Determination of fluoxetine in pharmaceutical preparations and biological samples using potentiometric sensors based on polymeric membranes

The construction and general performance of four novel potentiometric membrane sensors for the determination of fluoxetine have been described. The sensors are based on the formation of the ion association complex of fluoxetine with sodium tetraphenylborate and phosphotungstic acid as electroactive materialles, dispersed in a PVC matrix with dibuthyl sebacate (or diethyl sebacate) as a plasticizer. These sensors exhibit fast, stable and near-Nernstian response for the monocharged fluoxetine cation over wide concentration range from 3.0 × 10^?6 to 1.2 × 10^?2 M and pH 4.0–7.5. No interferences are caused by many inorganic and organic species. Direct potentiometric determinations of 5–100 μg/mL of fluoxetine in drug and urine samples show good recovery of fluoxetine. The developed membrane electrodes have been used as an end point indicator electrode; the potentiometric titration of fluoxetine with sodium tetraphenylborate as a titrant has been monitored.     

Determination of the ionic composition of perfluorinated sulfocationite membranes based on Donnan potential estimates

An express method for determination of the membrane ionic composition is developed based on the possibility of estimating the Donnan potential difference on the individual studied-solution/membrane interface. The Donnan potential is assessed using a new device one end of which serves as the ion-selective electrode sensor. The ionic composition of perfluorinated sulfocationite membranes is determined in systems with mixed inorganic electrolyte solutions by comparing the Donnan potentials in a given system and the reference system. As the reference, a system with perfluorinated sulfocationic membrane and individual potassium chloride solutions is used. The time of one measurement does not exceed 10–15 min. The accuracy and sensitivity of determination were 3% and 0.02 mmol/g, respectively.     

Studies of the rates of thermal decomposition of glycine,alanine, and serine

Rates of thermal decomposition of glycine, alanine, and serine are described by the equation of first order reaction in the temperature range 200–300°C. Apparent rate constants and apparent activation energies of decomposition of α-amino acids were evaluated. It was found that the main gaseos reaction product is carbon dioxide.     

Structures of glycine, enantiopure alanine, and racemic alanine adlayers on Cu(110) and Cu(100) surfaces

We have determined the structures of dense adlayers of glycine and alanine on the Cu(110) and Cu(100) surfaces using plane wave density functional theory. These calculations resolve several experimental controversies regarding these structures. Glycine exists on Cu(110) as a single adlayer structure, while on Cu(100) two distinct glycine adlayers coexist. The glycine structures serve as useful starting points for constructing alanine adlayer structures. We considered separately the adsorption of enantiopure alanine and racemic alanine on each surface. Adlayers of enantiopure alanine are found to be closely related to the adlayers observed for glycine. Racemic alanine adlayers on Cu(110) are structurally analogous to those observed for glycine on this surface and adopt a pseudo-racemate ordering. On Cu(100), in contrast to glycine, racemic alanine is found to adopt a single adlayer structure that is an ordered racemate. Spontaneous segregation of molecular enantiomers does not occur in racemic adsorbed mixtures on either surface. Consideration of the orientationally distinct domains that may exist for each adlayer on these surfaces provides important information for the interpretation of the adlayer domain boundaries that are commonly observed in scanning tunneling microscopy images of amino acid adlayers. Examining this set of amino acid adlayers provides useful insight into the range of subtle behaviors that can arise in these and related systems where chiral molecules form ordered adlayers on flat metal surfaces.     

Interfacial potential difference in electromembrane systems with MA-41 anion-exchange membranes and alkaline solutions of glycine

A method for estimating the interfacial potential difference in electromembrane systems with alkaline solutions of amino acids is developed. Systems with MA-41 anion-exchange membranes and aqueous sodium hydroxide solutions with glycine are investigated. It is found that in solutions with alkali concentrations greater than 0.010 M, the interfacial potential difference decreases logarithmically, i.e., in agreement with the Donnan relationship. At the same time, in weakly alkaline solutions of glycine, in which the amino-acid concentration exceeds that of the alkali, an increase in the interfacial potential difference is observed. This may be due to a disturbance of the equilibrium of the protolytic reaction in the membrane on the side where hydroxide ions form.     

