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PubMedDate not supplied

Molecular recognition by natural macrocycles. I. d-tubocurarine as a host molecule for organic anions.

The binding of 8-anilino-1-naphthalenesulfonate and 15 anions of substituted benzoic, aliphatic dicarboxylic, and N-acetyl-alpha-amino acids to a macrocyclic alkaloid d-tubocurarine in aqueous solution has been studied by fluorometry, conductometry, and 1H NMR. The binding constants vary from ca. 50 to 3300 M-1 depending on the guest structure, charge and hydrophobicity. The results of fluorescence and NMR studies show that the host-guest complexation of the anions of aromatic acids involves the formation of a salt bridge between the quaternary nitrogen of the alkaloid and the anionic group of the guest as well as hydrophobic/Van der Waals interactions between the guest and host aromatic moieties. The binding of dianions of aliphatic dicarboxylic acids most probably is purely electrostatic. In general, d-tubocurarine possesses binding ability comparable to that of synthetic cyclophanes. It binds enantiospecifically anions of N-acetyl-alpha-amino acids and discriminates between positional isomers of anions of hydroxy and carboxy substituted benzoic acids.

Anions↗

PubMed1976

[Investigations in streaming blood of the alert animal and man. II. Investigations on the mechanism of water resorption from the gut into the portal vein blood in the rat (author's transl)].

Arteriovenous differences (AVD) of electrolyte concentration in the gut are followed by continuous conductometry of ultrafiltrates of portal venous and aortic blood. Intraduodenal application of tap water amounting to 0.5, 1,0 or 2,0% body weight lead to extremely variable reactions both in time course and dimension of AVD. Whereas large and fast changes can be described by a single exponential term, small and slow changes show saturation effects. Evidence is presented, that these differences correspond to permeability changes, both for water in the direction: lumen to blood and for electrolytes in the direction: blood to lumen. Thirst has an enhancing effect on AVD. A dual resorptive pathway is proposed to fit the data.

Animals↗

PubMed2004

Transport properties and association behaviour of the zwitterionic drug 5-aminolevulinic acid in water. A precision conductometric study.

The behavior of the hydrochloride salt of 5-aminolevulinic acid (ALA-HCl) with respect to transport properties and dissociation in aqueous solution at 25 degrees C has been studied using precision conductometry within the concentration range 0.24-5.17mM. The conductivity data are interpreted according to elaborated conductance theory. The carboxyl group appears to be, in practice, undissociated. The dissociation constant, K(a), of the NH(3)(+) form of the amino acid molecules is determined to 6.78x10(-5) (molarity scale); pK(a)=4.17. The limiting molar conductivity of the ALA-H(+) ion, lambda(0)=33.5cm(2)Omega(-1)mol(-1); electric mobility u=3.47x10(-4)cm(2)V(-1)s(-1), is close to the electric mobilites of the acetate and benzoic ions.

Aminolevulinic Acid↗

PubMed2002

Improvements of conductivity measurements of electrolyte solutions using a new conductometric cell design.

A new conductometric cell design, for precise conductance measurements, has been developed and tested using aqueous lidocaine hydrochloride as a model system. A small portion of a stock solution in the conductivity cell is diluted stepwise by pure solvent. The resistance of the cell is measured by means of a precision conductance bridge. Contrary to conventional technique in precision conductometry, the temperature is allowed to change during the measurements and corrected to the desired standard temperature. The temperature is determined using a thermistor immersed in the cell solution, which is agitated during the entire experiment. Using this new approach, significant improvements over conventional conductivity technique were observed. The time required for the measurements was considerably reduced, by a factor of at least ten. The amounts, especially of costly drugs, required in the measurements are also reduced. The pK(a) value obtained, 7.28, is close to the previously reported conductometrically determined average, 7.18. The precision of the single conductivity value is equally high, if not higher, as that obtained using conventional conductivity technique.

Anesthetics, Local↗

PubMed2006

Cationic prevesicle and vesicle nanoaggregates: an experimental and theoretical study.

