases. In case 2), the concentration of neutralizing monovalent cations becomes very large and the electrostatic free energy difference between secondary structures becomes small as the interhelical spacing decreases. Such high ionic concentrations will be expected to modify the stability of DNA by changing water activity as well as by screening electrostatic interactions. This may be the root of the decreased thermal stability of DNA in the presence of high concentrations of magnesium ions.     

Energy-dependent transport of manganese into yeast cells and distribution of accumulated ions.

Manganese transport into yeast cells is energy-dependent. It is dependent on endogenous sources of energy and is inhibited by olygomycin (12.5-25 microgramg/ml), 2,4-dinitrophenol (1 mM), 2-deoxyglucose (1-50 mM) and sodium azide (1-10 mM), but is stimulated by cyanide and glucose. The stimulating effect of glucose is eliminated by N-ethylmaleimide and iodoacetate, which apparently inhibit the transport of glucose itself. About 75% of the manganese accumulated in the presence of glucose is found in yeast protoplasts and nearly 25% in the cell walls. A major portion of the accumulated manganese is found in vacuoles. The concentration of osmotically free manganese in the cytosol did not exceed 2 mM, but the concentration in vacuoles was up to 14 mM. The tonoplast is assumed to have a transport system for divalent cations, thereby regulating their concentration in the cytosol.     

Kinetics of Ca2+ and Sr2+ uptake by yeast. Effects of pH, cations and phosphate.

The uptake of Ca2+ and Sr2+ by the yeast Saccharomyces cerevisiae is energy dependent, and shows a deviation from simple Michaelis-Menten kinetics. A model is discussed that takes into account the effect of the surface potential and the membrane potential on uptake kinetics. The rate of Ca2+ and Sr2+ uptake is influenced by the cell pH and by the medium pH. The inhibition of uptake at low concentration of Ca2+ and Sr2+ at low pH may be explained by a decrease of the surface potential. The inhibition of Ca2+ and Sr2+ uptake by monovalent cations is independent of the divalent cation concentration. The inhibition shows saturation kinetics, and the concentration of monovalent cation at which half-maximal inhibition is observed, is equal to the affinity constant of this ion for the monovalent cation transport system. The inhibition of divalent cation uptake by monovalent cations appears to be related to depolarization of the cell membrane. Phosphate exerts a dual effect on uptake of divalent cations: and initial inhibition and a secondary stimulation. The inhibition shows saturation kinetics, and the inhibition constant is equal to the affinity constant of phosphate for its transport mechanism. The secondary stimulation can only partly be explained by a decrease of the cell pH, suggesting interaction of intracellular phosphate, or a phosphorylated compound, with the translocation mechanism.     

Persister cells, the biofilm matrix and tolerance to metal cations in biofilm and planktonic Pseudomonas aeruginosa

In this study, we examined Pseudomonas aeruginosa ATCC 27853 biofilm and planktonic cell susceptibility to metal cations. The minimum inhibitory concentration (MIC), the minimum bactericidal concentration (MBC) required to eradicate 100% of the planktonic population (MBC 100), and the minimum biofilm eradication concentration (MBEC) were determined using the MBEC trade mark-high throughput assay. Six metals - Co(2+), Ni(2+), Cu(2+), Zn(2+), Al(3+) and Pb(2+)- were each tested at 2, 4, 6, 8, 10 and 27 h of exposure to biofilm and planktonic cultures grown in rich or minimal media. With 2 or 4 h of exposure, biofilms were approximately 2-25 times more tolerant to killing by metal cations than the corresponding planktonic cultures. However, by 27 h of exposure, biofilm and planktonic bacteria were eradicated at approximately the same concentration in every instance. Viable cell counts evaluated at 2 and 27 h of exposure revealed that at high concentrations, most of the metals assayed had killed greater than 99.9% of biofilm and planktonic cell populations. The surviving cells were propogated in vitro and gave rise to biofilm and planktonic cultures with normal sensitivity to metals. Further, retention of copper by the biofilm matrix was investigated using the chelator sodium diethlydithiocarbamate. Formation of visible brown metal-chelates in biofilms treated with Cu(2+) suggests that the biofilm matrix may coordinate and sequester metal cations from the aqueous surroundings. Overall, our data suggest that both metal sequestration in the biofilm matrix and the presence of a small population of 'persister' cells may be contributing factors in the time-dependent tolerance of both planktonic cells and biofilms to high concentrations of metal cations.     

