Potentiometric sensor arrays for the individual determination of penicillin class antibiotics using artificial neural networks

Arrays of potentiometric sensors with plasticized polymer membranes based on tetraalkylammonium organic ion exchanges with anions of penicillin class antibiotics (benzylpenicillin, ampicillin, oxacillin, and amoxicillin) have been proposed for the individual determination of antibiotics in model mixtures and pharmaceutical preparations. The following cross-sensitivity parameters of sensors have been estimated: the average slope of the electrode function (54 < S _av < 61), the nonselectivity factor (2.8 < F < 74.6), and the reproducibility factor (31.9 < K < 61.2). Artificial neural networks have been applied to the treatment of analytical signals from the multisensor system in the concentration range 2.5 × 10⁻⁴–1 × 10⁻¹ M. The average error of the individual determination of penicillin class antibiotics is 5–7%.     

Physicochemical properties of tetraalkylammonium tetraphenylborates and tetraalkylammonium dodecylsulfates

Physicochemical parameters (composition, conventional solubility products, and thermal stability) of ionic associates of tetraalkylammonium dodecylsulfates and tetraalkylammonium tetraphenylborates are determined. The usefulness of these compounds as electrode-active components of membranes in potentiometric sensors that selective to tetraalkylammonium salts is assessed.     

Solubilities of the Silver Halides in Aqueous Mixtures of Methanol, Acetonitrile, and Dimethylsulfoxide

 The solubilities of the silver halides in three non-aqueous solvents: methanol, acetonitrile, and dimethylsulfoxide, and in their aqueous mixtures, are reviewed. Values for the solubility product, K _SO , the enthalpies of solution, Δ _sol H ^o , and the equilibrium products for AgX ⁽ⁱ⁻¹⁾⁻ _i silver halide complexes, β _i , are listed and, where possible, compared. The solvent systems provide examples for three types of mixed aqueous solvent system: aqueous alcohol mixtures and aqueous mixtures with dipolar aprotic solvents that are weakly or strongly basic. The experimental data are discussed in terms of the solvation of the silver and halide ions in the mixed solvents.     

Solubilities of the Silver Halides in Aqueous Mixtures of Methanol,Acetonitrile, and Dimethylsulfoxide

Summary.  The solubilities of the silver halides in three non-aqueous solvents: methanol, acetonitrile, and dimethylsulfoxide, and in their aqueous mixtures, are reviewed. Values for the solubility product, K _SO , the enthalpies of solution, Δ _sol H ^o , and the equilibrium products for AgX ⁽ⁱ⁻¹⁾⁻ _i silver halide complexes, β _i , are listed and, where possible, compared. The solvent systems provide examples for three types of mixed aqueous solvent system: aqueous alcohol mixtures and aqueous mixtures with dipolar aprotic solvents that are weakly or strongly basic. The experimental data are discussed in terms of the solvation of the silver and halide ions in the mixed solvents. E-mail: Earle.Waghorne@ucd.ie Received September 30, 2002; accepted October 29, 2002 Published online April 7, 2003 RID="a" ID="a" Dedicated to Prof. Heinz Gamsj?ger on the occasion of his seventieth birthday     

Interaction between 1,2-dimethyl-3-hydroxypyrid-4-one and europium(III)

The interaction of the clinically used 1,2-dimethyl-3-hydroxypyrid-4-one with trivalent europium Eu(III), was investigated by using potentiometric and spectroscopic methods. The stability constants of the EuL_n ⁽³⁻ⁿ⁾⁺ complexes determined by spectroscopic and potentiometric measurements were found to be log β₁₁ = 6.5±0.3 and log β₁₂ = 12.0±0.5. However, at pH ≥ 5, hydrolysis of the Eu-L complexes starts, resulting in the formation of needle-type, yellow crystals. The low solubility of the Eu-L complexes in the neutral pH range is disadvantageous with respect to the use of deferiprone as chelating agent for decorporation of trivalent f-elements.     