The formation of spontaneous mixed prevesicles and vesicles consisting of a cationic double-chain surfactant, didecyldimethylammonium bromide (di-C(10)DMAB), and a cationic single-chain alkyltrimethylammonium bromide with 10 and/or 14 carbon atoms (decyltrimethylammonium bromide, C(10)TAB, and/or tetradecyltrimethylammonium bromide, C(14)TAB) has been investigated by means of a series of (i) highly precise experimental techniques, such as conductometry, transmission electronic microscopies (TEM and cryo-TEM), laser Doppler electrophoresis (LDE), and steady-state fluorescence spectroscopy and (ii) theoretical models, such as the DLVO theory and two of its main further modifications, Inoues's and Sogami's models. Two new potentials, based on the combination of DLVO or Inoue potentials with that of Sogami, have been proposed and checked. This theoretical analysis has been carried out not only for the aggregates studied in this work but also for other di-C(m)DMAB + C(n)TAB (m = 10, 12 and n = 10, 12, 14) systems previously reported by us. In respect to the experimental study, special emphasis has been devoted to the prevesicle domain. We have confirmed the existence of two critical aggregation concentrations in the very diluted concentration domain, where the conductivity plot shows a zigzag pattern: the so-called mixed critical aggregate concentration, CAC* and the mixed critical vesicle concentration, CVC*. Contrarily, only CVC* is detected. The pre-CAC* nanoaggregates, with a variety of sizes and shapes, do not show a clear aggregation pattern, but even at such low concentrations a small number of nanoaggregates with a clear and ordered aggregation pattern has been visualized. In the postvesicle domain, the aggregates (vesicles) are unilamellar and spherical with a medium polidispersity and a net averaged surface density charge of around 14 x 10(-3) (pure vesicles) and 24 x 10(-3) C m(-2) (mixed vesicles). The hydrophobicities of the lipidic bilayer and the surface of the vesicles resemble those of media with dielectric constants of around 30 and 75, respectively. Finally, theoretical predictions confirm the stability of the pure and mixed vesicles studied in this work and in other works previously reported.

Algorithms↗

PubMedDate not supplied

[Interaction of pancreatic RNAase A with platinum ions].

Complex formation between RNAase and Pt(II) has been studied by gel filtration and conductometry, and spectrophotometry. Existence of three strong binding locations with the binding location of 2 times 10-4 M-1 has been stated. Effect of Pt(II) on hydrolase activity of RNAase Ahas been investigated. An inhibiting effect showed itself at high concentrations of platinum ions. A conclusion has been made concerning the nature of groups participating in complex formation with Pt(II). Modification of three methionin residues does not influence the enzyme activity at 24-48 hour preincubation.

Binding Sites↗

PubMed1975

[On the role of pentachlorocyclohexene in the metabolism and action of hexachlorocyclohexane. I. Synthesis of beta-pentachlorocyclohexene and its identification as the monodehydrochlorination product of alpha-hexachlorocyclohexane (author's transl)].

It is the aim of a series of investigations to test whether or not beta-pentachloro-1-cyclohexene is an intermediate in the biodegradation of alpha-hexachlorocyclohexane. This paper describes attempts to synthesize this intermediate by chemical methods. 1) Pentachlorocyclohexene was synthesized by partial additive chlorination of chlorobenzene. Combined gas chromatography-mass spectrometry revealed that at least five different isomers of pentachlorocyclohexene had been formed. 2) Treatment of alpha-hexachlorocyclohexane with alkaline buffer (pH 8) produced trichlorobenzenes and, in small yield (4%), a pentachlorocyclohexene. This was isolated and identified as the beta-isomer by melting point (71.8 - 72.6 degrees C, uncorr.), IR- and mass spectrum. Dehydrochlorination of beta-pentachlorocyclohexene produced the trichlorobenzene isomers in a pattern which is characteristic of alpha-hexachlorocyclohexane. The position of the chlorine substituents in the beta-pentachlorocyclohexene molecule as judged from NMR studies is e-aeee. This confirms that it is the monodehydrochlorination product of alpha-hexachlorocyclohexane. The configurations of gamma- and delta-pentachlorocyclohexene, determined for comparison, are e-eeaa and e-eeee, respectively. The kinetics of dehydrochlorination of both alpha-hexachlorocyclohexane and beta-pentachlorocyclohexene in alkaline acetone/water (3 + 2) was studied by means of conductometry. Both reactions are of second order: kappa alpha-HCH 0.0495 [1 times mol- minus 1 times s- minus 1[; kappa beta-PCH 0.905 [1 times mol- minus 1 times s- minus 1] (3.6 degrees C). 3) Dehydrochlorination of alpha-hexachlorocyclohexane in pyridine/xylene (3 + 4) was also studied. An earlier report claiming that gamma-pentachlorocyclohexene (and not the beta isomer) is produced in this medium was confirmed, if the reaction was performed at high temperature (120 - 140 degrees C). Moreover, the ratio of trichlorobenzene isomers formed from alpha-hexachlorocyclohexane shifted to a pattern characteristic of the gamma (or gamma) isomer. However, at temperatures of 90 degrees C or less, beta-pentachlorocyclohexene was the main product. The results strongly suggest that in pyridine/xylene, the same isomer is primarily produced from alpha-hexachlorocyclohexane and is isomerized to the gamma, delta and at least two other isomers of pentachlorocyclohexene before further dehydrochlorination ensues. A simple method for the synthesis of beta-pentachlorocyclohexene is presented.

