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At least 937 records · Page 52Linked to original sources
PubMed1989

Structure and stability of the Boc(D-Pro-L-Pro)2OCH3 Na+ complex: a model of poly(D,L-proline), an alkali ion channel across membranes.

In order to elucidate the molecular mechanisms of alkali ions conduction of poly(D,L-proline), which shows the typical behavior of an ion channel across membranes, conformations of the tetrameric derivative Boc(D-Pro-L-Pro)2OCH3 in solution, in the presence of the Na+ ion, were investigated by CD and nmr spectroscopy. Both theoretical and experimental data indicate the formation in solution of a monomeric complex between the tetramer, in the all-trans conformation, and the cation. The results allow to confirm the channel model, previously proposed on the phenomenological behavior of poly(D,L-proline) in membranes.

Hydrogen-Ion Concentration↗
PubMed2004

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

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

Journal Article↗
PubMed2003

Synthesis and structural analysis of the polymetallated alkali calixarenes [M4(p-tert-butylcalix[4]arene-4H)(thf)x]2.n THF (M=Li, K; n=6 or 1; x=4 or 5) and [Li2(p-tert-butylcalix[4]arene-2H)(H2O)(mu-H2O)(thf)].3 THF.

To study the structures and reactivities of alkali metallated intermediates of calix[4]arenes, three compounds were isolated: [Li(4)(p-tert-butylcalix[4]arene-4H)(thf)(4)](2).6 THF (1), [Li(2)(p-tert-butylcalix[4]arene-2H)(H(2)O)(mu-H(2)O)(thf)].3 THF (2), and [K(4)(p-tert-butylcalix[4]arene-4H)(thf)(5)](2).THF (3). The structure of 1 is shown to be dependent on the coordinating solvent. Partial hydrolysis of 1 leads to the formation of 2. The potassium compound 3 features a different structure to that of 1, due to a higher coordination number as well as stronger cation-pi-bonding interactions.

Journal Article↗
PubMed2006

Effect of alkali metal coordination on gas-phase chemistry of the diphosphate ion: the MH(2)P(2)O(7) (-) ions.

Systematic experimental and theoretical studies on anionic phosphate species in the gas phase are almost nonexistent, even though they could provide a benchmark for enhanced comprehension of their liquid-phase chemical behavior. Gaseous MH(2)P(2)O(7) (-) ions (M=Li, Na, K, Rb, Cs), obtained from electrospray ionization of solutions containing H(4)P(2)O(7) and MOH or M salts as a source of M(+) ions were structurally assayed by collisionally activated dissociation (CAD) mass spectrometry and theoretical calculations at the B3LYP/6-31+G* level of theory. The joint application of mass spectrometric techniques and theoretical methods allowed the MH(2)P(2)O(7) (-) ions to be identified as having a structure in which the linear diphosphate anion is coordinated to the M(+) ion (I) and provides information on gas-phase isomerization processes in the [PO(3)...MH(2)PO(4)](-) clusters II and the [P(2)O(6)...M...H(2)O](-) clusters IV. Studies of gas-phase reactivity by Fourier transform ion cyclotron resonance (FTICR) and triple quadrupole (TQ) mass spectrometry revealed that the MH(2)P(2)O(7) (-) ions react with selected nucleophiles by clustering, proton transfer and addition-elimination mechanisms. The influence of the coordination of alkali metal ions on the chemical behavior of pyrophosphate is discussed.

Journal Article↗
PubMed2006

Unexpected structural diversity in alkali metal azide-crown ether complexes: syntheses, X-ray structures, and quantum-chemical calculations.

A series of alkali metal azide-crown ether complexes, [Li([12]crown-4)(N3)], [Na([15]crown-5)(N3)], [Na([15]crown-5)(H2O)2]N3, [K([18]crown-6)(N3)(H2O)], [Rb([18]crown-6)(N3)(H2O)], [Cs([18]crown-6)(N3)]2, and [Cs([18]crown-6)(N3)(H2O)(MeOH)], has been synthesised. In most cases, single crystals were obtained, which allowed X-ray crystal structures to be derived. The structures obtained have been compared with molecular structures computed by density functional theory (DFT) calculations. This has allowed the effects of the crystal lattice on the structures to be investigated. Also, a study of the M-N(terminal) metal-azide bond length and charge densities on the metal (M) and terminal nitrogen centre (N(terminal)) in these complexes has allowed the nature of the metal-azide bond to be probed in each case. The bonding in these complexes is believed to be predominantly ionic or ion-dipole in character, with the differences in geometries reflecting the balance between maximising the coordination number of the metal centre and minimising ligand-ligand repulsions. The structures of the crown ether complexes determined in this work show the subtle interplay of such factors. The significant role of hydrogen bonding is also demonstrated, most clearly in the structures of the K and Rb dimers, but also in the chain structure of the hydrated Cs complex.