Determination of sulfur-containing anions in alkaline solutions using arrays of DP-sensors based on hybrid perfluorinated membranes with proton-donor dopants

The influence of proton-donor properties and concentration of dopant nanoparticles introduced into Nafion and MF-4SС perfluorosulfonic cation-exchange membranes on the characteristics of cross-sensitive DP-sensors (sensors whose analytical signal is the Donnan potential) in alkaline solutions of sulfur-containing organic compounds were studied. The dopants were acid salts of heteropoly acids (HPAs) and hydrated silica SiO₂ and zirconia ZrO₂ surface-modified with sulfur-containing groups and an acid HPA salt. A correlation was revealed between the DP-sensor sensitivity to anions (and zwitter-ions) in alkaline solutions, size and proton-donor ability of the added particles, and diffusion permeability of hybrid membranes. Optimum compositions of membranes for arrays of cross-sensitive DP-sensors ensuring the simultaneous determination of cations and anions (and zwitter-ions) in the test solutions with an error of less than 18% were selected.     

Theoretical study of the electronic gas-phase spectrum of glycine, alanine, and related amines and carboxylic acids

A theoretical study on the origin of the common electronic excitations in amino acids is presented, focusing on the excited states of glycine, alanine and the related substructures formic acid, acetic acid, propionic acid, ammonia, methylamine, and ethylamine. Special attention is given to the valence excitation from the nonbonding lone-pair on the carboxylic oxygen atom to the antibonding pi-orbital (n(O) --> pi*(CO)) and the first Rydberg excitation from the nonbonding lone-pair on the nitrogen atom (n(N) --> 3s). From extensive calculations on formic acid and methylamine, different basis sets and electron correlation treatments are benchmarked using a hierarchy of coupled cluster (CC) methods, consisting of CCS, CC2, CCSD, CCSDR(3), and CC3, in combination with augmented correlation consistent basis sets. The dependence of the excitation energies on the size of the backbone structure in the two groups of molecules is investigated, and 0-0 transition energies for the n(O) --> pi*(CO) and n(N) --> 3s transitions are calculated for the smallest molecules. Excellent agreement with experimental values is found where secure experimental assignments are available. A few outstanding problems in the experimental assignments found in the literature are described for both the carboxylic acids and the amines. Final predictions for vertical excitation energies are given for all molecules, including glycine and alanine where no gas-phase experimental results are available. Finally, calculations on protonated amino acids are presented showing an isolation of the n(O) --> pi*(CO) from higher lying states by as much as 1.9 eV for alanine.     

Unraveling the role of stereo-electronic, dynamical, and environmental effects in tuning the structure and magnetic properties of glycine radical in aqueous solution at different pH values

A recently developed extended Lagrangian model employing localized basis functions and nonperiodic boundary conditions (GLOB/ADMP) was applied to the radicals issuing from the homolytic breaking of the C(alpha)-H(alpha) bond of glycine in aqueous solution at different pH values. Although the modifications of the structure and the magnetic properties of these species induced by the solvent are qualitatively reproduced by a static discrete-continuum model, magnetic parameters are further tuned by short-time dynamical effects (solute vibrations and solvent librations). The results delivered by GLOB/ADMP simulations for both hyperfine tensors and g-tensors are in remarkable agreement with their experimental counterparts, allowing a reliable disentanglement of the overall observables into well-defined contributions. The dominant role of out-of-plane vibrations in determining hyperfine splittings is confirmed and quantified, together with the remarkable sensitivity of the gyromagnetic tensor to bond lengths and valence angles defining the NC(alpha)C' moiety. Together with their specific interest for the title radical, our results suggest some interesting trends for other biologically significant radicals and point out the need of extending magneto-structural relationships to dynamical aspects.     

A thermochemical study of the coordination reactions of La(III) with alanine and glycine

The solid-state coordination reactions of lanthanum chloride with alanine and glycine, and lanthanum nitrate with alanine have been studied by classical solution calorimetry. The molar dissolution enthalpies of the reactants and the products in 2 mol L^-1 HCl solvent of these three solid-solid coordination reactions have been measured using an isoperibol calorimeter. From the results and other auxiliary quantities, the standard molar formation enthalpies have been determined to be Δ_f H _m ^θ[La(Ala)₃Cl₃·3H₂O(s), 298.2 K]= -3716.3 kJ mol^-1, Δ_f H _m ^θ [La(Gly)₃Cl₃·5H₂O(s), 298.2 K]= -4223.0 kJ mol^-1 and Δ_f H _m ^θ [La(Ala)₄(NO₃)₃·H₂O(s), 298.2 K]= -3867.57 kJ mol^-1, respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies on the adsorption of peptides of glycine/alanine on montmorillonite clay with or without co-ordinated divalent cations