Effects of divalent cations,temperature, osmotic pressure gradient,and vesicle curvature on phosphatidylserine vesicle fusion

Summary Fusion of phosphatidylserine vesicles induced by divalent cations, temperature and osmotic pressure gradients across the membrane was studied with respect to variations in vesicle size. Vesicle fusion was followed by two different methods: 1) the Tb/DPA fusion assay, whereby the fluorescent intensity upon mixing of the internal aqueous contents of fused lipid vesicles was monitored, and 2) measurement of the changes in turbidity of the vesicle suspension due to vesicle fusion. It was found that the threshold concentration of divalent cations necessary to induce vesicle fusion depended on the size of vesicles; as the diameter of the vesicle increased, the threshold value increased and the extent of fusion became less. For the osmotic pressure-induced vesicle fusion, the larger the diameter of vesicles, the smaller was the osmotic pressure gradient required to induce membrane fusion. Divalent cations, temperature increase and vesicle membrane expansion by osmotic pressure gradient all resulted in increase in surface energy (tension) of the membrane. The degree of membrane fusion correlated with the corresponding surface energy changes of vesicle membranes due to the above fusion-inducing agents. The increase in surface energy of 9.5 dyn/cm from the reference state corresponded to the threshold point of phosphatidylserine membrane fusion. An attempt was made to explain the factors influencing fusion phenomena on the basis of a single unifying theory.     

The effect of monovalent cations on calcium efflux in yeasts

The properties of the calcium efflux system in the yeast Saccharomyces cerevisiae were investigated. After growing the cells overnight in medium containing ⁴⁵Ca, the cells were transferred to medium containing glucose, Hepes buffer (pH 5.2) and monovalent cations. The presence of potassium or sodium in the medium induced efflux of calcium from the cells. The magnitude of the efflux was dependent on the concentration of these cations in the medium. The time course of calcium efflux was analyzed, and two types of exchangeable calcium pools, which turned over at different rates, were detected: ‘Fast turnover’ and ‘slow turnover’. Increase in the concentration of monovalent cations in the medium caused an increase in the fraction of cellular calcium which turned over at a fast rate, and activation of calcium efflux from the ‘slow turnover’ calcium pool. The specific changes in the parameters of calcium efflux induced by monovalent cations were different from those reported previously to be induced by divalent cations. Both processes, i.e. activation of calcium efflux by monovalent and by divalent cations, were found to be additive, indicating that they operate via different mechanisms. Experiments using the respiratory inhibitor Antimycin A, showed that stimulation of calcium efflux by monovalent cations is energy dependent. Lanthanum ions which are known to inhibit calcium influx into yeast cells, inhibitted the activation of calcium efflux by both divalent and monovalent cations. Determination of the cationic composition of the cells indicated that the stimulation of calcium efflux was accompanied by influx of potassium or sodium into the cells.     

Interactions between liposomes and cations in aqueous solution

An investigation on the dependence of electrophoretic mobilities of unilamellar vesicles of phosphatidylcholine-cholesterol-phosphatidylinositol (PC-Chol-PI) on the concentration of several cations with variations in the relation charge/radius in the range Na+, K+, Cs+, Mg2+, Ca2+, Ba2+, Al3+, and La3+ has been realized. Plots of zeta potential against ion concentration exhibit a maximum for all the cations under study, the position of the maximum is greatly affected by the charge of the ion. From the feature of these plots two phenomenon were observed: an initial binding of cations into the slipping plane for ion concentration below the maximum and a phenomenon of vesicle association for concentration above the maximum. To confirm these observations measurements on dynamic light scattering were performed to obtain the corresponding size distribution of the liposomes at different ion concentrations. Finally the ability of the Stern isotherm to describe the adsorption of the cations to vesicles was tested by two methods. The two main parameters of the theory: the total number of adsorption sites per unit area, N1, and the equilibrium constant, K; (and consequently the free energy of adsorption, deltaG0ads) were calculated for the different ions, showing good agreement. The equilibrium constants of adsorption have been found to obey a linear relationship with ion radius the slope of which decreases with the ion charge.     