Journal Article↗
PubMed2007

Reactions of Sn(NMe2)2 with MPHCy: the effects of alkali metal phosphide coupling (Cy=cyclohexyl; M=Li, Na, K, Rb).

The reactions of the SnII base Sn(NMe2)2 with CyPHM (Cy=cyclohexyl) produce a range of products, depending primarily on the alkali metal (M) involved. The 1:3 stoichiometric reaction of Sn(NMe2)2 with CyPHNa in the presence of the Lewis base donor PMDETA (PMDETA=(Me2NCH2CH2)2NMe) gives [(NaPMDETA)2{Sn(mu-PCy)}3] (3), containing the electron-deficient [{Sn(mu-PCy)}3]2- dianion. Natural bond order (NBO) and electron localisation function (ELF) calculations show that this species is described most appropriately by a two-electron, three-centre Sn3 bonding model. Evidence that 3 results from phosphide coupling is provided by the 1:1 reaction of Sn(NMe2)2 with CyPHNa in the presence of PMDETA, which gives 3 and trace amounts of (NaPMDETA)2[{Sn(mu-PCy)}2(mu-PCyPCy)] (4) (containing one PCyPCy2- dianion). Greater extents of phosphide coupling are observed as the size of the Group 1 metal is increased. Thus, the 1:3 reaction of Sn(NMe2)2 with CyPHK in THF gives the co-crystalline product {(K2 THF)2[{Sn(mu-PCyPCy)}2(mu-PCy)]}0.9{(K2 THF)2[{Sn(mu-PCy)}2(mu-PCyPCy)]}0.1 (5) (containing [{Sn(mu-PCyPCy)}2(mu-PCy)]2- and [{Sn(mu-PCy)}2(mu-PCyPCy)]2- dianions), whereas the analogous reaction of Sn(NMe2)2 with RbPHCy gives [RbPMDETA{(CyP)3SnP(H)Cy}] (6) (containing a cyclic {(CyP)3Sn} unit).

Journal Article↗
PubMed1991

Chirality of isomers of aldosterone formed in dilute alkali.

In the presence of dilute alkali at room temperature aldosterone undergoes rearrangement to form 11 beta,18:18,21-diepoxy-20,21-dihydroxy-4-pregnen-3-one (V). X-Ray crystallographic analysis demonstrates that isomers of both 18R, 20S, 21S and 18R, 20S, 21R configuration are formed rather than the 18R, 20R, 21R isomer postulated on the basis of examination of 1H-NMR spectra. The spectra appears to be consistent with the major component of the mixture. The 20S configuration observed is in agreement with the chirality assigned to the degradation product obtained when the same alkaline solution of aldosterone is subjected to reflux. The crystals of (V) are monoclinic P2(1), Z = 4 with a = 20.891(2), b = 6.3848(5), c = 16.067(2)A, beta = 122.09 degrees(1) with two molecules in the asymmetric unit. Molecule A has the 20S,21S configuration and the molecules in the second site are a mixture of the 20S, 21S and 20S, 21R configuration in the ratio of 3:2.

Aldosterone↗
PubMed2004

Alkali-stable high-pI isoelectric membranes for isoelectric trapping separations.

Alkali-stable, high-pI isoelectric membranes have been synthesized from quaternary ammonium derivatives of cyclodextrins and poly(vinyl alcohol), and bifunctional cross-linkers, such as glycerol-1,3-diglycidyl ether. The new, high-pI isoelectric membranes were successfully applied as cathodic membranes in isoelectric trapping separations in place of the hydrolytically more labile, polyacrylamide-based cathodic isoelectric membranes, and permitted the use of catholytes as alkaline as 1 M NaOH. The new high-pI isoelectric membranes have shown excellent mechanical stability, low electric resistance and long life times, even when subjected to electrophoresis with current densities as high as 80 mA/cm2.

Cross-Linking Reagents↗
PubMed2001

Electrospray ionization tandem mass spectrometric study of salt cluster ions. Part 1--investigations of alkali metal chloride and sodium salt cluster ions.

