Stability and solvation of alkali metal ion complexes with dibenzo-30-crown-10

Stability constants for the 1:1 complexes of dibenzo-30-crown-10 (DB30C10) with alkali metal ions have been determined at 25 °C in nitromethane and water by conductometry and capillary electrophoresis, respectively. Transfer activity coefficients of DB30C10 and its complexes from nitromethane to S (S = water, acetonitrile, propylene carbonate, methanol, and N,N-dimethylformamide) have been determined at 25 °C to evaluate the solvation properties. The stability constant in the poorly solvating solvent, nitromethane, decreases with increasing metal ion size, Na⁺ > K⁺ > Rb⁺ > Cs⁺, reflecting the intrinsic selectivity governed by electrostatic interaction between the metal ion and the ether oxygen atoms. It is also suggested that a part of the ether oxygen atoms does not bind to the metal ion in the Na(DB30C10)⁺ complex. The aqueous stability constant varies as Na⁺ ? K⁺ ≈ Rb⁺ ≈ Cs⁺; this selectivity pattern is similar to that in acetonitrile, propylene carbonate, and methanol. The complex stability in water is very low compared to that in the nonaqueous solvents, owing to hydrogen bonding of water to the oxygen atoms of the free crown ether. The transfer activity coefficient values show that DB30C10 shields all the metal ions effectively from the solvents and lead to the conclusion that the complexation selectivity in S receives a significant contribution from the solvation of the free metal ions. The Na(DB30C10)⁺ complex has specific interaction with water, causing much lower K⁺/Na⁺ selectivity in H₂O than in MeOH.     

Stabilities in nitromethane of alkali metal ion complexes with dibenzo-18-crown-6 and dibenzo-24-crown-8 and their transfer from nitromethane to other polar solvents

Stability constants for the 1:1 complexes of Na⁺, K⁺, Rb⁺, and Cs⁺ with dibenzo-18-crown-6 (DB18C6) and dibenzo-24-crown-8 (DB24C8) have been determined by conductometry at 25 °C in a poorly solvating solvent, nitromethane. For both the crown ethers, the stability constant decreases with increasing metal ion size, Na⁺ > K⁺ > Rb⁺ > Cs⁺, regardless of the size compatibility between the metal ions and the ligand cavities. A comparison of the results with those in several other solvents (S: acetonitrile, propylene carbonate, water, methanol, and N,N-dimethylformamide) leads to the conclusion that the selectivity sequence of these crown ethers in nitromethane agrees with the intrinsic one in the absence of a solvent. Transfer activity coefficients of the crown ethers and their complexes from nitromethane to S have been determined to evaluate the solute-solvent interactions. It is shown that DB24C8 shields the alkali metal ions more effectively from the solvents than DB18C6 because of the larger number of oxygen atoms and the more flexible structure of DB24C8. Regarding the complexation in nitromethane as a reference, the complex stability and selectivity in S are discussed. The selectivities of these crown ethers in water, methanol, and N,N-dimethylformamide, which apparently obey the size-fit concept, are largely due to the solvation of the free alkali metal ions.     

Experimental and theoretical study on the supramolecular complexes of 15-crown-5 with adrenaline

The electron transfer properties of supramolecular complexes of 15-crown-5 (15C5) with protonated adrenaline (PAd⁺) at different electrodes using cyclic voltammetry (CV) have been investigated in the article. The experimental results show that 15C5 will affect the electron transfer properties of adrenaline. The formed supramolecular complexes by ion-dipole and hydrogen bond interaction between PAd⁺ and 15C5 will slow down the diffusion ability of adrenaline and make it hard to donate electron and be oxidized.The interaction energies and NPA calculations for the supramolecular complexes of 15C5 with PAd⁺ at B3LYP/6-31+G(d) level have been performed. The calculational results confirm the experimental fact that 15C5 can form stable supramolecular complexes with PAd⁺.     

