Charge Density Of Cation Research Articles

Insight into Indole Interactions from Alkali Metal Chloride Effects on a Tryptophan Zipper β-Hairpin Peptide

Weakly solvated, low charge density, alkali metal cations (K+ and Rb+) destabilize tryptophan zipper (trpzip) peptides with an effectiveness (for Rb+) similar to that of the protein denaturant urea. An analysis of alkali metal cation effects on polypeptides stabilized predominantly either by hydrogen bonds or by the classical hydrophobic effect indicates that the alkali metals attenuate stabilizing interactions involving the tryptophan indole groups. Destabilization does not result from electrolyte screening of the electrostatic component of the indole-indole interaction, but is likely to involve direct interaction of the low charge density cation with the indole group in a cation-pi interaction. The observations highlight a general simplicity in the nature of molecular interactions in solution, in which stabilizing contributions to polypeptide and protein structures are attenuated by solutes of a complementary nature.

Tuning the Band Gap in Hybrid Tin Iodide Perovskite Semiconductors Using Structural Templating

Structural distortions within the extensive family of organic/inorganic hybrid tin iodide perovskite semiconductors are correlated with their experimental exciton energies and calculated band gaps. The extent of the in- and out-of-plane angular distortion of the SnI4(2-) perovskite sheets is largely determined by the relative charge density and steric requirements of the organic cations. Variation of the in-plane Sn-I-Sn bond angle was demonstrated to have the greatest impact on the tuning of the band gap, and the equatorial Sn-I bond distances have a significant secondary influence. Extended Hückel tight-binding band calculations are employed to decipher the crystal orbital origins of the structural effects that fine-tune the band structure. The calculations suggest that it may be possible to tune the band gap by as much as 1 eV using the templating influence of the organic cation.

Influence of Smectite Hydration and Swelling on Atrazine Sorption Behavior

Smectites, clay minerals commonly found in soils and sediments, vary widely in their ability to adsorb organic chemicals. Recent research has demonstrated the importance of surface charge density and properties of exchangeable cations in controlling the affinity of smectites for organic molecules. In this study, we induced hysteresis in the crystalline swelling of smectites to test the hypothesis that the extent of crystalline swelling (or interlayer hydration status) has a large influence on the ability of smectites to adsorb atrazine from aqueous systems. Air-dried K-saturated Panther Creek (PC) smectite swelled less (d(001) = 1.38 nm) than never-dried K-PC (d(001) = 1.7 nm) when rehydrated in 20 mM KCl. Correspondingly, the air-dried-rehydrated K-PC had an order of magnitude greater affinity for atrazine relative to the never-dried K-PC. Both air-dried-rehydrated and never-dried Ca-PC expanded to approximately 2.0 nm in 10 mM CaCl2 and both samples had similar affinities for atrazine that were slightly lower than that of never-dried K-PC. The importance of interlayer hydration status in controlling sorption affinity was confirmed by molecular modeling, which revealed much greater interaction between interlayer water molecules and atrazine in a three-layer hydrate relative to a one-layer hydrate. The entropy change on moving atrazine from a fully hydrated state in the bulk solution to a partially hydrated state in the smectite interlayers is believed to be a major factor influencing sorption affinity. In an application test, choice of background solution (20 mM KCl versus 10 mM CaCl2) and air-drying treatments significantly affected atrazine sorption affinities for three-smectitic soils; however, the trends were not consistent with those observed for the reference smectite. Further, extending the initial rehydration time from 24 to 240 h (prior to adding atrazine) significantly decreased the soil's sorption affinity for atrazine. We conclude that interlayer hydration status has a large influence on the affinity of smectites for atrazine and that air-drying treatments have the potential to modify the sorption affinity of smectitic soils for organic molecules such as atrazine.

Limiting transport properties of lanthanide and actinide ions in pure water

Summary The limiting transport properties, i.e. the limiting ionic conductivity (λ°) and the limiting diffusion coefficient (D°), of lanthanide and actinide ions at 298.15K have been calculated by means of the microscopic version of the Stokes–Einstein law involving (i) the effective charge and the ionic radius of the ions and (ii) the ion micro-viscosity, i.e. the viscosity of the hydrated water molecules in the vicinity of the ion. The latter quantity was derived from the variation of the dynamic properties of the water molecules in the first hydration shell with the surface charge density of common mono-atomic cations of various charges. The obtained results were found to be consistent with the experimental data given in the literature.

(CH3)3NCH2CH2NH3]SnI4: a layered perovskite with quaternary/primary ammonium dications and short interlayer iodine-iodine contacts.