Salt cluster ions of alkali metal chlorides ACl (A = Li(+), Na(+), K(+), Rb(+) and Cs(+)) and sodium salts NaB (B = I(-), HCOO(-), CH(3)COO(-), NO(2)(-), and NO(3)(-)), formed by electrospray ionization, were studied systematically by mass spectrometry. The influences on the total positive ion and negative ion currents of variation of solvent, solution concentration, desolvation temperature, solution flow-rate, capillary voltage and cone voltage were investigated. Only cone voltage was found to influence dramatically the distribution of salt cluster ions in the mass spectra observed. Under conditions of normal cone voltage of approximately 70 V, cluster ions having magic numbers of molecules are detected with high relative signal intensity. Under conditions of low cone voltage of approximately 10 V, the distribution of cluster ions detected is characterized by a relatively low average mass/charge ratio due to the presence of multiply charged cluster ions; in addition, there is a marked reduction in cluster ions having a magic number of molecules. Product ion mass spectra obtained by tandem mass spectrometry of cluster ions are characterized by a base peak having a magic number of molecules that is less than and closest to the number of molecules in the precursor ion. Structures have been proposed for some dications and some quadruply charged ions. At pH 3 and 11, the mass spectra of NaCl clusters show the presence of mixed clusters of NaCl with HCl and NaOH, respectively. The effects of ionic radius on 20 distributions of cluster ions for 10 salts were investigated; however, the fine structure of these effects is not readily discerned.

Journal Article↗
PubMed2002

Clustering of nucleobases with alkali metals studied by electrospray ionization tandem mass spectrometry: implications for mechanisms of multistrand DNA stabilization.

Self-clustering of the five common nucleobases was investigated by electrospray ionization tandem mass spectrometry and shown to provide insight into the non-covalent interactions between identical bases. Alkali and ammonium cations significantly increase self-aggregation of the nucleobases and lead to the formation of uniquely stable magic number clusters. Sodium adducts of guanine, thymine and uracil preferentially take the form of tetrameric (quartet) clusters. This gas-phase result correlates with previously reported solution-phase data on sodium cation stabilized guanosine, thymine and uracil quartet structures believed to be responsible for telomere stabilization. In the presence of potassium, cesium or ammonium cations, pentameric magic number clusters are formed from thymine and uracil, while in solution the nucleoside isoguanosine yields clusters of this favored size. The formation of magic number metaclusters occurs for thymine and uracil in the presence of ammonium cations. These doubly charged 10- and 15-mers are tentatively attributed to the formation of pentamer/ammonium cation/ pentamer sandwich structures.

Chromatography, High Pressure Liquid↗
PubMed1978

GLC determination of caffeine in plasma using alkali flame detection.

A rapid, specific, and sensitive GLC assay for caffeine in plasma was developed utilizing alkali flame-ionization detection. The procedure involves the addition of mepivacaine as an internal standard, alkalinization of the sample, and extraction with chloroform. Peak height ratio measurements produced linear standard curves in the 0.25-10.0-microgram/ml range. Absolute sensitivity from a 1.0-ml plasma sample was 0.1 microgram/ml. The relative standard deviation of a 2.0-microgram/ml pooled plasma standard run repeatedly over several months was 5.2%. The method is applicable to time-concentration studies in human and animal plasma following typical oral doses of caffeine.

Animals↗
PubMed1994

Alkali halide-assisted penetration of neostigmine across excised human skin: a combination of structured water disruption and a Donnan-like effect.

The penetration of neostigmine across excised human skin mounted in flow-through diffusion cells, delivered from a 0.28 M aqueous solution, was below detection limits. The presence of either NaCl or LiCl in the donor solution caused significant fluxes of neostigmine, with permeability coefficients (Kp's) in the range of 10(-6) cm min-1. Paradoxically, low concentrations of NaCl or LiCl (0.25 and 0.5 M) were more effective in this respect than the 1 M solution, which was the least effective concentration in the range of 0.25-3 M. Thus, the dependence of the experimental Kp values on inorganic ion concentration followed a biphasic course, suggesting the participation of two distinctive mechanisms in the penetration-enhancement process. The early phase corresponding to 0.25 and 0.5 M NaCl or LiCl is being partly ascribed to a decrease in the viscosity of lamellar water caused by the influx of the respective hydrated ions, hydration of LiCl or NaCl being more extensive at low alkali halide concentration that at higher ones (reference cited). The late phase corresponding to 2 and 3 M LiCl and NaCl is partly ascribed to a Donnan-like effect whereby the presence of a large excess of poorly diffusible common ion (Na+ or Li+) enhances the partitioning into the skin of the more diffusible ion, in this case neostigmine cation. The presence of inorganic ions at different concentrations had no effect on the partial molal volume of neostigmine bromide (Vi infinity = 223.5 cm3 mol-1), which was practically the same for all concentrations of either LiCl and NaCl.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzalkonium Compounds↗
PubMed1993

DNA binding and cleavage of a novel antitumor antibiotic dynemicin A: detection of binding and cleavage sites by exonuclease III digestion and alkali-induced cutting reactions.