Sodium-potassium exchange in sea urchin egg. I. Kinetic and biochemical characterization at fertilization

Biochemical and kinetic characteristics of the Na⁺-K⁺ exchange were studied in Paracentrotus lividus eggs. Measurement of the ⁸⁶Rb uptake shows that ouabain-sensitive ⁸⁶Rb uptake is dramatically stimulated within the first minute following fertilization. The Na⁺-K⁺ pump-mediated K⁺ entry presents a maximal rate at 8 min postfertilization and then decreases to reach a plateau within 30 min. We assess that the steep rise in cell K⁺ occurring at fertilization (J.P. Girard, P. Payan, C. Sardet, Exp. Cell. Res. 142:215–221, 1982) does not originate from a net entry of external K⁺. Measured 30 min postfertilization, the half-maximal activation by K⁺ of the ouabain-sensitive Na⁺-K⁺ exchange is 5–6 mM and the ouabain lC₅₀ is 5.10^?5 M. Egg cortices from unfertilized and fertilized eggs show comparable Na⁺-K⁺ ATPase activity with a 50% ouabain-sensitive fraction. V_m and K_m for Na⁺ and K⁺ of the enzyme are of the same order of magnitude in cortices of unfertilized and fertilized eggs. Cortical Na⁺-K⁺ ATPase from unfertilized eggs shows a ten fold increase of activity between pH 6.7 and pH 7.7. The results strongly suggest that the plasma membrane of unfertilized eggs contains a preexisting Na⁺-K⁺ transporting system which is obligatorily stimulated at fertilization.     

Thermodynamics and coordination characteristics of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 extraction complex

The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)₂·(H₃O⁺·χH₂O)₂·UO₂Cl₄²⁻ (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (K_ex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log K_ex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol⁻¹. The value of the entropy change, ΔS°, was calculated from ΔH° and K_ex and equals −690 J K⁻¹ mol⁻¹, whereas ΔG° = −6.45 kJ mol⁻¹. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K⁻¹ mol⁻¹, ΔH° = −101 kJ mol⁻¹ and ΔG° = −8.37 kJ mol⁻¹), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H₅O₂⁺ ions in it, in contrast to the H₃O⁺ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H₅O₂⁺ ions is held in the crown ether cavity by four hydrogen bonds. The H₅O₂⁺ ion has a central bond. The uranium atom forms UO₂Cl₄²⁻ as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above.     

Density functional theory study of the potassium complexation of an unsymmetrical 1,3-alternate calix[4]-crown-5-N-azacrown-5 bearing two different crown rings

Theoretical studies of an unsymmetrical calix[4]-crown-5-N-azacrown-5 (1) in a fixed 1,3-alternate conformation and the complexes 1·K⁺(a), 1·K⁺(b), 1·K⁺(c) and 1·K⁺K⁺ were performed using density functional theory (DFT) at the B3LYP/6-31G* level. The fully optimized geometric structures of the free macroligand and its 1:1 and 1:2 complexes, as obtained from DFT calculations, were used to perform natural bond orbital (NBO) analysis. The two main types of driving force metal–ligand and cation–π interactions were investigated. NBO analysis indicated that the stabilization interaction energies (E ₂) for O…K⁺ and N…K⁺ are larger than the other intermolecular interactions in each complex. The significant increase in electron density in the RY* or LP* orbitals of K⁺ results in strong host–guest interactions. In addition, the intermolecular interaction thermal energies (ΔE, ΔH, ΔG) were calculated by frequency analysis at the B3LYP/6-31G* level. For all structures, the most pronounced changes in the geometric parameters upon interaction are observed in the calix[4]arene molecule. The results indicate that both the intermolecular electrostatic interactions and the cation–π interactions between the metal ion and π orbitals of the two pairs that face the inverted benzene rings play a significant role.     