The organic-inorganic hybrid [(CH(3))(3)NCH(2)CH(2)NH(3)]SnI(4) presents a layered perovskite structure, templated by an organic dication containing both a primary and a quaternary ammonium group. Due to the high charge density and small size of the organic cation, the separation of the perovskite layers is small and short iodine-iodine contacts of 4.19 A are formed between the layers. Optical thin-film measurements on this compound indicate a significant red shift of the exciton peak (630 nm) associated with the band gap, as compared with other SnI(4)(2)(-)-based layered perovskite structures.

Ion-exchange Behavior between Sodium Ion and Other Metal Cations in Hydrated Molybdenum Bronzes

Ion-exchange behavior between sodium ion and various metal cations (alkali, alkaline earth and a few transition metal cations) in hydrated molybdenum bronzes were investigated in [NaCl] +n [M (n+) Cln] =1M solution. The selectivity of ion exchange was in the order of Cs>Rb>K>Li for alkalin metal and Ba>Sr>Ca>Mg for alkaline earth metals. Transition metal cations gave similar ion exchange behavior to each other. The effect of the change in structure was hardly observed. The results suggest that the ion exchange behavior depends on the property of metal ions. The change in MoO3 interlayer spacing depends on the hydration energy, i.e. the surface charge density of metal cations. Hydrated sodium bronze can exchange with other metals and can extract metal ions in the solution. This property may be useful as the ion exchanger

Binding of acetylcholine and tetramethylammonium to a cyclophane receptor: anion's contribution to the cation-pi interaction.

The interaction of the lipophilic cyclophane 1 with several acetylcholine (ACh) and tetramethylammonium (TMA) salts has been investigated in deuteriochloroform to ascertain the influence of the counterion on the cation-pi interaction. Reliable association constants have been measured for 17 salts of commonly used anions; corresponding binding free energies -DeltaG degrees ranged from over 8 kJ mol(-1) down to the limit of detection. The dramatic dependence of the binding energy on the anion showed that the latter takes part in the process with a passive and adverse contribution, which inhibits cation binding even to complete suppression in unfavorable cases. Thermodynamic parameters for the association of 1 with TMA picrate demonstrate that binding is enthalpic in origin, showing a substantial enthalpy gain (DeltaH degrees = -16.7 kJ mol(-1)) and an adverse entropic contribution (DeltaS degrees = -27.9 J mol(-1) K(-1)). A correlation has been found between the "goodness" of anions as binding partners and the solubility of their salts. Conversion of the anion into a more charge-dispersed species, for example, conversion of chloride into dialkyltrichlorostannate, improves cation binding substantially, indicating that charge dispersion is a main factor determining the influence of the anion on the cation-pi interaction. DFT computational studies show that the variation of the binding free energy of TMA with the counterion is closely accounted for by the electrostatic potential (EP) of the ion pair: guest binding appears to respond to the cation's charge density exposed to the receptor, which is determined by the anion's charge density through a polarization mechanism. A value of -DeltaG degrees = 38.6 kJ mol(-1) has been extrapolated for the free energy of binding of TMA to 1 in chloroform but in the absence of a counterion. The transmission of electrostatic effects from the ion pair to the cation-pi interaction demonstrates that host-guest association is governed by Coulombic attraction, as long as factors (steric, entropic, solvation, etc.) other than pure electrostatics are not prevalent.

Structure/function relationship in the polyplexes containing cationic polypeptides for gene delivery.

Various cationic polypeptides of linear or highly branched structures were synthesized by introducing tertiary or quaternary ammonium groups and hydroxyl groups to poly(L-lysine) (PL) or polyamideamine (PAMAM) dendrimers. These polycations were mixed with plasmid DNA to form polyplexes and subjected to in vitro gene introduction experiment. The transient gene expression was greatly affected by the side groups of PL derivatives or the surface cation charge density of PAMAM dendrimers. This difference in gene expression was found to result from two independent factors as follows: one is the cellular uptake of the polyplexes and the other is the compaction of the polyplexes. Lower charge density of PAMAM dendrimers suppressed the polyplex formation and cellular uptake, resulting the lower gene expression. Only the polycations that form polyplexes compacted at an adequate extent lead an effective gene expression, suggesting that the physicochemical properties of the polyplexes defined by the chemical structures of the polycations play an important role in the effective gene transfer.