Biodegradation, Environmental↗

PubMed2000

Stability and electrophilicity of phosphorus analogues of arduengo carbenes--an experimental and computational study

A variety of differently substituted 1,3,2-diazaphospholenium salts and P-halogeno-1,3,2-diazaphospholenes (X = F, Cl, Br) were synthesized, and their molecular structures, bonding situation, and Lewis acid properties were characterized by experimental (single-crystal X-ray diffraction, NMR and IR/Raman spectroscopy, MS, conductometry, titrations with Lewis bases) and computational methods. Both experimental and computational investigations confirmed that the structure and bonding in the diazaphospholenium cations of OTf and BF4 salts resembles that of neutral Arduengo carbenes and that the cations should not be described as genuinely aromatic. P-Halogenodiazaphospholenes are, in contrast to earlier assumptions, molecular species with covalent P-X bonds whose bonding situation can be expressed in terms of hyperconjugation between the six pi electrons in the C2N2 unit and the sigma*(P-X) orbital. This interaction induces a weakening of the P-X bonds, whose extent depends subtly on substituent influences and contributes fundamentally to the amazing structural similarity of ionic and covalent diazaphospholene compounds. A further consequence of this effect is the unique polarizability of the P-Cl bonds in P-chlorodiazaphospholenes, which is documented in a considerable spread of P-X distances and bond orders. Measurement of the stability constants for complexes of diazaphospholene compounds with Lewis bases confirmed the lower Lewis acidities and higher stabilities of diazaphospholenium ions as compared with nonconjugated phosphenium ions; this had been inferred from computed energies of isodesmotic halide-transfer reactions, and permitted also to determine equilibrium constants for P-Cl bond dissociation reactions. The results suggest, in accord with conductance measurements, that P-chlorodiazaphospholenes dissociate in solution only to a small extent. On the basis of these findings, the unique solvatochromatic behavior of NMR chemical shifts of these compounds was attributed to solvent-dependent P-Cl bond polarization rather than to shifts in dissociation equilibria.

Journal Article↗

PubMed2002

Deprotonation and dimerization of maleimide in the triplet state: a laser flash photolysis study with optical and conductometric detection.

The photochemistry of maleimide in aqueous solution is governed by the coexistence of up to three different triplet states, the keto triplet (lambda(max)=250, 330 nm, lambda(min)=290 nm, pK(a)=4.4+/-0.1, tau=5 micros), the deprotonated or enolate triplet (lambda(max)=360, 260 nm, lambda(min)=320 nm, shoulder at 370-380 nm) and a dimer triplet. This biradical is formed by the addition of the keto triplet to the double bond of a ground state maleimide in competition with electron transfer, (k( (3)MI+MI)=2.6 x 10(9) dm(3) mol(-1) s(-1)). Its spectrum is identical to that of the maleimide H-adduct radical (lambda(max)=370-380 (broad), 255 nm (narrow), lambda(min)=290 nm) and its lifetime is 110 ns. While protolysis is confined to maleimide and aqueous solutions, the dimer triplet is also found in acetonitrile. Dimer triplet formation is also observed with N-ethylmaleimide. Time-resolved conductometry and buffer experiments were used to characterise excited state protolysis. Multi-wavelength "global analysis" of the time profiles allowed the separation of the transient spectra and study of the kinetics of the monomer and dimer triplets. The cyclobutane dimer yield (determined by GC) is independent of maleimide concentration. This indicates that the dimer triplet does not contribute significantly to the initiation of free-radical polymerisation. Time-dependent Hartree-Fock calculations agree with the experimental data and further confirm the proposed mechanisms.

Journal Article↗

PubMed2007

Thin films composed of multiwalled carbon nanotubes, gold nanoparticles and myoglobin for humidity detection at room temperature.

Optically transparent and electrically conductive nanocomposite thin films consisting of multiwalled carbon nanotubes (MWCNTs), gold nanoparticles (GNPs) and myoglobin molecules that glue GNPs and MWCNTs together are fabricated for the first time on glass substrates from aqueous solution. The nanocomposite thin film is capable of varying its resistance, impedance or optical transmittance at room temperature in response to changes in ambient humidity. The conductometric sensitivity to relative humidity (RH) of the nanocomposite thin film is compared with those of the pure and Mb-functionalized MWCNT layers. The pure MWCNT layer shows a small increase in its resistance with increasing RH due to the effect of p-type semiconducting nanotubes present in the film. In contrast, a four times higher sensitivity to RH is observed for both the nanocomposite and Mb-functionalized MWCNT thin films. The sensitivity enhancement is attributable to swelling of the thin films induced by water absorption in the presence of Mb molecules, which increases the inter-nanotube spacing and thereby causes a further increase of the film resistance. A humidity change as low as DeltaRH=0.3 % has been readily detected by conductometry using the nanocomposite thin film.