Interaction of some Pd(II) complexes with Na+/K+-ATPase: Inhibition,kinetics, prevention and recovery

The aim of this work was to investigate the influence of [PdCl₄]^{2 ?} , [PdCl(dien)]⁺ and [PdCl(Me₄dien)]⁺ complexes on Na⁺/K⁺-ATPase activity. The dose-dependent inhibition curves were obtained in all cases. IC₅₀ values determined by Hill analysis were 2.25 × 10^{? 5} M, 1.21 × 10^{? 4} M and 2.36 × 10^{? 4} M, respectively. Na⁺/K⁺-ATPase exhibited typical Michelis-Menten kinetics in the presence of Pd(II) complexes. Kinetic parameters (V_max, K_m) derived using Eadie–Hofstee transformation indicated a noncompetitive type of Na⁺/K⁺-ATPase inhibition. The inhibitor constants (K_i) were determined from Dixon plots. The order of complex affinity for binding with Na⁺/K⁺-ATPase, deducted from K_i values, was [PdCl₄]^{2 ?} >[PdCl(dien)]⁺>[PdCl(Me₄dien)]⁺. The results indicated that the potency of Pd(II) complexes to inhibit Na⁺/K⁺-ATPase activity depended strongly on ligands of the related compound. Furthermore, the ability of SH-donor ligands, l-cysteine and glutathione, to prevent and recover the Pd(II) complexes-induced inhibition of Na⁺/K⁺-ATPase was examined. The addition of 1 mM l-cysteine or glutathione to the reaction mixture before exposure to Pd(II) complexes prevented the inhibition by increasing the IC₅₀ values by one order of magnitude. Moreover, the inhibited enzymatic activity was recovered by addition of SH-donor ligands in a concentration-dependent manner.     

Influence of dielectric constant on protonation equilibria of maleic acid and L-asparagine in acetonitrile water-mixtures

Abstract

The protonation constants of maleic acid and L-asparagine have been studied pH-metrically in various concentrations (0–50% v/v) of acetonitrile–water mixtures maintaining an ionic strength of 0.16 mol L^-1 at 30⁰C. The protonation constants have been calculated using the computer program MINIQUAD75 and are selected based on statistical parameters. Linear variation of step-wise protonation constants (log K) with the reciprocal of the dielectric constant of the solvent mixture has been attributed to the dominance of the electrostatic forces.     

Excitation and deexcitation processes of Eu(III) benzo-15-crown-5 complex studied on comparing with its first coordination sphere

The coordination sphere and the deexcitation mechanism of the Eu(III) benzo-15-crown-5 complex, Eu(B15C5), were studied with references of the Eu(III) complexes with a similar coordination sphere; the dibenzo-18-crown-6 complex, Eu₃(B₂18C6)₂, and the cryptand[2.2.1] complex, Eu([2.2.1]). NMR spectroscopy reveals that the Eu(B15C5) complex is quite stable in acetonitrile solution whereas only 40% of the Eu(III) ion forms the complex in the equimolar Eu(NO₃)₃ and B₂18C6 acetonitrile solution. The coordination sphere of the Eu(III) complexes in acetonitrile solutions were also discussed by the degenerate ⁷F₀→⁵D₀ transition energy levels. The Eu(B15C5) have a negative shift compared with the europium(III) nitrate in acetonitrile and it is explained by the coordination of both nitrate ions and the crown ether ligand. Energy transfer from the n–π* excited state located in the catechol structure to the central europium ion was first observed as the sensitized luminescence of ⁵D₀→⁷F_J. The excited state lifetime of the Eu(B15C5) complex was first determined as 201 μs in the present study.     

Molecular simulation of Tyr105 phosphorylated pyruvate kinase M2 to understand its structure and dynamics