Interaction of metal cations with Ca2+-transport sites of the plasma membrane Ca2+ pump of secretory cells of gastric glands

Investigation of Ca2+ transport by calcium pump of the cell plasma membrane of the gastric glands isolated from guinea pigs and its inhibition by metal cations has been performed. The mainly competitive type of Ca2+ translocation inhibition by the calcium pump by metals cations (0.025-1.00 mM) was determined. Potency of inhibition increases in such an order (I50, mM): Ba2+ (0.336) < Sr2+ (0.251) < Mn2+ (0.099) < Co2+ (0.029) < Cd2+ (0.016). It was shown by one-factor dispersion analysis that potency of inhibition depends on ionic radii and hydration enthalpy of metal cations and also on stability constants of their complexes with oxygen-containing bioligands (acetic, aspartic and glutamic acid) (hx2 = 83.73-85.95). Dependence of the inhibition constants (I50) on ionic radii is most adequately described by the parabolic equation, such a dependence on hydration enthalpy and stability constants with oxygen-containing bioligands--by exponential or multiplicative equations. The conclusion has been made that selective Ca2+ translocation by the calcium pump and its inhibition by metal cations is determined by the interaction between energy of their interaction with cation-binding sites of the transport system and energy of hydration. Energetics of such interactions depends on the steric factors. The physicochemical model of the Ca2+ selective translocation by calcium pump and its inhibition by metal cations has been proposed.     

重金属离子对猪红细胞膜Ca~(2+)-ATP酶的作用

猪红细胞膜Ca~(2+)-ATP酶是一种钙调蛋白(CaM)依赖酶,其活力又依赖巯基的完整性。实验应用Ca~(2+)-ATP酶这一模型体系观察到重金属离子,Pb~(2+)、Cd~(2+)和Hg~(2+)都能替代Ca~(2+),激活CaM,从而激活Ca~(2+)-ATP酶;其最大刺激活力分别为85％、80％和30％,半刺激浓度分别为32、27和0.7μmol/L。当三种重金属离子的浓度增加时,则与Ca~(2+)-ATP酶的巯基结合,抑制酶的活力,Pb2~(2+)、Cd~(2+)和Hg~(2+)的半抑制浓度分别为370、440和2μmol/L。抑制作用为渐进性过程,而刺激作用为即时效应。抑制作用可为巯基化物,特别是二巯基化物所逆转。研究结果提示,CaM可能是重金属中毒最初作用的靶分子,而重金属中毒不仅使CaM“开关”失灵,还可能导致细胞内Ca~(2+)的调节全面失控。     

Differential Cytotoxicities of N-Methyl-β-Carbolinium Analogues of MPP+ in PC12 Cells: Insights into Potential Neurotoxicants in Parkinson's Disease

N-Methylated β-carbolinium cations that can form in vivo from environmental or endogenous β-carbolines are putative neurotoxic factors in Parkinson's disease. The cytotoxicities of 11 N-methylated β-carbolinium cations and N-methyl-4-phenylpyridinium cation (MPP⁺), the experimental parkinsonian neurotoxicant which the carbolinium cations structurally resemble, were examined using rat pheochromocytoma (PC12) cells cultured in “low energy” N-5 medium; cell death was estimated by released lactate dehydrogenase activity and viable cell protein. Of the eight N-monomethylated β-carbolinium cations utilized, only 2-methyl-harmalinium (harmaline-2- methiodide) was as cytotoxic as MPP⁺. Also, three N²(β), N⁸(indole)-dimethylated β-carbolinium cations displayed cytotoxic effects, with the simplest, 2,9-dimethylnorhar- manium, approaching the effectiveness of MPP⁺ in PC12 cells cultured in N-5 medium. However, when PC12 cells grown in higher energy Dulbecco's modified Eagle's medium were utilized with selected effective cations, it was observed that the cultures were relatively resistant to MPP⁺ and 2,9-dimethylnorharmanium, but remained vulnerable to 2-methylharmalinium. The results are interpreted to mean that different cytotoxic mechanisms exist for the two most potent β-carbolinium cations—namely, a mechanism for the 2,9-dimethyl-β-carbolinium species that, as with MPP⁺, is conditional on mitochondrial ATP depletion, but a different (or additional) mechanism for 2- methylharmalinium that is independent of mitochondrial inhibition. The possible accumulation of these cytotoxic cations in Parkinson's disease is discussed in the context of these findings.     

