Crystal structures of leucites – past, present, and future?

We have studied the molecular determinants of ion permeation through the TRPV4 channel (VRL-2, TRP12, VR-OAC, and OTRPC4). TRPV4 is characterized by both inward and outward rectification, voltage-dependent block by Ruthenium Red, a moderate selectivity for divalent versus monovalent cations, and an Eisenman IV permeability sequence. We identify two aspartate residues, Asp(672) and Asp(682), as important determinants of the Ca(2+) sensitivity of the TRPV4 pore. Neutralization of either aspartate to alanine caused a moderate reduction of the relative permeability for divalent cations and of the degree of outward rectification. Neutralizing both aspartates simultaneously caused a much stronger reduction of Ca(2+) permeability and channel rectification and additionally altered the permeability order for monovalent cations toward Eisenman sequence II or I. Moreover, neutralizing Asp(682) but not Asp(672) strongly reduces the affinity of the channel for Ruthenium Red. Mutations to Met(680), which is located at the center of a putative selectivity filter, strongly reduced whole cell current amplitude and impaired Ca(2+) permeation. In contrast, neutralizing the only positively charged residue in the putative pore region, Lys(675), had no obvious effects on the properties of the TRPV4 channel pore. Our findings delineate the pore region of TRPV4 and give a first insight into the possible architecture of its permeation pathway.

Leucites are tetrahedrally coordinated silicate framework structures with some of the silicon framework cations that are partially replaced by divalent or trivalent cations. These structures have general formulae A2BSi5O12 and ACSi2O6, where A is a monovalent alkali metal cation, B is a divalent cation, and C is a trivalent cation. There are also leucite analogs with analogous tetrahedrally coordinated germanate framework structures. These have general formulae A2BGe5O12 and ACGe2O6. In this paper, the Rietveld refinements of three synthetic Ge-leucite analogs with stoichiometries of AAlGe2O6 (A = K, Rb, Cs) are discussed. KAlGe2O6 is I41/a tetragonal and is isostructural with KAlSi2O6. RbAlGe2O6 and CsAlGe2O6 are $I\bar{4}3d$ cubic and are isostructural with KBSi2O6.

The Smaller the Better: Hosting Trivalent Rare-Earth Guests in Cu–P Clathrate Cages

Leucites are silicate framework structures with some of the silicon framework cations partially replaced by divalent or trivalent cations. A monovalent extraframework alkali metal cation is also incorporated to balance the charges. We have previously reported Pbca leucite structures with the stoichiometries Cs2X2+Si5O12 (X = Mg, Mn, Co, Ni, Cu, Zn, Cd) and Rb2X2+Si5O12 (X = Mg, Mn, Ni, Cd). These orthorhombic leucite structures have all the silicon and non-silicon framework cations completely ordered onto separate crystallographic sites. This structure has five distinct Si sites and 1 X site; there are also two distinct sites for the extra-framework Cs or Rb. We have recently synthesised leucite analogues with two different extra-framework cations, these have the stoichiometry RbCsX2+Si5O12 (X = Mg, Ni, Cd). The initial Rietveld refinements assumed 50% Cs and 50% Rb on each of the two extra-framework cation sites. The refined structures for X = Ni and Cd have (within error limits) complete extra-framework cation site disorder. However, for X = Mg there is partial ordering of the extra-framework cation sites, the site occupancies are:- Cs1 0.37(3), Rb1 0.63(3), Cs2 0.63(3), Rb2 0.37(3).

Leucites are tetrahedrally coordinated silicate framework structures with some of the silicon framework cations partially replaced by divalent or trivalent cations. These structures have general formulae A2BSi5O12 and ACSi2O6; where A is a monovalent alkali metal cation, B is a divalent cation, and C is a trivalent cation. In this paper, we report the Rietveld refinements of three more synthetic leucite analogues with stoichiometries of Cs2NiSi5O12, RbGaSi2O6, and CsGaSi2O6. Cs2NiSi5O12 is Ia$\bar{3}$d cubic and is isostructural with Cs2CuSi5O12. RbGaSi2O6 is I41/a tetragonal and is isostructural with KGaSi2O6. CsGaSi2O6 is $I\bar{4}3d$ cubic and is isostructural with RbBSi2O6.

Influence of ionic strength, anions, cations, and natural organic matter on the adsorption of pharmaceuticals to silica