Microstructure of dibenzo-18-crown-6 (DB18C6) and DB18C6/Li⁺ complex in different solvents (water, methanol, chloroform, and nitrobenzene) have been analyzed using radial distribution function (RDF), coordination number (CN), and orientation profiles, in order to identify the role of solvents on complexation of DB18C6 with Li⁺, using molecular dynamics (MD) simulations. In contrast to aqueous solution of LiCl, no clear solvation pattern is found around Li⁺ in the presence of DB18C6. The effect of DB18C6 has been visualized in terms of reduction in peak height and shift in peak positions of g_Li-Ow. The appearance of damped oscillations in velocity autocorrelation function (VACF) of complexed Li⁺ described the high frequency motion to a “rattling” of the ion in the cage of DB18C6. The solvent-complex interaction is found to be higher for water and methanol due to hydrogen bond (HB) interactions with DB18C6. However, the stability of DB18C6/Li⁺ complex is found to be almost similar for each solvent due to weak complex-solvent interactions. Further, Li⁺ complex of DB18C6 at the liquid/liquid interface of two immiscible solvents confirm the high interfacial activity of DB18C6 and DB18C6/Li⁺ complex. The complexed Li⁺ shows higher affinity for water than organic solvents; still they remain at the interface rather than migrating toward water due to higher surface tension of water as compared to organic solvents. These simulation results shed light on the role of counter-ions and spatial orientation of species in pure and hybrid solvents in the complexation of DB18C6 with Li⁺. Graphical Abstract

DB18C6/Li⁺ complex in pure solvents (water, methanol, chloroform, and nitrobenzene) and water/nitrobenzene interface     

Substituent control of selective alkali metal ion extraction by amidocrown η-butadienyl complexes of molybdenum(II)

Substituted η³-butadienyl complexes containing amide-armed crowns (X) of general formula [MoCl(CO)₂(η³-CH₂C(COX)CCH₂)(phen)]_n (phen=1,10-phenanthroline) were prepared and investigated for their ability to extract alkali metal ions from a mixed phase system. Reaction of the chlorocarbonyl precursor (1) with 1-aza-15-crown-5, 4^′-aminobenzo-15-crown-5, 2-aminomethyl-15-crown-5, 4^′-aminobenzo-18-crown-6 or 2-aminomethyl-18-crown-6 gave monomeric complexes (n=1), and addition of sodium tetraphenylboron to the 15-crown-5-substituted complexes gave the corresponding sodium salts. Dinuclear complexes (n=2) were formed by reaction of 1 and 1,7-diaza-15-crown-5 or 4,4^′(5^′)-diaminobenzo-15-crown-5. Comparison of amidobenzo- and 2-amidomethyl-15-crown-5-substituted complexes showed enhanced sodium transport properties for the latter, and spectroscopic and molecular modeling studies suggested complexation occurred by concerted action of the amide and crown.     

Endothelin 1 Stimulates Na+,K+-ATPase and Na+-K+-Cl− Cotransport Through ETA Receptors and Protein Kinase C-Dependent Pathway in Cerebral Capillary Endothelium

Abstract: The effect of endothelins (ET-1 and ET-3) on ⁸⁶Rb⁺ uptake as a measure of K⁺ uptake was investigated in cultured rat brain capillary endothelium. ET-1 or ET-3 dose-dependently enhanced K⁺ uptake (EC₅₀ = 0.60 ± 0.15 and 21.5 ± 4.1 nM, respectively), which was inhibited by the selective ET_A receptor antagonist BQ 123 (cyclo-d -Trp-d -Asp-Pro-d -Val-Leu). Neither the selective ET_B agonists IRL 1620 [N-succinyl-(Glu⁹,-Ala^11,15)-ET-1] and sarafotoxin S6c, nor the ET_B receptor antagonist IRL 1038 [(Cys¹¹,Cys¹⁵)-ET-1] had any effect on K⁺ uptake. Ouabain (inhibitor of Na⁺,K⁺-ATPase) and bumetanide (inhibitor of Na⁺-K⁺-Cl^? cotransport) reduced (up to 40% and up to 70%, respectively) the ET-1-stimulated K⁺ uptake. Complete inhibition was seen with both agents. Phorbol 12-myristate 13-acetate (PMA), activator of protein kinase C (PKC), stimulated Na⁺,K⁺-ATPase and Na⁺-K⁺-Cl^? cotransport. ET-1-but not PMA-stimulated K⁺ uptake was inhibited by 5-(N-ethyl-N-isopropyl)amiloride (inhibitor of Na⁺/H⁺ exchange system), suggesting a linkage of Na⁺/H⁺ exchange with ET-1-stimulated Na⁺,K⁺-ATPase and Na⁺-K⁺-Cl^? cotransport activity that is not mediated by PKC.     