Kinetic study of cell disruption by ionic polymers with varied charge density

The effect of charge density and hydrophobicity of the polymeric cations on cell disruption is studied thermodynamically and kinetically by using the budding yeast protoplast. It is found that cationic polymers drastically disrupt the cells above a certain concentration while nonionic and anionic ones do not. Reduced charge density of copolymers of cationic and nonionic monomers resulted in decreased cell disruption. However, it is further experimentally proved that the disruption of the cells occurs only when the polycation has a certain hydrophobicity. The stronger the hydrophobicity of the cationic polymer, the more cooperatively the cells are disrupted.

Effect of ion-exchanged alkali metal cations on the photolysis of 2-pentanone included within ZSM-5 zeolite cavities: a study of ab initio molecular orbital calculations

The Norrish Type I and Type II reactions in the photolysis of 2-pentanone included within the alkali metal cation-exchanged ZSM-5 zeolite have been investigated by experimental and theoretical approaches. Changes in the molecular environment of the zeolite cavities by exchanging the cations had significant effects not only on the adsorption state but also on the photochemical reactions of the ketones included within the zeolite cavities. The yields of the photolysis decreased and the ratio of the Type I/Type II reactions increased, respectively, by changing the ion-exchanged cations from Cs+ to Li+. The observed IR and phosphorescence spectra of the adsorbed ketones and the ab initio molecular orbital calculations of this host-guest system indicate that the ketones interact with two different adsorption sites, i.e. the surface OH groups and alkali metal cations, while the interaction between the ketones and cations increased by changing the cations from Cs+ to Li+. Molecular orbital calculations were also carried out and indicated that the zeolite framework promotes the delocalization of the charge density of the alkali metal cations which can modify the interaction between the adsorbed ketones and cations, resulting in significant changes in the photolysis of these ketones.

Rate Enhancement by Cations in Supercritical Water Oxidation of 2-Chlorophenol

Interactions between cations and Cl species in the supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) has been investigated in a high-pressure quartz-lined reactor at 673−773 K. Experimentally, we found that the S/D ratio (defined as the amount of 2CP converted to CO2 and H2O/disappearance of 2CP) for SCWO of 2CP was enhanced by cations (such as Li+, Na+, K+, Ca2+, Fe2+, or Fe3+). The S/D ratio increased as the charge density of cations increased (Fe3+> Fe2+ > Li+ > Ca2+ > Na+ > K+). Due to the extremely low solubility of metal salts in the supercritical water, abstraction of Cl in 2CP via an intermediate ((OH)PhClδ-- - - - -Mδ+) was postulated. Formation of these metal chloride (such as KCl, CaCl2, and FeCl3) precipitates in the SCWO of 2CP were identified by X-ray diffraction (XRD) spectroscopy. Since the formation of toxic highly chlorinated phenols and heavy polycyclic aromatic hydrocarbons (PAHs) was notably reduced, abstraction of Cl of 2CP by cations may occur in the early stage of the SCWO process.

The Plant Cell Wall Polysaccharide Rhamnogalacturonan II Self-assembles into a Covalently Cross-linked Dimer

The location of the 1:2 borate-diol ester cross-link in the dimer of the plant cell wall polysaccharide rhamnogalacturonan II (RG-II) has been determined. The ester cross-links the apiofuranosyl residue of the 2-O-methyl-D-xylose-containing side chains in each of the subunits of the dimer. The apiofuranosyl residue in each of the two aceric acid-containing side chains is not esterified. The site of borate esterification is identical in naturally occurring and in in vitro synthesized dimer. Pb2+, La3+, and Ca2+ increase dimer formation in vitro in a concentration- and pH-dependent manner. Pb2+ is the most effective cation. The dimer accounts for 55% of the RG-II when the monomer (0.5 mM) is treated for 5 min at pH 3.5 with boric acid (1 mM) and Pb2+ (0.5 mM); at pH 5 the rate of conversion is somewhat slower. Hg2+ does not increase the rate of dimer formation. A cation's charge density and its ability to form a coordination complex with RG-II, in addition to steric factors, may regulate the rate and stability of dimer formation in vitro. Our data provide evidence that the structure of RG-II itself determines which apiofuranosyl residues are esterified with borate and that in the presence of boric acid and certain cations, two RG-II monomers self-assemble to form a dimer.