The adsorption of simple peptides of glycine/alanine from their aqueous solutions onto montmorillonite, Ca²⁺ and Mg²⁺ exchanged montmorillonite clay studied UV spectrophotometrically at constant pH 7.02 and temperature 23 °C. The percent binding of Gly², Gly³, Gly⁴ and Gly-ala is calculated in terms of their optical density. The adsorption parameters, i.e. X_m and K_L have been calculated from Langmuir adsorption isotherm. Similar adsorption behaviour was observed with or without divalent cation exchanged adsorbent, but the percent binding and monolayer capacity appear to depend on the molecular weight, i.e. number of aliphatic carbon atoms of the adsorbates. The adsorption was significantly affected by the concentration of peptide, pH and temperature of the system. Equilibrium constant (K) and the free energies of adsorption (−ΔG) were determined from the isotherm measured under static conditions. Tetra glycine (Gly⁴) has positive −ΔG and K>1 showing greater adsorptibility, whereas for other peptides, −ΔG values were negative and K<1, thus showing very weak adsorption. A linear dependence of −ΔG on the number of aliphatic carbon atoms (n_c) from Gly² to Gly⁴ in adsorbate molecule was found. Thermodynamic data strongly support the quantitative data obtained from Langmuir adsorption isotherm. Ca²⁺ montmorillonite exhibited relatively better adsorption as compared to Mg²⁺ exchanged form or montmorillonite without Ca²⁺ or Mg²⁺. Results have shown that clay minerals might have played a significant role in prebiotic formation of proteins via adsorption of simple bio-oligomers on their surface.     

Charge transfer study through the determination of the ionization energies of tetrapeptides X3-Tyr, X = Gly, Ala, or Leu. Influence of the inclusion of one glycine in alanine and leucine containing peptides

The energies of the fundamental and several excited states of tetrapeptide radical cations were determined at the outer valence Green's function (OVGF) level, at three geometries corresponding to the lowest energy conformations: two for the neutral and one for the cation. The conformations were optimized at the density functional theory level within the B3LYP framework. It was found that, from a purely energetic point of view, a charge initially created on the tyrosine chromophore could migrate without any geometrical change and without further activation once the excited electronic state of the ionized chromophore was formed. This migration could reach the NH(2) terminus for the neutral conformations but should stop at the adjacent peptide link for the cation conformation. These results stress the probable influence of the electronic coupling between the states rather than the existence of a barrier on the charge pathway to explain the difference between the peptides in the charge-transfer process leading to the loss of an iminium [NH(2)=CHR](+) cation. The dissociation energy of the asymptote related to the formation of this NH(2) terminus iminium cation was calculated for few species and it appears that the excess energy available for dissociation is significant when starting from the lowest energy conformations of the neutral or the cation, provided that the charge transfer is effective. It was also found that the amino acids did not conserve their energetic properties and their zero order energy levels turned to a complete new energetic scheme corresponding to the conformation of the peptide.     

Gas-phase basicity of glycine,alanine, proline,serine, lysine,histidine and some of their peptides by the thermokinetic method

The thermokinetic method is applied to a set of six amino acids (glycine, alanine, proline, serine, lysine, histidine) and 30 of their di- and tri-peptides for which experimental proton transfer rate constants were available. The comparison between the presently determined gas-phase basicities, GBs, of the amino acids with values obtained from equilibrium constant determination is generally good (a mean deviation of appoximately 3 kJ mol(-1) is observed). Derived proton affinities values are discussed. The gas-phase basicities of peptides provided by the present study correct several previously estimated values thus offering a more firm basis for structural discussion. Composite reaction efficiency curves indicate the existence, for several peptides, of at least two non-interconverting populations of protonated forms.     