Molecular dynamics simulation in aqueous solution of N-methylazetidinone as a model of β-lactam antibiotics

In this article, we analyze the results of a molecular dynamics simulation in aqueous solution of the N-methylazetidinone molecule, often used to model β-lactam antibiotics. The radial distribution functions (RDFs) corresponding to the most interesting atoms, in terms of reactivity, are presented. We focus our study on the effect of a polar environment on the molecule. The solvent structure around the system is compared to the structure of β-lactam-water complexes, as obtained in a previous study of reaction mechanisms for the neutral and alkaline hydrolyses of N-methylazetidinone. Two types of complexes have been considered which are related to different hydrolysis mechanisms having similar energy barriers at the rate-limiting step of the reaction path. In the first type, the β-lactam-water interaction takes place through the oxygen carbonyl atom and there is agreement between the maxima of the RDFs obtained here and the ab initio structure of the complexes previously reported. In the second type, the interaction takes place through the nitrogen atom and we do not predict a coordination layer around the β-lactam nitrogen atom. The results suggest that in aqueous solution hydrolysis of the carbonyl group is the most probable starting point for the overall hydrolysis reaction. Some discussion on the use of cluster models to represent the solvent effect is included. Received: 29 July 1998 / Accepted: 13 October 1998 / Published online: 1 February 1999     

Potentiometric sensors with membranes based on ionic liquid tetradecylammonium triethylammonio-<Emphasis Type="Italic">closo</Emphasis>-dodecaborate

Polymer compositions based on the ionic liquid tetradecylammonium triethylammonium-closododecaborate (TTCD) are proposed as the main components of membranes of potentiometric sensors (ion-selective electrodes) for determining ions [B₁₂H₁₁N(C₂H₅)₃]⁻. Two types of polymer compositions are considered: conventional, polyvinyl chloride (PVC)-liquid ion-exchanger (solution of TTCD in o-nitrophenyl octyl ether) and another, PVC-TTCD (ionophore-plasticizer). The optimal composition of membranes for both types of electrodes are proposed, and their main electroanalytical parameters, such as selectivity, effect of pH, range of linear response, reproducibility, and stability of potential, were measured. A comparative analysis of the electroanalytical parameters of potentiometric sensors with membranes of two types is given. The detection limits for the electrodes of types I and II are 9 × 10⁻⁷ and 4 × 10⁻⁷ M. It is shown that [B₁₂H₁₁N(C₂H₅)₃]⁻ anions can be determined by potentiometric titration with indicator electrodes of different types.     

Complexation between iron(III) and nicotinamide in aqueous dimethyl sulfoxide

The stability constants of iron(III) complexes with nicotinamide in water-DMSO mixtures (X _DMSO = 0–0.75) were determined by potentiometric titration at 25.0 ± 0.1°C and an ionic strength of 0.25 (NaClO₄). The contributions from the solvation of the reagents to the Gibbs energy of complexation transfer were analyzed. The stabilities of iron(III), copper(II), and silver(I) complexes with nicotinamide were compared. The observed decrease in the stability constants was attributed to the stabilization of iron(III) solvate complexes as the DMSO content increases.     

Silver and lead all-plastic sensors—polyaniline vs. poly(3,4-ethyledioxythiophene) solid contact

Silver and lead selective all-plastic ion-selective electrodes were obtained using poly(vinyl chloride)-based membranes and either poly(3,4-ethylenedioxythiophene) or polyaniline dispersion cast on an insulating plastic support as transducer and electrical lead. The effect of interactions of applied conducting polymer with analyte ions on potentiometric responses was evaluated and correlated with changes in elemental composition and element distribution within the ion-selective membrane and the conducting polymer transducer revealed in course of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) experiments. In the case of silver selective electrodes, potentiometric responses obtained are much dependent on the oxidation state of the polymer placed beneath the ion-selective membrane. For semi-oxidized polymer (poly(3,4-ethylenedioxythiophene)-based electrodes, linear responses with detection limit equal to 10^−5.4 M were obtained. For a more oxidized polyaniline (of higher conductivity), although the electrodes were pretreated exactly in the same way and tested in parallel, super Nernstian potential slope was recorded within the AgNO₃ activities range form 10⁻⁶ to 10⁻⁷ M. These responses were consistent with results of LA-ICP-MS, revealing profoundly higher silver signals intensities for poly(3,4-ethylenedioxythiophene) underlying silver selective membrane. It seems highly probable that silver is accumulated in this polymer layer as Ag⁰ due to spontaneous redox reaction leading to oxidation of the polymer; however, this process requires also the presence of silver ions at the interface. In fact, when reduced (deprotonated) polyaniline was used as transducer, the potentiometric responses of the sensor were, within the range of experimental error, the same as obtained for poly(3,4-ethylenedioxythiophene)-based sensor. On the other hand, for lead(II) selective sensors, the difference in responses of electrodes prepared using poly(3,4-ethylenedioxythiophene) or polyaniline was less pronounced, which is in accordance with the elemental composition of these sensors.     