A metabolic osmotic model of human erythrocytes

A metabolic osmotic model of red blood cells is presented which takes into account the main reaction steps of glycolysis and the passive and active fluxes of ions across the cell membrane. Cellular energy metabolism and osmotic behaviour are linked by the ATP consumption for the active transport of cations as well as by the osmotic action of the glycolytic intermediate 2,3-diphosphoglycerate (2,3-DPG). The model is based on a system of differential equations describing the metabolic reactions and transport processes. Further, two algebraic conditions for the osmotic equilibrium and the electroneutrality of the cell are considered. Using realistic system parameters the model allows the calculation of a great number of dependent variables, among them the cell volume, the concentrations of metabolites and ions and the transmembrane potential. Only stationary states are considered.The parameter dependence of important model variables is characterized by control coefficients. The main results are: (a) The volume of erythrocytes is mainly determined by the permeabilities of the leak fluxes of cations, the content of hemoglobin and the activity of the hexokinase-phosphofructokinase system of glycolysis; (b) Changes of volume affect the glycolytic rate mainly by changing the concentration of ATP which is a regulator of glycolysis; (c) A change in the membrane area may affect the other cell properties only if it is connected with variations of the number of active and leak sites of the membrane.     

Calcium and magnesium effect on divalent cation deficientHydrilla verticillata thylakoid electron transport activity

The role of divalent cations like magnesium (Mg²⁺) and calcium (Ca²⁺) was irrvestigated on energy distribution process ofHydrilla verticillata thylakoids. Effect of these cations was tested on relative quantum yield of photosystem (PS) II catalyzed electron transport activity, room and liquid nitrogen temperature fluorescence emission properties and thylakoid light scattering characteristics. The electron transport activity was found to be stimulated in the presence of these cations in a light intensity independent manner. The concentration of cation required for maximum stimulation was nearly 10–12 mM. Comparatively, Ca²⁺ was more effective than Mg²⁺. Cation induced stimulation in electron transport activity was not accompanied by increase in chlorophylla fluorescence intensity either at room (25°C) or liquid nitrogen (77°K) temperatures. Furthermore, 540 nm absorption and 90° light scattering properties of thylakoids remained insensitive towards divalent cations. These facts together suggest that divalent cations inHydrilla thylakoids are not effective in supporting the excitation distribution between the interacting photosystem complexes.     

Ion permeability and strength of cell contacts

Summary The effects of several simple parameters (pH, concentration of bivalent cations, osmotic pressure, and temperature) on the ion permeability and mechanical properties of cell contacts have been investigated. It has been shown that the mechanical properties of a cell contact make it possible to describe it as a viscoelastic system. The main contribution to cell adhesion is made by the tight junction. Two populations of acidic centers have been identified on the cell membrane surface. One population interacts with bivalent cations to assure cell adhesion. The other population of weaker acidic centers regulating ion permeation is involved in the cell membrane's interaction of the repulsion type. An intimate correlation has been established between changes in passive transepithelial ion permeability and cell adhesion in response to changes in pH and in bivalent cation concentration. Such a correlation is possible if the tight junction is the principal contributor to the passive ion permeability and mechanical strength of the cell contacts.     