We here extend the cluster‐plus‐glue‐atom model, originally developed for metallic glasses, to the interpretation of silicate glass compositions. By referring to β‐SiO 2 crystal structure and to the widely recognized random network model, our model identifies a 16‐basic‐unit (or 32‐cation) composition formula {M 1+ 2 O} n –{M 3+ 2 O 3 } 16−( m + n ) –{M 4+ 2 O 4 } m , where monovalent unit {M 1+ 2 O} and quadrivalent unit {M 4+ 2 O 4 } cannot coexist and the trivalent units {M 3+ 2 O 3 } are constructed from pure trivalent cations such as {(Al,B) 2 O 3 } and from combinations of monovalent and divalent cations with the base quadrivalent Si such as {(Na,K) 2/3 Si 4/3 O 3 } and {(Mg,Ca)SiO 3 }. After analyzing some glasses of historical importance, it is pointed out that most of soda–lime–silica and aluminosilicate glasses satisfy closely {M 3+ 2 O 3 } 16 , as exemplified by ancient glasses as well as by modern ones, such as Container glass, 1980, Jena Standard Glass, and Corning Gorilla glasses of the first generation. On the other hand, borosilicate glasses feature extra {Si 2 O 4 } units ranging from 4 to 9.5 in their 16‐unit formulas, as exemplified by Corning E‐glass and Schott thermometer glass with {Si 2 O 4 } 4 and Corning Pyrex with {Si 2 O 4 } 9.5 containing the highest silica proportion. The number of monovalent cations in 32‐cation formulas follows linear dependences on the average cation valence e / c and on the 16‐unit parameters 2 m − 4 n , showing that, except alkali‐free glasses, the compositions of soda–silicate glasses converge to 2 N 2 + N 3 ≈ 8, where N 2 and N 3 are, respectively, the numbers of divalent and trivalent cations in the 32‐cation formula. The revelation of the composition rule shows the capacity of the cluster‐plus‐glue‐atom model in understanding the compositions of complex glassy systems.

Effect of cation valence on the retrogradation, gelatinization and gel characteristics of maize starch

Our observation reveals the effects of divalent and trivalent cations on the higher-order structure of giant DNA (T4 DNA 166 kbp) by fluorescence microscopy. It was found that divalent cations, Mg(2+) and Ca(2+), inhibit DNA compaction induced by a trivalent cation, spermidine (SPD(3+)). On the other hand, in the absence of SPD(3+), divalent cations cause the shrinkage of DNA. As the control experiment, we have confirmed the minimum effect of monovalent cation, Na(+) on the DNA higher-order structure. We interpret the competition between 2+ and 3+ cations in terms of the change in the translational entropy of the counterions. For the compaction with SPD(3+), we consider the increase in translational entropy due to the ion-exchange of the intrinsic monovalent cations condensing on a highly charged polyelectrolyte, double-stranded DNA, by the 3+ cations. In contrast, the presence of 2+ cation decreases the gain of entropy contribution by the ion-exchange between monovalent and 3+ ions.

Cation specificity and effects of pH and temperature on membrane aggregation of porcine intestinal brush borders were studied in terms of the turbidity change of the membrane suspensions due to the addition of polyvalent cations. Monovalent cations (K+ and Na+) did not induce the membrane aggregation at 5 mM or below. Divalent and trivalent cations induced membrane aggregation even below 0.5 mM in the following orders of effectiveness: Hg2+ much greater than Cd2+ greater than Zn2+ greater than Mn2+ greater than Ca2+ greater than or equal to Ba2+ greater than Sr2+ greater than or equal to Mg2+ and Lu3+ greater than Yb3+ greater than Nd3+ = Er3+ = Tb3+ greater than Ce3+ greater than La3+, respectively. The aggregation of membranes induced by these polyvalent cations was reversible. Membrane aggregation induced by either divalent or trivalent cations followed saturation kinetics with increasing cation concentration and the apparent dissociation constants of these cations for the membranes were estimated to be at 4-6 mM for divalent cations (Ca2+, Mn2+, and Sr2+) and 1-2 mM for trivalent cations (Lu3+, Tb3+, and La3+). Cd2+ and Hg2+ had peculiarly much lower dissociation constants than any other divalent cations tested, being rather comparable to trivalent ones. The membrane aggregation induced by Ca2+, Mn2+, or Sr2+ was independent of pH in the range of 6.6 to 8.0, whereas that induced by Tb3+ or Cd2+ was linearly enhanced with increasing pH, very steeply above pH 7.0. The membrane aggregation induced by Cd2+, Hg2+, or Tb3+ was enhanced steadily with increasing temperature between 15 and 45 degrees C. On the other hand, Ca2+-induced aggregation showed a peak at around 30 degrees C in the temperature dependence profile, still being enhanced above 40 degrees C.

The Iso-Competition Point, a New Concept for Characterizing Multivalent versus Monovalent Counterion Competition, Successfully Describes Cation Binding to DNA

In this study, grand canonical Monte Carlo simulation was carried out to systematically study the effects of extra-framework cations on the capacity of storage and separation of carbon dioxide (CO2) and methane (CH4) in cation-exchanged rho-zeolite-like metal-organic framework (rho-ZMOF). Monovalent (Na+, NH4 + and Li+), divalent (Mg2+ and Sr2+) and trivalent (Al3+) cations with different ion radii were adopted as the representative extra-framework cations. The simulations of the single-component adsorption of CO2 molecules indicate that the varieties in extra-framework cations do not bring evident differences in the dispersion interaction contributions to the isosteric adsorption heats of CO2 but rather the electrostatic interaction contributions. Higher the valence a cation has, stronger the electrostatic interaction with CO2 is and, therefore, a higher storage capacity, though the corresponding accommodation number of the extra-framework cations is smaller. For the cations having the same valence, the storage capacity decreases with the increase in the accessible surface area and total free volume of the host structure. The single-component adsorption isotherms of CO2 and CH4 can be described by a dual-site Langmuir–Freundlich equation. Under typical operating conditions (298 K and 1 atm) in a pressure swing adsorption (PSA) process, the simulation results of the adsorption of CO2 and CH4 mixture demonstrate that the Al-exchanged rho-ZMOF exhibits an unprecedented high selectivity of CO2 over CH4 up to 112, compared with other metal-organic frameworks and nanoporous materials reported to date. Our results suggest that the variation of extra-framework cations is an efficient way to improve the adsorption capacity of the rho-ZMOF for the storage and separation of CO2 and CH4 mixtures using the PSA process at ambient temperature and pressure.