Potentiometric sensors for Ag+ based on poly(3-octylthiophene) (POT)

Potentiometric ion sensors were prepared from the conjugated polymer poly(3-octylthiopene) (POT). The influence of additional membrane components, including silver 7,8,9,10,11,12-hexabromocarborane (AgCB₁₁H₆Br₆) and potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTpFPB) as lipophilic salts, and [2.2.2]p,p,p-cyclophane as silver ionophore, was studied. The membrane components were dissolved in chloroform and membranes were prepared by solution casting on glassy carbon disk electrodes. For comparison, POT-based potentiometric sensors were also prepared by galvanostatic electrosynthesis of POT from the 3-octylthiophene monomer. All the POT-based ion sensors fabricated by solution casting show Nernstian or slightly sub-Nernstian response to Ag⁺, even those based only on POT without any additional membrane components. The potentiometric response of electrochemically polymerized POT depends on the film thickness and the doping anion incorporated in the conducting polymer during polymerization. It is of particular importance that chemically synthesized undoped POT (without any additives) shows a sensitive and selective potentiometric response to Ag⁺ ions although UV-vis results show that POT remains in its undoped form, i.e., POT is not oxidized by Ag⁺. This indicates that undoped POT can exhibit good sensitivity and selectivity to Ag⁺ also in the absence of metallic silver in the polymer film. In this case, the potentiometric response is related to interactions between Ag⁺ and the conjugated polymer backbone. Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, 13–16, 2005     

On-line bioaffinity-electrospray mass spectrometry for simultaneous detection,identification, and quantification of protein-ligand interactions

We describe here an on-line combination of a surface acoustic wave (SAW) biosensor with electrospray ionization mass spectrometry (SAW-ESI-MS) that enables the direct detection, identification, and quantification of affinity-bound ligands from a protein-ligand complex on a biosensor chip. A trapping column was used between the SAW-biosensor and the electrospray mass spectrometer equipped with a micro-guard column, which provides simultaneous sample concentration and desalting for the mass spectrometric analysis of the dissociated ligand. First applications of the on-line SAW-ESI-MS combination include (1), differentiation of β-amyloid (Aβ) epitope peptides bound to anti-Aβ antibodies; (2), the identification of immobilized Substance P peptide-calmodulin complex; (3), identification and quantification of the interaction of 3-nitrotyrosine-modified peptides with nitrotyrosine-specific antibodies; and (4), identification of immobilized anti-α-synuclein-human α-synuclein complex. Quantitative determinations of protein-ligand complexes by SAW yielded dissociation constants (K_D) from micro-to low nanomolar sample concentrations. The on-line bioaffinity-ESI-MS combination presented here is expected to enable broad bioanalytical application to the simultaneous, label-free determination and quantification of biopolymer-ligand interactions, as diverse as antigen-antibody and lectin-carbohydrate complexes.     

Computational thermal and kinetic analysis

The complexation of β-lactam antibiotics, amoxicillin (AMPC), ampicillin (ABPC) and benzylpenicillin (PCG), with 2-hydroxypropyl-β-cyclodextrin (HPCD) was studied at various pH values using microcalorimetry, ¹H NMR spectroscopy, and molecular dynamic simulation. In the strong acid solution, two different types of inclusion complex with a 1:1 stoichiometry, Complex I with a phenyl ring of β-lactam antibiotics penetrated into the cavity of HPCD and Complex II with a penam included in the cavity, were formed by hydrophobic interaction, and Complex II was more stable than Complex I. In aqueous solution at pH≥4.5, only Complex I was formed, where the penam of PCG was more deeply penetrated into the cavity to keep it stable than those of AMPC and ABPC. The charged carboxyl-group on the penam was less affinity to form Complex II.     