Polyamine replacement by magnesium ions in BHK-21/C13 cells

Cultures of BHK-21/C13 cells, whose growth was inhibited by deprivation of serum, were stimulated to grow by addition of serum to the culture medium. Addition of MgCl(2) to the medium, to increase the concentration of Mg(2+) ions by 15mm, 30min before addition of serum, had no effect on the stimulation of cell growth, but inhibited the accumulation of cellular spermidine, so that the spermidine/spermine molar ratio was lower in these cultures than in cultures that had received no additional cations. The increase in the activity of ornithine decarboxylase that occurs 4-5h after serum ;step-up' was substantially diminished by increasing the concentration of Mg(2+) ions, but not of Na(+) or K(+) ions, in the medium by 30mm, 30min before addition of serum, and this inhibition was maintained for at least 24h. Methylglyoxal bis(guanylhydrazone), added to serum-deprived cultures to a concentration of 20mum, 30min before addition of serum, severely inhibited the increase in cell growth. The inhibitory effects of the drug were prevented by simultaneous addition of spermidine to the medium (to 100mum), and were partly prevented by the simultaneous addition of Mg(2+) ions (to 30mm). Mg(2+) ions were particularly effective in overcoming the inhibitory effect of methylglyoxal bis(guanylhydrazone) on the synthesis of DNA. Thus although a certain lack of specificity for cations exists in BHK-21/C13 cells, in that Mg(2+) ions can be substituted for polyamines, particularly spermidine, to some extent, there are cellular processes for which the requirement for polyamines as cations is specific.     

Molecular origin of the cation selectivity in OmpF porin: single channel conductances vs. free energy calculation

Ion current through single outer membrane protein F (OmpF) trimers was recorded and compared to molecular dynamics simulation. Unidirectional insertion was revealed from the asymmetry in channel conductance. Single trimer conductance showed particularly high values at low symmetrical salt solution. The conductance values of various alkali metal ion solutions were proportional to the monovalent cation mobility values in the bulk phase, LiCl相似文献   

The effect of chitosan and other polycations on tight junction permeability in the human intestinal Caco-2 cell line(1)

Chitosan is a polycationic compound widely employed as dietary supplement and also present in pharmaceutical preparations. Although it has been approved for human consumption, its possible side effects have not been widely investigated and the available data in the literature are still controversial. Several polycationic substances have been shown to affect tight junction permeability in epithelial cell models in vitro. In this study we have compared the effects of chitosan and other polycations (polyethylenimine, poly-L-lysines of different molecular weights) on the integrity of tight junctions and of the actin cytoskeleton in the human intestinal Caco-2 cell line. We have measured trans-epithelial electrical resistance and paracellular passage of the extracellular marker inulin, and we have localized F-actin and tight junctional proteins (ZO1 and occludin) in cell monolayers treated with various concentrations of each polycation. Fluorescent poly-L-lysines were also employed to determine their association with the cell monolayer. Our results indicate that all polycations investigated are able to induce a reversible increase in tight junction permeability. This effect is concentration and energy dependent, affected by the extracellular concentration of divalent cations (calcium, magnesium and manganese) and it is associated with morphological changes in the F-actin cytoskeleton, as well as in the localization of tight junctional proteins. Chitosan, in particular, was the only cationic polymer that displayed an irreversible effect on tight junctions at the highest concentration tested (0.01%). These results indicate that oral ingestion of chitosan may have more widespread health effects by altering intestinal barrier function, thus allowing the entrance into the circulation of potentially toxic and/or allergenic substances.     

Inhibition of cell-wall autolysis and pectin degradation by cations.

Modification of cell wall components such as cellulose, hemicellulose and pectin plays an important role in cell expansion. Cell expansion is known to be diminished by cations but it is unknown if this results from cations reacting with pectin or other cell wall components. Autolysis of cell wall material purified from bean root (Phaseolus vulgaris L.) occurred optimally at pH 5.0 and released mainly neutral sugars but very little uronic acid. Autolytic release of neutral sugars and uronic acid was decreased when cell wall material was loaded with Ca, Cu, Sr, Zn, Al or La cations. Results were also extended to a metal-pectate model system, which behaved similarly to cell walls and these cations also inhibited the enzymatic degradation by added polygalacturonase (EC 3.2.1.15). The extent of sugar release from cation-loaded cell wall material and pectate gels was related to the degree of cation saturation of the substrate, but not to the type of cation. The binding strength of the cations was assessed by their influence on the buffer capacity of the cell wall and pectate. The strongly bound cations (Cu, Al or La) resulted in higher cation saturation of the substrate and decreased enzymatic degradability than the weakly held cations (Ca, Sr and Zn). The results indicate that the junction zones between pectin molecules can peel open with weakly held cations, allowing polygalacturonase to cleave the hairy region of pectin, while strongly bound cations or high concentrations of cations force the junction zone closed, minimising enzymatic attack on the pectin backbone.     