Dephosphorylation and aromatic nucleophilic substitution in nonionic micelles. The importance of substrate location

Reactions of OH-and F-with p-nitrophenyl diphenyl phosphate (pNPDPP) are inhibited by very dilute dodecyl (10) and (23) polyoxyethylene glycol (C12E10and C12E23, respectively), but rate constants become independent of surfactant concentrations at concentrations above the critical micelle concentration. Low charge density anions, e.g., ClO4-, inhibit and low charge density cations, e.g., (n-C7H15)4N+, accelerate reactions, probably by controlling concentrations of nucleophiles in the palisade layer. Diphenyl phosphorofluoridate, generated by attack of F-, is not detected but is rapidly hydrolyzed to phenyl phosphorofluoridate or diphenyl phosphate ion with loss of phenol or F-. The products are different in DMSO containing modest amounts (<15 vol%) of water and no surfactant and are phenyl phosphorofluoridate and difluorophosphate ions generated by attack of F-on the initial phosphorofluoridate. These differences are consistent with the micellar palisade layer being water-rich. Although the nonionic surfactants do not intervene nucleophilically in reactions of pNPDPP, considerable amounts of ether are formed in the reaction of 2,4-dinitrochlorobenzene (DNCB), in C12E10and C12E23at high pH by attack of alkoxide ion with the relatively hydrophilic DNCB located close to the micellar surface. The differences in the chemistries of reactions of pNPDPP and DNCB appear to be associated largely with differences in locations of these substrates in the nonionic micelles.Key words: fluoridates, p-nitrophenyl diphenyl phosphate, 2,4-dinitrochlorobenzene, nonionic micelles, palisade layer.

Inner versus outer sphere complexation of f-elements

The evidence from a variety of experimental techniques for formation of outer and inner sphere complexes of f-element cations with inorganic and organic ligands, is reviewed. The very weakly basic ligands favor outer sphere complexation although the value of ligand pKa at which inner sphere formation becomes favored increases with metal cation charge density (z/r). The need to understand the limitations of different methods for measuring stability constants in evaluation of outer versus inner sphere complexation is emphasized.

Surface charge density measurements on mercury electrodes covered by phospholipid monolayers

A novel method to measure the surface charge density at a hanging mercury drop electrode (HMDE) coated with a selfassembled phospholipid monolayer mimicking a biological membrane is described. The charge density is obtained by analogical integration of the capacitive current which flows at constant applied potential as a consequence of a slight contraction of the mercury drop. The contraction must be carried out while keeping the neck of the lipidcoated mercury drop in contact with the lipid film spread on the surface of the electrolytic solution. The validity of this procedure is tested by comparison with differential capacity measurements carried out at a lipidcoated HMDE, fully immersed into the solution, under otherwise identical conditions. The potential of this method in the investigation of the properties of selfassembled lipid monolayers, either pure or incorporating lipophilic species, is briefly outlined. The method is applied to the determination of the charge density of tetraphenylphosphonium cations adsorbed in the polar head region of a phosphatidylserine monolayer supported by the HMDE.

Location and bonding of cations in ETS-10 titanosilicate molecular sieve: A multinuclear n.m.r. investigation

ETS-10 is a large pore titanosilicate molecular sieve with unique properties due to its peculiar pore structure and charge distributions. Extensive use of multinuclear n.m.r. in the present work has enabled elucidation of the effect of cations on the local structure and electronic environment in ETS-10. MAS 23Na n.m.r. shows that Na+ ions in NaETS-10 reside in both the 12- and 7-member ring channels to balance the negative charges from the octahedral TiO62 units. The cations bond weakly to the framework and can be exchanged reversibly without damaging the framework structure. After proton exchange, Na ions in both channels are replaced by more delocalized protons, as evidenced by efficient 1H-29Si cross-polarization, significant high field shift and line broadening of 29Si n.m.r. signals, and the great similarity between CP/MAS and MAS 29Si n.m.r. spectral patterns. Proton exchange causes a significant cation charge density dilution in the system, which could affect the properties of the molecular sieve, such as acidity and adsorption behavior. MAS 1H n.m.r. shows that silanol hydroxyl groups are the major H-containing species in HETS-10. Contrary to the prediction based on a currently established ETS-10 framework structure model, one Si(3Si, 1Ti) site was found to have quite different properties from the other three Si(3Si, 1Ti) sites.