Polymeric adsorbents with peptide pendants as artificial receptors for β-lactam antibiotics by mimicking the binding sites of β-lactamases

A series of polymeric adsorbents with peptide pendants were designed as the artificial receptors of β-lactam antibiotics by mimicking the structures of binding site in β-lactamases. Crosslinked poly(N, N-dimethyl acrylamide) gel as a carrier was prepared by suspension copolymerization of N, N-dimethyl acrylamide and N. Ń-bisacryl-diaminoethane and then functionalized with ethylenediamine after partial hydrolysis. Using solid-phase peptide synthesis with symmetrical anhydride of protected amino acid step by step, various peptide pendants were respectively anchored onto the functionalized carrier. The adsorption properties of these peptide-containing adsorbents for β-lactam antibiotics such as ampicillin and cefotaxime were then studied. The results showed that only those adsorbents in which peptide chains contained more than one lysine residues could obviously adsorb both β-lactams and that static interaction as well as hydrogen bond played an important role during the adsorption. Project supported by the National Natural Science Foundation of China (No. 59493207).     

A biomimetic sensor surface to detect anti-β<Subscript>2</Subscript>-glycoprotein-I antibodies as a marker for antiphospholipid syndrome

A biomimetic sensor has been developed, that allows for quantification of autoantibodies related to the antiphospholipid syndrome (APS). Autoantibodies directed against the β₂-glycoprotein-I (β₂GP-I) are known as the best markers for diagnosis of APS, however, detection of such antibodies is still a challenge. The epitopes of β₂GP-I are exposed upon binding to negatively charged membranes. The surface of the sensor chips was therefore modified with such type of membranes, on which β₂GP-I molecules were subsequently immobilized as recognition elements. Using the label-free method, reflectometric interference spectroscopy, it was possible to quantify anti-β₂GP-I antibodies and to calibrate the sensor chip in buffer. A mild regeneration procedure allows for many consecutive measurements without stripping off the membrane in between.     

Sodium sulfite: A promising reagent in the electrochemical oxidation of metallic silver

Among the complexing agents suitable for the electrochemical silver recovery, a promising one is sodium sulfite forming silver complexes with high stability constants: according to our data, log β₁ ∼ 5.3, log β₂ ∼ 8.2. This reagent is accessible and cheap, but nevertheless it almost never has been used for these purposes. We studied the stability of the sodium sulfite solutions in air and showed a possibility of using them for a long period. Using the methods of potentiodynamic polarization and potentiostatic electrolysis, we have studied characteristics of the process of metallic silver oxidation in the sulfite media. The anodic polarization process of the silver dissolution is shown to occur with 100% yield with respect to the consumed current. Simultaneously, on the cathode deposited up to 90% of dissolved silver.     

Thermodynamic properties of chloro-complexes of silver chloride in aqueous solution

Available data on the solubility of silver chloride in aqueous solutions of HCl, NaCl, KCl, LiCl, and NH₄Cl, along with potentiometric measurements of the activity of Ag⁺ in aqueous NaCl–NaClO₄ mixtures, have been analyzed to obtain the thermodynamic properties of the (AgCl)⁰, AgCl ₂ ^– , AgCl ₃ ^2– , and AgCl ₄ ^3– complexes. Results obtained include the stability constants of the complexes and 25°C, the virial parameters needed to calculate the activity coefficients of the complexes and the heats of the reactions by which they are formed. These results are sued to calculate tables of recommended values for the solubility of silver chloride in the host solutions as a function of concentration from 0 to 150°C and to make a critical evaluation of the reliability of previously published data.     

