Electroosmosis through a Cation-Exchange Membrane: Effect of an ac Perturbation on the Electroosmotic Flow

Electroosmosis experiments through a cation-exchange membrane have been performed using NaCl solutions in different experimental situations. The influence of an alternating (ac) sinusoidal perturbation, of known angular frequency and small amplitude, superimposed to the usual applied continuous (dc) signal on the electroosmotic flow has been studied. The experimental results show that the presence of the ac perturbation affects the electroosmotic flow value, depending on the frequency of the ac signal and on the solution stirring conditions. In the frequency range studied, two regions have been observed where the electroosmotic flow reaches a maximum value: one at low frequencies (Hz); and another at frequencies of the order of kHz. These regions could be related to membrane relaxation phenomena.     

Application of the JKR Method to the Measurement of Adhesion to Langmuir-Blodgett Cellulose Surfaces

The JKR method has been applied for studying adhesion between poly(dimethylsiloxane) (PDMS) caps and Langmuir–Blodgett cellulose surfaces including the substrate, hydrophobized mica, and two flat mineral surfaces, bare mica and glass. The self-adhesion of PDMS caps and oxidized PDMS caps are included as a reference to compare with literature data. The results of the measurements have been compared with previous studies using the surface force apparatus and similar systems. A satisfactory agreement is obtained for simple systems showing no, or very limited, hysteresis between loading and unloading curves. In several cases, however, a large hysteresis is found between loading and unloading curves, with a larger adhesion measured from the pull-off force than from the JKR-curve determined on loading. This is, for instance, the case for PDMS against cellulose. The situation is analogous to that found in wetting studies showing a large hysteresis between advancing and receding contact angles.     

Synthesis,characterization and antimicrobial studies on 13-membered-N6-macrocyclic transition metal complexes containing trimethoprim

Asymmetrical macrocyclic complexes of Mn^II, Co^II, Ni^II, Cu^II and Zn^II have been synthesized by the template process using bis(benzil)ethylenediamine as precursor. Bis(benzil)ethylenediamine reacts with transition metal chlorides and trimethoprim in a 1:1:1 molar ratio in methanol to give several solid metal complexes of the general composition [M(L)X₂] (M = Mn^II, Co^II, Ni^II, Cu^II and Zn^II, L = ligand and X = Cl^?). They were characterized by physicochemical and spectroscopic techniques. Based on analytical, spectral and magnetic moments, all the complexes are identified as distorted octahedral structures. All the complexes are of the [M(L)X₂] type. The shifts of the ν(CN) (azomethine) stretches have been monitored. To find out the donor sites of the ligands, the activity data show that the metal complexes are more potent than the parent ligand. The [M(L)X₂] complexes showed a broad spectrum of antimicrobial activity in vitro against both gram-positive and gram-negative human pathogenic bacterial isolates and the antimicrobial spectrum enhanced only with a combination of metal chlorides and trimethoprim complex. From the results it is imperative that the synthesized macrocyclic [M(L)X₂] complexes exhibit potent broad spectrum antibacterial activity.     

Variational solution of the single-particle Dirac equation in the field of two nuclei using relativistically adapted Slater basis functions

A variational method for solving the time-independent single-particle Dirac equation in the Coulomb field of two nuclei is described. A minimax variational principle and basis functions that have the proper analytic behavior, i.e. behave like r ^γ,γ non-integer, in the neighborhood of a nucleus, are used. A momentum space integration scheme for computing the necessary two-center integrals is described. Results are given for a standard test problem on two nuclei with Z=90 with an internuclear separation of 2.0/Z. The results confirm those of a previous calculation [F.A. Parpia and A.K. Mohanty, Chem Phys Lett 238: 209 (1995)]. Received: 13 May 1998 / Accepted: 22 June 1998 / Published online: 28 August 1998     

Chemiluminescence of Peracetic Acid in Alkaline Medium and its Application to Dihydralazine Sulfate Determination

The chemiluminescence (CL) of peracetic acid (PAA) in alkaline medium is very weak but is strongly enhanced after the addition of dihydralazine sulfate (DHZS). Based on this phenomenon, a simple, rapid and highly sensitive flow-injection CL method for the determination of DHZS was developed. The CL emission was linearly related to the DHZS concentration in the range of 20–4000 ng mL⁻¹ with a detection limit (3σ) of 1.2 ng mL⁻¹. As a preliminary application, the proposed method was successfully applied to the determination of DHZS in pharmaceutical preparations; the recovery of DHZS in human urine was between 96.5% and 102.2%. A detailed CL mechanism was proposed and singlet molecular oxygen (¹O₂) was suggested to be produced in the CL reaction process.     