We found here that dynemicin A effectively breaks DNA strands under alkaline pH condition. Binding of dynemicin A to double-stranded DNA clearly interrupts the digestion reaction of exonuclease III. The DNA association sites of dynemicin A correspond considerably well to its cleavage sites. Dynemicin A seems to intercalate preferentially into the relaxed region of the DNA double helix. On the other hand, the alkali-product of dynemicin A was chromatographically identified with dynemicin N, suggesting a DNA cleavage mechanism similar to the reductant- and light-induced activation systems of dynemicin A. In order to detect drug binding to the relaxation structure in the DNA duplex, the present exonuclease III-digestion-stop-sequence method is useful.

Anthraquinones↗
PubMed1993

Evaluation and control of mercury vapor exposure in the cell house of chlor alkali plants.

A pilot study was carried out in the cell houses of three chlor alkali plants to assess level of exposure to mercury vapors among workers by air and biological monitoring. Overall airborne mercury concentrations (mg/m3) were found to range from 0.05 to 0.42 (mean, 0.21, n = 68), 0.03 to 0.16 (mean, .08, n = 49), and 0.02 to 0.17 (mean, 0.04, n = 26), whereas urinary mercury levels (mg/liter) of the exposed workers of the respective plants ranged from 0.076 to 0.592 (mean, 0.207, SD, 0.107, n = 19), 0.015 to 0.220 (mean, 0.070, SD, 0.054, n = 16), and 0.013 to 0.275 (mean, 0.06, SD, 0.054, n = 23). Unattended mercury spillage on the floor and improper sealing of the lids of the end boxes of electrolysis cells were found to be main factors attributing to prevalence of mercury vapors in excess of the permissible exposure limit of 0.05 mg/m3. Based on the deficiencies observed, appropriate control measures have been suggested to reduce airborne mercury vapor concentrations in the work environment.

Air Pollutants, Occupational↗
PubMed2000

Relaxation Spectra and Viscoelastic Behavior of a Model Hydrophobically Modified Alkali-Soluble Emulsion (HASE) Polymer in Salt/SDS Solutions.

The viscoelastic behavior of a semidilute hydrophobically modified alkali-soluble emulsion (HASE)-C20 polymer in NaCl and NaCl/SDS (sodium dodecyl sulfate) solutions was determined using a Rheometric fluids rheometer and the data were converted to relaxation spectra. The dynamic moduli can be fitted with a multiple modes Maxwell model. In the presence of increasing amounts of NaCl, the moduli decrease, where G', decreases more rapidly than G". However, in the presence of SDS and 0.4 M NaCl, the dynamic moduli increase to a maximum at a critical concentration and decrease thereafter. The relaxation spectra suggest that the structure of the polymer network is complex and it contains two to six relaxation times, depending on the NaCl or SDS/0.4 M NaCl concentrations. With increasing NaCl concentrations, the fastest peak shifts to longer times while the slowest peak decreases. This corresponds to the destruction of the network as the polymer backbone collapses to form clusters with a larger aggregation number. For HASE in SDS/0.4 M NaCl solutions, the lifetime of both the hydrophobic junction (fastest peak) and network relaxation (slowest peak) shift to longer times, which suggests the strengthening of active junctions by bound SDS molecules. However, beyond a critical SDS concentration, the relaxation time of the polymer and hydrophobic junction decreases to an asymptotic value. Copyright 2000 Academic Press.

Journal Article↗
PubMed1997

Exchange Energy on Alkali Diatomic Molecules: Period Effects

The exchange energy in the long-range near-dissociation region is analyzed for the ground electronic states of homo- and heteronuclear alkali diatomic molecules. The analysis is based on the use of the functional form Ae-ar to represent this exchange term. A period effect is observed in the values of both parameters and the exponent parameter a appears to approach an asymptotic limit for the heavier molecules. The results obtained in this study also suggest that some care must be taken in using even this function to represent the exchange energy. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1998

Analytical Formulas for Long-Range Energies of the 16 Omega(+/-)g,u States of Alkali Dimers Dissociating into M(ns) + M(np 2PJ)

Analytical formulas for long-range energies of the 16 Omega(+/-)g,u states of alkali dimers dissociating into M(ns) + M(np 2PJ) are displayed in terms of the long-range Cn coefficients for the 2S+1Lambda(+)g,u corresponding states, and including exchange interaction energy contributions. Corrections due to retardation effects for resonant-dipole interactions are also included. For values of the internuclear distance R large enough so that only Coulombic interactions in R-3 are nonnegligible, the variation with R of the long-range energy is seen to be different from the usually assumed C3/R3 form. Present formulas can be used both to predict accurately long-range energies and to fit experimental energies now available in such long-range of R from photoassociative spectroscopy. As an example, they have been used, together with more accurate values of Cn coefficients than those used some 10 years ago, to predict again the three states of Rb2 displaying structures (well and barrier) at long range, i.e., 0(-)g(3/2), 1u(3/2), and 1u(1/2). Copyright 1998 Academic Press.

Journal Article↗