Electrochemical determination of ascorbic acid using the poly-cysteine film-modified electrode

A poly(L-cysteine) thin film was prepared onto electrode surface via electropolymerization. In pH 7.0 phosphate buffer, L-cysteine was oxidized during the cyclic potential sweep between −0.60 and 2.00 V, forming a thin film at the glassy carbon electrode (GCE) surface. The electrochemical behaviors of ascorbic acid at the bare GCE and the poly(L-cysteine) film-coated GCE were investigated. The oxidation peak potential of ascorbic acid shifts to more negative potential at the poly(L-cysteine) film-modified GCE. Moreover, the oxidation peak current significantly increases at the poly(L-cysteine) film-modified GCE. These phenomena indicate that poly(L-cysteine) film shows highly-efficient catalytic activity to the oxidation of ascorbic acid. Based on this, a sensitive and simple electrochemical method was proposed for the determination of ascorbic acid. The oxidation peak current of ascorbic acid is proportional to its concentration over the range from 1.0 × 10⁻⁶ to 5.0 × 10⁻⁴ mol l⁻¹. The limit of detection is evaluated to be 4.0 × 10⁻⁷ mol l⁻¹.     

1,3,5,7-Tetramethyl-8-(<Emphasis Type="Italic">N</Emphasis>-hydroxysuccinimidyl butyric ester)difluoroboradiaza-<Emphasis Type="Italic">s</Emphasis>-indacene as a new fluorescent labeling reagent for HPLC determination of amino acid neurotransmitters in the cerebral cortex of mice

1,3,5,7-Tetramethyl-8-(N-hydroxysuccinimidyl butyric ester)difluoroboradiaza-s-indacene (TMBB-Su), a new BODIPY-based fluorescent probe, was designed and synthesized for the labeling of amino compounds. It was used as a pre-column derivatizing reagent for determination of amino acid neurotransmitters by high-performance liquid chromatography (HPLC). The fluorescence quantum yield in acetonitrile increased from 0.84 to 0.95 when it reacted with amino acid neurotransmitters. Derivatization of TMBB-Su with seven amino acid neurotransmitters was completed within 30 min at 25 °C in 24.0 mmol L⁻¹ pH 7.8 boric acid buffer. The separation was performed on a C₁₈ column with methanol–water–buffer 55:35:10 (v/v) as mobile phase (buffer: 0.10 mol L⁻¹ H₃Cit–0.10 mol L⁻¹ NaOH). Interference from the other concomitant amino acids was eliminated successfully by means of pH gradient elution. With fluorescence detection at 494 and 504 nm for excitation and emission, respectively, the limits of detection (signal-to-noise ratio = 3) were from 2.1 to 12.0 nmol L⁻¹. The proposed method has been used to determine amino acid neurotransmitters in the cerebral cortex of mice with cerebral ischemia at the convalescence stage with satisfactory recoveries varying from 94.9 to 105.2%.     

Molecular recognition on supramolecular systems (XXXV)

A novel β-cyclodextrin derivative4 bearing a pyridinio group on the primary side was synthesized by the reaction of 2-aminopyridine with 6-β-cyclodextrin monoaldehyde3, and its complexation stability constants with several aliphatic amino acids have been determined in phosphate buffer solution ( pH = 7.2, 0.1 mol·L⁻¹) at 25 °C by using spectrofluormetric titrations. The stoichiometry is 1:1 for the inclusion complexation of amino acids with compound4. Circular dichroism study indicates that the aromatic moiety was embedded shallowly into the cyclodextrin cavity. As a spectral probe, the pyridinio group in the modified cyclodextrin can recognize not only differences of the size and shape of amino acid molecules, but also theL/D-amino acid chiral isomer. As compared with mono-[6-(1-pyridinio)-6-deoxy]-β-cyclodextrin5, compound4 switched the enantiomer preference forL- toD-isomer, and showed the highest enantioselectivity of 5.4 forD/L-serine. These results are discussed from the viewpoints of geometric compensation, induced-fit concept and cooperation of several weak interactions.     