Gold↗

PubMed2006

Effect of alkali metal hydroxides on the enantioseparation of amines using di-O-isopropylidene-keto-L-gulonic acid as the selector in NACE.

The present work demonstrates the importance of the ionic composition in the BGE for enantioseparation. (-)-2,3:4,6-di-O-Isopropylidene-2-keto-L-gulonic acid ((-)-DIKGA) has been used as the chiral selector in methanolic and ethanolic BGEs. The influence of added alkali metal hydroxides on the EOF and the chiral separation of amines (atenolol, isoprenaline, pindolol and propranolol) have been studied. The ion-pair formation constants in ethanol were determined by precision conductometry for the enantiomers of pindolol with (-)-DIKGA, for Li(+), Na(+) and Cs(+) with (-)-DIKGA, and also for the corresponding alkali metal hydroxides. The effective mobilities and the enantiomeric mobility differences were affected by the type of alkali metal hydroxide (LiOH, NaOH, KOH, RbOH or CsOH) added to the BGE. The effective mobility and mobility difference were increased with decrease in solvated radius of the alkali metal cation. These differences could partly be correlated to the ion-pair formation constants of the alkali metal cations with the chiral selector, affecting the equilibrium concentration of the free selector. The electroosmosis was also affected by the alkali metal hydroxide added to the BGE. The cathodic electroosmosis decreased with decreasing solvated radius of the alkali metal cation added to the BGE. Interestingly, the cathodic EOF was even reversed, i.e. became anodic in the ethanolic BGEs containing KOH, RbOH or CsOH and the methanolic ones with RbOH and CsOH.

Amines↗

PubMed2001

Electrical Conductivity of Water/Sodium Bis(2-ethylhexyl) Sulfosuccinate/n-Heptane and Water/Sodium Bis(2-ethylhexyl) Phosphate/n-Heptane Systems: The Influences of Water Content, Bis(2-ethylhexyl) Phosphoric Acid, and Temperature.

Conductance behaviors of reverse micelles/microemulsions have been investigated by means of electrical conductometry, with em-phasis on the influences of water content, bis(2-ethylhexyl) phos-phoric acid (HDEHP) content, and temperature. Two systems were prepared and studied, i.e., sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in n-heptane (H(2)O/AOT/n-heptane) and sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane (H(2)O/NaDEHP/ n-heptane). The conductance behaviors of AOT+HDEHP and NaDEHP+HDEHP systems are dependent upon water content (W(0), expressed as the molar ratio of solubilized water and surfactant(s)), HDEHP content, and temperature. With an increase of water content, the conductivity-W(0) curves of the AOT+HDEHP system show maximum conductivity and no percolation conductance, different from that of the AOT system. The conductivity of the NaDEHP+HDEHP system varies with the NaDEHP to HDEHP ratio. For the systems with the NaDEHP to HDEHP ratios of 80 : 20 and 70 : 30, both maximum conductivity and percolation phenomenon can be observed, while for the system with the NaDEHP to HDEHP ratio of 90 : 10, neither maximum conductance nor percolation conductance occurs. With varying temperature, markedly different conductance behaviors were observed in AOT+HDEHP and NaDEHP+HDEHP systems. Percolation conductance occurs in the AOT+HDEHP system and the onset temperature for percolation conductance decreases with increasing water content and/or HDEHP content. In the H(2)O/(NaDEHP, HDEHP)/n-heptane system, however, the conductivity decreases markedly with increasing temperature until a minimum, followed by a slow recovery of the conductivity. These phenomena could be attributed to the different aggregation states of the surfactants and the different transition mechanisms of charge carriers in respective systems at different water contents, HDEHP contents, and temperatures. Copyright 2001 Academic Press.

Journal Article↗

PubMed2002

Adsorption and association in aqueous solutions of dissymmetric gemini surfactant.

A gemini surfactant with two hydrocarbon chains differing in length and with an ethylene spacer N,N-dimethyl-N-(2-(N',N'-dimethyl-N'-dodecylammonio) ethyl) tetradecylammonium dibromide, 12-2-14, was synthesized and its physicochemical properties were studied by surface tension, conductometry, potentiometry, viscosimetry, and light scattering measurements, as well as by optical microscopy. Surface properties and thermodynamic parameters lie between those obtained for its symmetric counterparts, while association in solution exhibited peculiar properties, i.e., high polydispersity and the coexistence of three populations of differently sized aggregates.