Biofilm susceptibility to metal toxicity

This study compared bacterial biofilm and planktonic cell susceptibility to metal toxicity by evaluating the minimum inhibitory concentration (MIC), the planktonic minimum bactericidal concentration (MBC), and minimum biofilm eradication concentration (MBEC) using the MBEC device. In total, 17 metal cations and oxyanions, chosen to represent groups VIB to VIA of the periodic table, were each tested on biofilm and planktonic cultures of Escherichia coli JM109, Staphylococcus aureus ATCC 29213, and Pseudomonas aeruginosa ATCC 27853. In contrast to control antibiotic assays, where biofilm cultures were 2 to 64 times less susceptible to killing than logarithmically growing planktonic bacteria, metal compounds killed planktonic and biofilm cultures at the same concentration in the vast majority of combinations. Our data indicate that, under the conditions reported, growth in a biofilm does not provide resistance to bacteria against killing by metal cations or oxyanions.     

Effects of polyamines on the thermotropic behaviour of dipalmitoylphosphatidylglycerol

Using differential scanning calorimetry the thermotropic behaviour of the acidic phospholipid dipalmitoylphosphatidylglycerol (DPPG) was studied as a function of pH, concentration of NaCl and the concentration of organic polyvalent cations, polyamines. Although changes in hydration forces and hydrophobic effect may be involved the measured pH/[NaCl]/Tm phase diagram for DPPG suggests that, even at pH 8.5, DPPG is incompletely deprotonated. In terms of energetics however, the observed changes of about 1 degree C are relatively minor corresponding to a bilayer free energy change of 30 cal/mol. The effects of spermine and spermidine on the phase transition of DPPG were strongly concentration dependent even in the micromolar range. The observed effects correlated to the number of positive charges in these cations and thus the greatest effect was observed for spermine. Specific and nonspecific interactions between DPPG and spermine are suggested to exist which are both reversed by high concentrations of NaCl. These results indicate that under proper conditions changes in the intracellular concentrations of polyamines may be capable of triggering an isothermal phase change in membrane acidic phospholipids.     

Mass-Action Expressions of Ion Exchange Applied to Ca, H, K, and Mg Sorption on Isolated Cells Walls of Leaves from Brassica oleracea

The cation exchange properties of cell walls isolated from collard (Bassica oleracea var acephala D.C.) leaves were investigated. Cation sorption on cell walls was described by mass-action expressions of ion exchange, rather than by the traditional Donnan equilibrium. The mass-action expressions enable the selectivity of the wall for one cation over another to be determined unambiguously from ion exchange isotherms. We found that: (a) the cation composition of the wall varied as a function of the solution cation concentration, solution cation composition, and pH in a way predicted by mass action; (b) the affinity of the wall for divalent cations increased as the equivalent fraction of divalent cation on the wall increased, and as the concentration of divalent cations in solution increased; (c) the selectivity of the wall for any metal cation pair was not altered by the concentration of H⁺ in solution or on the wall; (d) H⁺ sorption on the wall may be treated as a cation exchange reaction making it possible to calculate the relative affinity of the wall for metal cation pairs from H⁺-metal (Me) titration curves; and (e) the relative affinity of the wall for the cations we studied was: H⁺ (K⁺ ≥ Ca²⁺) > Mg²⁺. A cation-exchange model including surface complexes is consistent with observed cation selectivity. We conclude that metal cations interact with the wall to minimize or eliminate long-range electrostatic interactions and suggest that this may be due to the formation of site-specific cation-wall surface complexes.