Formation of bismuth(III) N-methylthiourea complexes in aqueous solutions

The formation constants of bismuth(III) methylthiourea complexes in aqueous solution were determined at 298 K and an acidity of 2 M HClO₄ using the “ligand-oxidized ligand species” potentiometric method and the methylthiourea (mtu)-symmetric dimethylformamidine disulfide (mFDS) redox pair. The formation function was obtained, and the conditional (β _n ^*) and true (β _n ) formation constants of Bi(mtu) _n ³⁺ (1 ≤ n ≤ 8) were calculated. The value of β₁ and the formation of complexes with coordination numbers higher than six was confirmed by spectrophotometry.     

Thermophysical and thermoacoustical properties of benzaldehyde mixtures with ethylbenzene at 303.15, 308.15, and 313.15 K and a pressure of 0.1 MPa

Ultrasound velocity (u), density (ρ) and viscosity (η) measurements of benzaldehyde + ethylbenzene mixtures have been carried out at 303.15, 308.15, and 313.15 K. These values have been used to calculate the excess molar volume (V ^E), deviation in viscosity (δη), and deviation in isentropic compressibility (δβ_s), deviations in ultrasound velocity (δu), excess free volume (δV _f), excess intermolecular free length (δL _f) and excess Gibbs free energy of activation of viscous flow (δG ^E). McAllister’s three body interaction model is used for correlating kinematic viscosity of binary mixtures. The excess values were correlated using the Redlich-Kister polynomial equation to obtain their coefficients and standard deviations. The thermophysical properties under the study were fit to the Jouyban-Acree model. The observed variation of these parameters helps in understanding the nature of interactions in these mixtures. Further, theoretical values of the ultrasound speed were evaluated using theories and empirical relations.     

On the active site of mononuclear B1 metallo β-lactamases: a computational study

Metallo-β-lactamases (MβLs) are Zn(II)-based bacterial enzymes that hydrolyze β-lactam antibiotics, hampering their beneficial effects. In the most relevant subclass (B1), X-ray crystallography studies on the enzyme from Bacillus Cereus point to either two zinc ions in two metal sites (the so-called ‘3H’ and ‘DCH’ sites) or a single Zn(II) ion in the 3H site, where the ion is coordinated by Asp120, Cys221 and His263 residues. However, spectroscopic studies on the B1 enzyme from B. Cereus in the mono-zinc form suggested the presence of the Zn(II) ion also in the DCH site, where it is bound to an aspartate, a cysteine, a histidine and a water molecule. A structural model of this enzyme in its DCH mononuclear form, so far lacking, is therefore required for inhibitor design and mechanistic studies. By using force field based and mixed quantum–classical (QM/MM) molecular dynamics (MD) simulations of the protein in aqueous solution we constructed such structural model. The geometry and the H-bond network at the catalytic site of this model, in the free form and in complex with two common β-lactam drugs, is compared with experimental and theoretical findings of CphA and the recently solved crystal structure of new B2 MβL from Serratia fonticola (Sfh-I). These are MβLs from the B2 subclass, which features an experimentally well established mono-zinc form, in which the Zn(II) is located in the DCH site. From our simulations the εεδ and δεδ protomers emerge as possible DCH mono-zinc reactive species, giving a novel contribution to the discussion on the MβL reactivity and to the drug design process.     

Alanine- and taurine-salicylal Schiff base complexes of magnesium

The complexes of α-alanine-salicylal Schiff base of magnesium (α-ASSM), β-alanine-salicylal Schiff base of magnesium (β-ASSM) and taurine-salicylal Schiff base of magnesium (TSSM) were synthesized. The formulae of the complexes are Mg[OC₆H₄CHNCH(CH₃)COO]·2H₂O, Mg[OC₆H₄CHNCH₂CH₂COO]·2H₂O and Mg[OC₆H₄CHNCH₂CH₂SO₃]·2H₂O. The crystal structure belongs to orthorhombic system with the lattice parameters: a=1.6954 nm, b=2.0873 nm and c=2.3037 nm for the β-ASSM, to orthorhombic system with the lattice parameters: a=1.5586 nm, b=1.8510 nm and c=2.6240 nm for the β-ASSM, to monoclinic system with the lattice parameters: a=1.3232 nm, b=1.4960 nm, c=2.1543 nm and β=98.04° for the TSSM, respectively. The results of the thermal decomposition processes and infrared spectra of the complexes show that the complexes may possess different coordination structures.