Kinetic studies and thermodynamic results for complex formation and solvation of thallium(I) cryptates in acetonitrile and in acetonitrile-water mixtures

Stability constants and dissociation rate constants of a range of thallium(I) cryptates in acetonitrile and of the cryptate Tl(2,2,2)⁺ in water-acetonitrile mixtures have been measured at 25°C. Solvation free energies of transfer for Tl⁺ from water to acetonitrile and to water-acetonitrile mixtures have been estimated from polarographic measurements using the ferrocene assumption. The results allow the calculation of transfer free energy data for the stable cryptate, TlCry⁺, and for the transition state (Tl⁺…Cry)^≠.In mixtures of water and acetonitrile the stability constant of Tl(2,2,2)⁺ increases substantially with increasing acetonitrile content. This variation arises almost equally from an increase in the formation rate constant and a decrease in dissociation rate constant. Alternatively, the increase of stability constants for Tl(2,2,2)⁺ with increasing mole fraction of acetonitrile results from a strongly decreasing transfer free energy of the cryptate which surpasses in magnitude the increasing transfer free energies of the reactants.     

Stability of sodium and potassium complexes of valinomycin

Stability constants of sodium and potassium complexes of valinomycin in some alcohols and water—organic solvent mixtures have been determined by titration, using circular dichroism to monitor complex formation. Constants range from 10¹ to 10⁶ M⁻¹. Stability of the potassium and sodium complexes increases with decreasing dielectric constant, but the ratio of the constants remains about 10³–10⁴. As others have shown, a similar selectivity for K⁺ is observed in a number of other types of measurements involving valinomycin. These include the permeability and conductance ratios which characterize the selectivity of cation transport through membranes and the ratio of salt extraction equilibrium constants. On the basis of data presented here, and elsewhere, it is suggested that conformational constraints within the depsipeptide part of the complexes aid ion selectivity and that differences in cation solvation and carbonyl ligand binding energies make an important, roughly equal, contribution.     

Spectrophotometric studies on the solvent extraction of lithium picrate with dibenzo-14-crown-4 and its analogs

The distributions of lithium cation, picrate anion and dibenzo-14-crown-4 (DB14C4) and its analogs between water and various solvents, as well as the formation of ion pairs of lithium picrate and crown in these solvents, were studied spectrophotometrically at 24 ± 1 °C. The solvents used included benzene, chloroform, dichloromethane, 1,2-dichloroethane, nitromethane, and nitrobenzene. A 1:1 complex cation was formed between Li⁺ and crown among these solvents. Three different kinds of ion pairs of Li⁺-crown complex cation and picrate anion could be determined from a series of absorption maximum shifts. The effects of substituent groups on DB14C4 and solvents upon the extraction constant are presented. A plausible extraction mechanism is also suggested.     

Complexes of lanthanoid salts with macrocyclic ligands. Part 25. Lanthanoid trifluoroacetate complexes with 12-crown-4, 15-crown-5, and 18-crown-6 ethers