Journal Article↗

PubMed1978

[Investigations in the streaming blood of the alert animal and man. III. Postprandial and therapeutically induced changes of electrolyte concentration and hematocrit in man during hemodialysis (author's transl)].

Arterial haematocrit and blood electrolyte concentrations are measured continuously by conductometry in three hemodialysis patients. Haematocrit elevation during meals, particularly when rich in carbohydrates, indicate fluid losses into the gut. Simultaneous biphasic responses in electrolyte concentration indicate osmotic and diffusional shifts of water and electrolytes. Similarly, blood volume and electrolyte concentration changes can be followed during and after intravenous infusion of colloid and crystalloid solutions. The quantitative aspects of such measurements are discussed.

Adult↗

PubMed2000

Individual sample conditioning in flow analysis. Determination of N-total in plant materials.

A flow-batch system allowing in-line individual sample matrix matching is proposed for analysis of sample lots with high variability in acidity. The feasibility of the approach is demonstrated in the spectrophotometric determination of total nitrogen in Kjeldahl digests, using a column with a slightly soluble reagent (AgCl). The solutions are sequentially injected by means of an 8-port selecting valve and processed in a mixing chamber that is also used as a monitoring unit. The system yields reproducible results (r.s.d. usually < 2.5%) and the sampling rate is 14 samples/h. The analytical curve is linear within 1.00 and 6.00% N (dry basis), and the regression coefficient is > 0.999 (n = 6). Results are in agreement with certified values of standard reference materials and with results obtained by conductometry.

Acids↗

PubMed1993

Relation between the physicochemical characteristics of collagen and its interactions with chitosan: I.

The interaction between bovine atelocollagen and a high molecular weight fully deacetylated chitosan has been studied. Considering the apparent mass of an anionic equivalent of collagen and the results of various techniques (potentiometry, conductometry and IR spectroscopy), a purely electrostatic mechanism is demonstrated. The role of charge density and chain length of collagen chains is discussed, and although the interaction is weak and hindered by gel formation in collagen solutions, a polyanion-polycation complex is obtained. Circular dichroism spectroscopy experiments show an organization in the solid state of the complex which is quite different to those of collagen and chitosan.

Acetylation↗

PubMed1983

Recent applications of electrochemical techniques to the analysis of pharmaceuticals.

The past decade has revealed a remarkable and renewed interest in the application of electrochemical techniques to the analysis of drugs and pharmaceuticals. In addition, several of these techniques are now being employed to elucidate electrode processes by studying the mechanisms involved in organic redox reactions. The renewed interest in electrochemical techniques can be attributed in part to more sophisticated instrumentation and to an increased understanding of the techniques themselves. This paper selectively reviews recent applications of the following electrochemical modes in the analysis of drugs and pharmaceuticals: amperometry, conductometry, coulometry, ion-selective electrodes, potentiometry, stripping voltammetry (anodic and cathodic) and voltammetry.

Journal Article↗

PubMed2004

A strategy for establishing accurate quantitation standards of oligonucleotides: quantitation of phosphorus of DNA phosphodiester bonds using inductively coupled plasma-optical emission spectroscopy.

A novel approach for quantitation of DNA (oligonucleotides) with an unprecedented accuracy of approximately 1% is reported. Quantitation of DNA is commonly performed by measuring UV absorption or fluorescence from dyes intercalated into DNA. Both methods need accurate quantitation standards to yield more comparable results between laboratories. For establishing technically authentic standards for DNA quantitation, a new measurement approach carrying an inherent capability of absolute quantitation is demanded. The proposed approach is based on the stoichiometric existence of phosphorus (P) in DNA. The quantity of P from the phosphodiester backbone of a purified oligonucleotide was accurately determined using inductively coupled plasma-optical emission spectroscopy (ICP-OES) with yttrium internal standard via acid digestion. The number of moles of oligonucleotides was then calculated from that of P using the stoichiometry. The major issues regarding the validity of the suggested approach were (i) effective removal of extra P sources, (ii) quantitative recovery of P through the digestion process, and (iii) oligomeric purity of the target oligonucleotide. These issues were investigated experimentally using various analytical techniques such as matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS), capillary electrophoresis, electrical conductometry, UV spectrometry, and gravimetry. In conclusion, it is feasible to certify pure oligonucleotide reference materials with uncertainties less than 1% using the proposed approach.

DNA↗