Reduction of soluble colloidal MnO<Subscript>2</Subscript> by <Emphasis Type="SmallCaps">DL</Emphasis>-malic acid in the absence and presence of nonionic TritonX-100

Kinetics of oxidation of DL-malic acid by water soluble colloidal MnO₂ (prepared from potassium permanganate and sodium thiosulfate solutions) have been studied spectrophotometrically in the absence and presence of nonionic Triton X-100 surfactant. The reaction is autocatalytic and manganese(II) (reduction product of the colloidal MnO₂) may be the autocatalyst. The order of the reaction is first in colloidal [MnO₂] as well as in [malic acid] both in the absence and presence of the surfactant. The reaction has acid-dependent and acid-independent paths and, in the former case, the order is fractional in [H⁺]. The effect of externally added manganese(II) is complex. The results show that the rate constant increases as the manganese(II) concentration is increased. It is not possible to predict the exact dependence of the rate constants on manganese(II) concentration, which has a series of reactions with other reactants. In the presence of TX-100, the observed effect on k is catalytic up to a certain [TX-100]; thereafter, an inhibitory effect follows. The catalytic effect is explained in terms of the mathematical model proposed by Tuncay et al. (in Colloids Surf A Physicochem Eng Aspects 149:279 3). Activation parameters associated with the observed rate constants (k_obs/k) have also been evaluated and discussed.     

On the conformational stability and dimerization of phosphotransferase enzyme I from Escherichia coli

The activity of enzyme I (EI), the first protein in the bacterial PEP:sugar phosphotransferase system, is regulated by a monomer–dimer equilibrium where a Mg²⁺-dependent autophosphorylation by PEP requires the homodimer. Using inactive EI(H189A), in which alanine is substituted for the active-site His189, substrate binding effects can be separated from those of phosphorylation. Whereas 1 mM PEP (with 2 mM Mg²⁺) strongly promotes dimerization of EI(H189A) at pH 7.5 and 20 °C, 5 mM pyruvate (with 2 mM Mg²⁺) has the opposite effect. A correlation between the coupling of N- and C-terminal domain unfolding, measured by differential scanning calorimetry, and the dimerization constant for EI, determined by sedimentation equilibrium, is observed. That is, when the coupling between N- and C-terminal domain unfolding produced by 0.2 or 1.0 mM PEP and 2 mM Mg²⁺ is inhibited by 5 mM pyruvate, the dimerization constant for EI(H189A) decreases from >10⁸ to <5 × 10⁵ or 3 × 10⁷ M⁻¹, respectively. With 2 mM Mg²⁺ at 15–25 °C and pH 7.5, PEP has been found to bind to one site/monomer of EI(H189A) with K_A′10⁶ M⁻¹ (ΔG′=−33.7±0.2 kJ mol⁻¹ and ΔH=+16.3 kJ mol⁻¹ at 20 °C with ΔC_p=−1.4 kJ K⁻¹ mol⁻¹). The binding of PEP to EI(H189A) is synergistic with that of Mg²⁺. Thus, physiological concentrations of PEP and Mg²⁺ increase, whereas pyruvate and Mg²⁺ decrease the amount of dimeric, active, dephospho-enzyme I.     

Determination of the Insect Repellent Bayrepel® in Pool and Lake Water by Gas Chromatography after Preconcentration with Solid-Phase Extraction and Stir-Bar-Sorptive Extraction

A method for the determination of the new insect repellent Bayrepel in bathing lakes and public pools is presented. Sample preconcentration is performed either by solid-phase extraction (SPE) or stir bar sorptive extraction (SBSE). Analysis of the enriched analyte using gas chromatography-mass spectrometry (GC-MS) led to almost identical limits of detection (LOD) of 25ngL^–1 for Bayrepel with both extraction methods. Because of the slightly reduced handling effort involved, the method based on preconcentration by SBSE was finally chosen for the analysis of real samples.     

An electron-diffraction study of the molecular structure of gaseous perbromyl fluoride and calculation of its force field and vibrational amplitudes

The molecular structure of FBrO₃ has been studied by gas-phase electron diffraction. Least-squares refinements of the molecular geometry using fixed spectroscopic amplitudes revealed two geometrical minima. Initially, the amplitudes employed were derived from diagonal force fields obtained by spectroscopic least-squares refinements to fit observed and calculated wave numbers; for each geometry there are two spectroscopic minima. In the lowest geometrical minimum the wave number agreement is poor, however, the introduction of the ∠OBrO/∠FBrO interaction force constant removed the discrepancies; the resulting force field is F(Br-O) = 6.92 ± 0.02 mdyn Å^?1F(Br-F) = 3.22 ± 0.03 mdyn Å^?1, F(∠OBrO) = 1.06 ± 0.02 mdyn Å, F(∠FBrO) = 0.81 ± 0.03 mdyn Å, F(∠OBrO/∠FBrO) = ?0.19 ± 0.02 mdyn Å. In the corresponding geometrical minimum r_g(Br-O) = 1.582 ± 0.001 Å, r_g(Br-F) = 1.708 ± 0.003 Å, r_α(∠OBrO) = 114.9 ± 0.3°, r_α(∠FBrO) = 103.3 ± 0.3°. Perpendicular amplitude correction coefficients, calculated for each force field employed, were used throughout to relate the interatomic distances through the r_α-structure. The geometries of the r_α^o- and r_e-structures are estimated.     

The magnetizability of excited-state hydrogen: temperature-independent paramagnetism in the B1Σ+u state

Components, of the magnetizability tensor for the B¹Σ⁺_u and a ³Σ⁺_g states ot the hydrogen molecule have been calculated for fixed bond lengths near equilibrium. Explicitly correlated “generalised James-Coolidge” type wavefunctions were used. It is found thilt in the B 1Σ⁺_u state the molecular magnetizability is a positive quantity.