Multifiber responses to olfactory stimulation with alcohols, acids and salts were recorded from the olfactory tract of carp. 1) Alcohols were found to be effective to the olfaction of carp, though the responses were small. We found a tendency that alcohols with longer carbon chain were more stimulative. 2) The magnitude of the initial excitatory response to the stimulation with HCI was large, while that to the stimulation with butyric acid was negligible. The pH value lower than 3.5 regard-less of the kind of acid, however, induced inhibitory responses, which may probably be nonspecific pharmacological effects rather than olfactory responses. 3) Monovalent cations and monovalent anions induced only the initial excitatory responses, and were not accompanied with the oscillatory responses. Divalent cations induced both the initial excitatory and oscillatory responses, but only inhibitory responses were observed with tri-valent cations. The responses to salts were concluded to be largely dependent upon the cations. The salts with monovalent and divalent cations were found to be olfactory stimulants in carp, while those with trivalent cations seemed to be injurious to the olfactory receptors.

The crystal structures and chemical compositions of uranyl-oxysalt minerals (primarily with sheet structural units) are interpreted in terms of the binary representation – bond-valence approach to the structure and chemistry of oxysalts. There is a coherent relation between the average basicity of the structural unit and [CN in ], the average number of bonds to oxygen atoms of the structural unit from the interstitial complex and adjacent structural units. This relation allows calculation of the range of Lewis basicity for specific structural units. The Lewis acidity of an interstitial complex is expressed graphically as a function of the amounts and coordination numbers of monovalent, divalent and trivalent interstitial cations and the amount of interstitial transformer (H 2 O) groups. The range in Lewis basicity for a specific structural unit may also be expressed graphically, and where there is overlap of the Lewis acidity and Lewis basicity, the valence-matching principle is satisfied, and the details of the possible interstitial complexes can be derived. There are three distinct types of interstitial (H 2 O) groups: transformer, non-transformer and inverse-transformer. Inverse-transformer (H 2 O) groups accept three bonds from cations, other (H 2 O) groups and (OH) groups of the structural unit. Their occurrence is rare and is limited to minerals with low bond-valence distribution factors. Detailed predictions of the number of transformer, non-transformer and inverse-transformer (H 2 O) groups in the minerals of the meta-autunite, uranophane, phosphuranylite, carnotite, zippeite and uranyl-hydroxy-hydrate groups (and synthetic analogues) are in good agreement with the observed chemical compositions.

We compared the effects of a series of di- and trivalent cations on various aspects of parathyroid function to investigate whether these polyvalent cations act on the parathyroid cell through a similar mechanism. Like high extracellular concentrations of Ca2+, high levels of barium (Ba2+), strontium (Sr2+), gadolinium (Gd3+), europium (Eu3+), terbium (Tb3+), and ytterbium (Yb3+) [corrected] each inhibited low calcium-stimulated PTH release and showed IC50 values (the concentration producing half of the maximal inhibitory effect) of 1.12 mM, 1.18 mM, 2.2 microM, 2.5 microM, 0.89 microM, and 15 microM, respectively. The inhibitory effects of both divalent (Ca2+ and Ba2+) and trivalent (Gd3+) cations were reversible by 76-100% after removal of the cation, suggesting that the polyvalent cation-mediated reduction in PTH release was not due to nonspecific toxicity. The same di- and trivalent cations produced an 80-90% decrease in agonist-stimulated cAMP accumulation with a similar order of potency as for their effects on PTH release. Preincubation overnight with pertussis toxin totally prevented the inhibitory effects of the trivalent cations on cAMP accumulation. The same di- and trivalent cations also increased the accumulation of inositol monophosphate, inositol bisphosphate, and inositol trisphosphate. Their effects on this parameter differed from those on PTH release and cAMP accumulation in several respects. First, Ba2+ and Sr2+, rather than being equipotent with Ca2+, were about 2-fold less potent in increasing the levels of inositol phosphates. Second, the trivalent cations were 5-50-fold less potent in raising inositol phosphates than in modulating PTH release and cAMP accumulation, and all were nearly equipotent. These results show that trivalent cations of the lanthanide series mimic the actions of divalent cations on several aspects of parathyroid function, and likely do so by interacting with the cell surface "Ca2(+)-receptor-like mechanism" through which extracellular Ca2+ has been postulated to act. The pharmacology of the effects of these polyvalent cations on cAMP and PTH release are similar and differ from that for their actions on inositol phosphate metabolism, raising the possibility that there might be more than one form of the putative Ca2+ receptor.