The lanthanoid trifluoroacetates, Ln(TFA)₃, react with 12-crown-4, 15-crown-5, and 18-crown-6 ethers to give complexes with various metal:ligand ratios, 1:1, 3:2, and 2:1. The following complexes have been isolated and characterized: Ln(CF₃CO₂)₃· (C₈He₁₆O₄), Ln = La, Ce, Pr; [Ln(CF₃CO₂)₃]₃· (C₈H₁₆O₄)₂, Ln = Pr, Eu, Er; [Ln(CF₃CO₂)₃]₂· (C₈H₁₆O₄), Ln = Pr, Nd, Sm; [Ln(CF₃CO₂)₃]₂· (C₁₀H₂₀O₅), Ln = La---Eu; Ln(CF₃CO₂)₃·(C₁₂H₂₄O₆), Ln = La---Eu; [Ln(CF₃CO₂)₃]₂·(C₁₂H₂₄O₆), Ln = Y, Eu---Er, Yb. Thermogravimetric data show that the 2:1 complexes are usually thermally more stable. The 2:1 complexes with the 15-membered polyether undergo a slow hydrolysis in the presence of traces of water, which yields the hydroxo complex [Ln₂(CF₃CO₂)₃(OH)(C₁₀H₂₀O₅)₂] [Ln₂(CF₃CO₂)₈]. The vibrational spectra confirm the coordination of the coronands; the Δν_as(CCO) shifts are not large, which point to a moderate interaction between the polyethers and the metal ions. Magnetic susceptibilities and X-ray powder diagrams have been measured.High-resolution excitation and emission spectra have been analysed for the europium-containing compounds. The spectrum of Eu(CF₃CO₂)₃·3H₂O indicates the presence of a single species with low symmetry, in agreement with the crystal structure data for the isostructural Pr-salt. The anhydrous salt Eu(CF₃CO₂)₃ generates an emission spectrum with broad bands and probably contains several, closely related polymeric species. The spectrum of [Eu(CF₃CO₂)₃]₂(C₁₀H₂₀O₅) is consistent with the presence of two chemically different sites for Eu(III); the emission bands are broad. The double salt AgEu(CF₃CO₂)₄·3CH₃CN has also been investigated; the observed transitions point to the presence of a species with idealized D_2d symmetry. The emission spectrum of [Eu(CF₃CO₂)₃]₂(C₁₂H₂₄O₆) displays sharp bands and reveals the presence of two different sites for the metal ion with efficient energy transfers between them. One of the species may have a relatively high symmetry.In solution, all the complexes are non-electrolytes in acetonitrile and propylene carbonate and close to 1:1 electrolytes in methanol. Some dissociation occurs in acetonitrile for the 2:1 complexes with 18-crown-6 ether. On the other hand, ¹H NMR spectra of the lanthanum 1:1 complexes with 12- crown-4 and 18-crown-6 ethers indicate no dissociation of the complexed polyether. Log β₁ is greater than 6 for both complexes; it is equal to 4.4 for the samarium 1:1 complex with 18-crown-6 ether.     

Formate Hydrogenlyase Is Needed for Proton-Potassium Exchange Through the F0F1-ATPase and the TrkA System in Anaerobically Grown and Glycolysing Escherichia coli

Anaerobically grown and glycolysing Escherichia coli produced H₂ and carried out H⁺-K⁺-exchange in two steps, the first of which had the fixed stoichiometry for DCCD-sensitive fluxes (2H⁺/K⁺), and the second one had a variable stoichiometry for DCCD-sensitive fluxes. H₂ production and the 2H⁺/K⁺-exchange were lost in the ΔfdhF or ΔhycA-H mutant. In the ΔfdhF mutant, H⁺-K⁺-exchange with K _m for K⁺-uptake of 2.3 mM and less K⁺-gradient between the cytoplasm and the medium were observed. H₂ production and H⁺-K⁺-exchange with a high K _m for K⁺-uptake were carried out in the uncD mutant; however, both H₂ production and H⁺-K⁺-exchange were lost in the Δunc or uncE mutant. H₂ production was observed in the trkA trkD kdpA mutant. It was displayed in protoplasts with increased membrane permeability when donor or acceptor of reducing equivalents—formate with DTT or NADH respectively—was added. The F₀F₁ and the TrkA(H) or the F₀ and the TrkA(G) had been assumed to form the united supercomplexes, functioning as a H⁺-K⁺-pump or antiporter respectively (for review see Bioelectrochem Bioenerg 33:1, 1994). Results allow the proposal that H₂ production by FHL has a relationship with the H⁺-K⁺-exchange through a H⁺-K⁺-pump and via an H⁺-K⁺-antiporter. Formate and NADH can serve as a donor and an acceptor of reducing equivalent respectively, for operation of such supercomplexes. Received: 12 December 1996 / Accepted: 19 March 1997     

Extraction properties for alkali metal picrates of macrocyclic trinuclear (p-cymene)ruthenium(II) and (pentamethylcyclopentadienyl)rhodium(III) complexes