Energetics and structure of glycine and alanine based model peptides: Approximate SCC-DFTB,AM1 and PM3 methods in comparison with DFT,HF and MP2 calculations

We calculate relative energies and geometries of important secondary structural elements for small glycine and alanine based polypeptides containing up to eight residues. We compare the performance of the approximate methods AM1, PM3 and self-consistent charge, density-functional tight-binding (SCC-DFTB) to density-functional theory (DFT), Hartree–Fock (HF) and MP2. The SCC-DFTB is able to reproduce structures and relative energies of various peptide models reliably compared to DFT results. The AM1 and PM3 methods show deficiencies in describing important secondary structure elements like extended, helical or turn structures. The discrepancies between different ab initio (HF, MP2) and DFT (B3LYP) methods for medium sized basis sets (6-31G*) also show the need for higher level calculations, since systematic errors found for small molecules may add up when investigating longer polypeptides.     

Determination of amino acids, vitamins, and drug substances in aqueous solutions using new potentiometric sensors with Donnan potential as analytical signal

Basic principles of a new potentiometric sensor are considered. Its analytical signal is the Donnan potential at the ion-exchange polymer/studied solution of electrolyte interface. Assessments of potential drops at individual interfaces are presented, same as estimates of diffusion potentials in the electrochemical circuit for measurement of the sensor response. It is shown that the overall contribution of the values of all potentials to EMF of the electrochemical circuit, except for the Donnan potential, at the ion-exchange polymer/studied solution interface are negligibly low as compared to the experimental values of the circuit EMF in the studied systems. Certain regularities of the Donnan potential formation are studied in the systems with the polymers of different structure and solutions containing inorganic ions and organic electrolytes in different ionic forms. The possibility is shown of using a sensor with such an analytical signal as the Donnan potential for assay of amino acids, vitamins, and drug substances in aqueous solutions. The sensor was used as a selective electrode for determination of lysine in aqueous solutions with neutral amino acids in the range of pH 5.0–7.0 and glycine in aqueous alkali solutions in the range of pH 9.00–11.00. The developed sensor is introduced as a cross-sensitive electrode into the array of multisensor systems for multicomponent quantitative analysis of the lysine monohydrochloride, thiamine chloride, novocaine hydrochloride, lidocaine hydrochloride solutions containing potassium and sodium chlorides. The measurement error of electrolytes in aqueous solutions did not exceed 10%.     

An experimental and theoretical investigation of the chemical shielding tensors of (13)C(alpha) of alanine, valine, and leucine residues in solid peptides and in proteins in solution

We have carried out a solid-state magic-angle sample-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic investigation of the (13)C(alpha) chemical shielding tensors of alanine, valine, and leucine residues in a series of crystalline peptides of known structure. For alanine and leucine, which are not branched at the beta-carbon, the experimental chemical shift anisotropy (CSA) spans (Omega) are large, about 30 ppm, independent of whether the residues adopt helical or sheet geometries, and are in generally good accord with Omega values calculated by using ab initio Hartree-Fock quantum chemical methods. The experimental Omegas for valine C(alpha) in two peptides (in sheet geometries) are also large and in good agreement with theoretical predictions. In contrast, the "CSAs" (Deltasigma) obtained from solution NMR data for alanine, valine, and leucine residues in proteins show major differences, with helical residues having Deltasigma values of approximately 6 ppm while sheet residues have Deltasigma approximately 27 ppm. The origins of these differences are shown to be due to the different definitions of the CSA. When defined in terms of the solution NMR CSA, the solid-state results also show small helical but large sheet CSA values. These results are of interest since they lead to the idea that only the beta-branched amino acids threonine, valine, and isoleucine can have small (static) tensor spans, Omega (in helical geometries), and that the small helical "CSAs" seen in solution NMR are overwhelmingly dominated by changes in tensor orientation, from sheet to helix. These results have important implications for solid-state NMR structural studies which utilize the CSA span, Omega, to differentiate between helical and sheet residues. Specifically, there will be only a small degree of spectral editing possible in solid proteins since the spans, Omega, for the dominant nonbranched amino acids are quite similar. Editing on the basis of Omega will, however, be very effective for many Thr, Val, and Ileu residues, which frequently have small ( approximately 15-20 ppm) helical CSA (Omega) spans.     

Reaction of 4-chloro-3-nitrocoumarin with glycine and alanine,and the synthesis of 1-benzopyrano[3,2-c]pyrimidine-3,5-dione

A considerable improvement of the yields obtained in substitution of halogens in 4-chloro-3-nitrocoumarin by amino acid or amino acid ester residues was achieved over literature claims. Electrochemical reduction of the nitro group in these products brought about a ring closure leading to benzopyranopyrimidinones, which are obtained in excellent yields.