DRIFTS and Raman Investigation of N2 and O2 Adsorption on Zeolites at Ambient Temperature

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and Raman spectroscopy were used to examine N2 and O2 adsorption on cation-exchanged (K, Na, Sr, Ca, and Li) low silica X (LSX) zeolites. IR and Raman absorption bands were observed for the molecular vibration of adsorbed N2 and O2 at room temperature and atmospheric pressure. The intensity (in Kubelka-Munk units) of the IR band increased with N2 pressure and mirrored the adsorption isotherm for N2. Both O2 and N2 displayed a similar dependence of the molecular vibrational frequency on cation charge density, which suggests that both gases are interacting directly with the cations. The vibrational frequencies for adsorbed N2 and O2 were more sensitive to the cation charge density than to framework Al content. Infrared studies of N2 and O2 on mixed cation forms of LSX show that N2 interaction was localized near individual cations within the sorption cavity of the zeolite. Thus, adsorbed N2 can be used to probe accessibility of specific cations within the zeolite framework. The spectroscopic data are consistent with the theory that the stronger interaction of N2 over O2 is caused by the stronger influence of the electric field with the larger quadrupole of N2.

Mixed-alkali effect and short-range interactions in amorphous poly(ethylene oxide) electrolytes

When dissolved in polyethylene oxide) (PEO), the alkali metal imides LiN(CF 3SO 2) 2 (LiTFSI), NaN(CF 3SO 2) 2 (NaTFSI) and KN(CF 3SO 2) 2 (KTFSI) yield rubbery electrolytes over a wide range of concentrations at T > 50 °C. Both the glass transition temperature ( T g ) and the dc bulk conductivity of these electrolytes exhibit a weak dependence on the cation size. Although T g of similar electrolytes involving less delocalized anions ( eg, thiocyanate and triflate anions) also exhibits a weak dependence on the cation size, their conductivity decreases markedly with increasing cation charge density. To clarify this point, a conductivity study was performed on PEO amorphous electrolytes containing binary salt mixtures (KTFSI-LiTFSI and KSCN-LiSCN) in various molar ratios. Over the range of the examined concentrations (0.6–1.6mol/kg), the conductivity data of the PEO-KTFSI-LiTFSI electrolytes are comparable to those of the one-salt electrolytes. In turn, the conductivity data of the PEO-KSCN-LiSCN electrolytes exhibit a strong, negative deviation from additivity. By analogy with molten salt binary mixtures, it is proposed that both this deviation and the cation-size effect on the conductivity of the PEO-MSCN one-salt electrolytes are caused by low-energy, short-range interactions associated with anion polarization.

The Effect of Different Metal Cation Binding on the Proton Pumping in Bacteriorhodopsin

AbstractThe first section of this paper is a detailed summary of studies made by us and others on metal cations binding to deionized bacteriorhodopsin (dIbR) and its variants. Our studies include the luminescence experiments of Eu3+ binding to dIbR and potentiometric studies of Ca2+ binding to dIbR, to deionized bR mutants, to bacterioopsin, and to dIbR with its C‐terminus removed. The results suggest the presence of two classes of binding sites, one class has two high‐affinity constants, and one has one low‐affinity constant. For Ca2+ binding, there is one metal cation in each of the two high‐affinity sites which are coupled to the charged aspartates 85 and 212 (known to be in the retinal cavity) but not coupled to each other. The low‐affinity class can accommodate 0–6 Ca2+ ions and most of them are bound to the surface. Mg2+ has a slightly smaller value for its binding constant to the highest‐affinity site. Thus, one expects more Ca2+ than Mg2+ bound to the two high‐affinity sites. In the second section, we summarize our recent study on the effect of metal cation charge density (Ca2+, Mg2+, Eu3+, Tb3+, Ho3+, Dy3+) on the kinetics of both Schiff base deprotonation and proton transport to the extracellular surface. For all metal cations, the apparent rate constant of the slow components of the deprotonation process is the same as that for the transport process at 22 °C. The temperature studies, however, show this apparent equality to be fortuitous and to result from cancellation of the contribution of the energy and entropy of activation. Thus, while the entropy of activation is positive for the deprotonation process, it is negative for the proton transport process. These kinetic parameters depend weakly on the charge density, but in an opposite sense for the two processes. These results suggest that the deprotonation is not the rate‐limiting step for the proton transport process. A possible mechanism is proposed in which a hydrated metal cation is used to induce the deprotonation of the protonated Schiff base and to dissociate one of its H2O molecules to donate the proton in the L → M process.

Crown ether-containing styryl dyes

The method for the synthesis of indoles from nitropyridinium salts has been improved. A method for synthesizing novel derivatives of indoleninium quaternary salts has been developed. The condensation of indoleninium salts with formyl derivatives of crown ethers leading to styryl dyes is described. Photoisomerization and complexing of crown-ether dyes with ions of alkaline and alkaline-earth metals have been studied. The shift of the long-wave absorption maximum has been observed to depend on the size and charge density of the metal cation.