The solvent extraction properties of macrocyclic trinuclear organometallic complexes, [(p-cymene)Ru(pyO₂)]₃ and [Cp^∗Rh(pyO₂)]₃, for Li⁺, Na⁺, and K⁺ picrates have been investigated in a dichloromethane-water system at 25 °C. The extraction rates of the alkali metal picrates with these macrocyclic complex ligands are unusually slow; the shaking times required to attain equilibrium are at least 1 h for [(p-cymene)Ru(pyO₂)]₃ and 20-40 h for [Cp^∗Rh(pyO₂)]₃. From analysis of the equilibrium data, the extraction constants (K_ex = [ML⁺A⁻]_o/[M⁺][L]_o[A⁻]; M⁺ = alkali metal ion, L = macrocyclic ligand, A⁻ = picrate ion, o = organic phase) have been determined. The log K_ex value varies in the sequences, Li⁺ (5.72) > Na⁺ (4.50) > K⁺ (2.88) for [(p-cymene)Ru(pyO₂)]₃ and Li⁺ (4.79) > Na⁺ (2.70) ≈ K⁺ (2.69) for [Cp^∗Rh(pyO₂)]₃. The K_ex values of 6,6-dibenzyl-14-crown-4 (DBz14C4), which is one of the best Li⁺-selective crown ethers, have also been determined for comparison. It is revealed that [Cp^∗Rh(pyO₂)]₃ is much superior to DBz14C4 both in the extractability for Li⁺ and the selectivity for Li⁺ over Na⁺.     

Human Brain Capillary Endothelium: Modulation of K+ Efflux and K+, Ca2+ Uptake by Endothelin

This report describes K⁺ efflux, K⁺ and Ca²⁺ uptake responses to endothelins (ET-1 and ET-3) in cultured endothelium derived from capillaries of human brain (HBEC). ET-1 dose dependently increased K⁺ efflux, K⁺ and Ca²⁺ uptake in these cells. ET-1 stimulated K⁺ efflux occurred prior to that of K⁺ uptake. ET-3 was ineffective. The main contributor to the ET-1 induced K⁺ uptake was ouabain but not bumetanide-sensitive (Na⁺-K⁺-ATPase and Na⁺-K⁺-Cl^– cotransport activity, respectively). All tested paradigms of ET-1 effects in HBEC were inhibited by selective antagonist of ET_A but not ET_B receptors and inhibitors of phospholipase C and receptor-operated Ca²⁺ channels. Activation of protein kinase C (PKC) decreased whereas inhibition of PKC increased the ET-1 stimulated K⁺ efflux, K⁺ and Ca²⁺ uptake in HBEC. The results indicate that ET-1 affects the HBEC ionic transport systems through activation of ET_A receptors linked to PLC and modulated by intracellular Ca²⁺ mobilization and PKC.     

Complex formation with alkali and alkaline earth metal ions of cyclic octapeptides,cyclo(Phe-Pro)4, cyclo(Leu-Pro)4, and cyclo[Lys(Z)-Pro]4

Complex formation with alkali and alkaline earth metal ions of cyclic octapeptides, cyclo(Phe-Pro)₄, cyclo(Leu-Pro)₄, and cyclo[Lys(Z)-Pro]₄ was investigated in relation to conformation. In an alcohol solution, cyclo(Phe-Pro)₄ did not form complexes. However, cyclo(Leu-Pro)₄ and cyclo[Lys(Z)-Pro]₄ formed complexes selectively with Ba²⁺ and Ca²⁺ ions. Changing the solvent from alcohol to acetonitrile, the complexation behavior was very different. In acetonitrile, cyclo(Phe-Pro)₄ was found to form a complex with Ba²⁺, and CD spectra of cyclo(Leu-Pro)₄ and cyclo[Lys(Z)-Pro]₄ changed sharply on complexation with K⁺. Rate constants of the complex formation between the cyclic octapeptides and metal salts were in the range of 0.7–12 L mol^?1 min^?1 in an alcohol solution. One of the two types of complex formation in acetonitrile was much faster than that in an alcohol solution.