Thermal decomposition of solid state poly(β-L-malic acid)

Thermal decomposition process of solid state poly(β -L-malic acid) was traced by DSC combined with FT-IR. Melting temperature of this partially crystallized polymer was detected at 46-60°C. The thermal decomposition initiated at ca 185°C accompanied by an evolution of gaseous products. In contrast to the cleavage reaction in the aqueous polymer solutions which gives L-malic acid and corresponding dimer of L-malic acid, the solid state poly(β -L-malic acid) decomposed at above the decomposition temperature giving not the constituent L-malic acid but fumaric acid at the first stage of the reaction then, maleic and maleic anhydride. This revised version was published online in July 2006 with corrections to the Cover Date.     

Biochemical Characterization of an Arginine 2,3‐Aminomutase with Dual Substrate Specificity

The radical S‐adenosylmethionine (SAM) aminomutases represent an important pathway for the biosynthesis of β‐amino acids. In this study, we report biochemical characterization of BlsG involved in blasticidin S biosynthesis as a radical SAM arginine 2,3‐aminomutase. We showed that BlsG acts on both L‐arginine and L‐lysine with comparable catalytic efficiencies. Similar dual substrate specificity was also observed for the lysine 2,3‐aminomutase from Escherichia coli (LAM_EC). The catalytic efficiency of LAM_EC is similar to that of BlsG, but is significantly lower than that of the enzyme from Clostridium subterminale (LAM_CS), which acts only on L‐lysine rather than on L‐arginine. Moreover, we showed that enzymes can be grouped into two major phylogenetic clades, each corresponding to a certain C3 stereochemistry of the β‐amino acid product. Our study expands the radical SAM aminomutase members and provides insights into enzyme evolution, supporting a trade‐off between substrate promiscuity and catalytic efficiency.     

Studies on synthesis and molecular dynamics simulation of dendrimers containing amino acids and peptides

A series of poly(ether-amide) dendrimers with amino acids and peptides as the peripheral functional groups was synthesized, and their structures were confirmed by nuclear magnetic resonance (NMR) and electrospray ionization-mass spectrometry (ESI-MS) spectrometry. Molecular dynamics simulation of the peptide dendrimers in solution was performed, indicating that, the prior conformations of the dendrimers were atom number dependent, i.e., with the increases of the atom number, the prior conformations were more spherical. Also, the amino acid α-C atom radial distribution indicated that, with larger peripheral groups, more back-folding of the dendrimers occurred. __________ Translated from Acta Chimica Sinica, 2007, 65(1): 21–26 [译自: 化学 通报]     

λ -Benzyl-L-Glutamate Graft Copolymers of Cellulose and Poly[Arylene Ether Sulfone]

Macroinitiators with primary amino substituents were synthesized by one of the following techniques: a) cyanoethylation of cellulose followed by diborane reduction to produce aminopropylcellulose, 1; b) nitration, then SnCl₂ reduction of poly(arylene ether sulfone), 5, to produce poly(2-aminoarylene ether sulfone), 2; c) phthalimidation of 5 followed by hydrazinolysis to yield poly(2-aminomethylarylene ether sulfone), 3; and d) LiAlH₄ reduction of poly(cyanophenylene arylene ether) to poly(aminomethylphenylene arylene ether), 4. Heterogeneous grafting of Λ-benzyl-L-glutamate-N-carboxyanhydride, 8, to Polymer 1 resulted in a nonrandom distribution of amino acid residues; α-helical conformations were detected at low BLG-NCA/NH₂ ratios (<5 amino acids). Using molar ratios ranging from 1 to 100 of 8, relative to the amine concentration, grafting to Polymers 3 and 4 was effected in anhydrous THF at room temperature under homogeneous conditions. If reaction times between 24 and 48 h are utilized, high grafting efficiencies (>80%) are obtained. The conformation of the polypeptide chain was evaluated by NMR and infrared spectroscopy. Polypeptides grafted to Polymers 3 and 4 appeared to adopt the expected conformation for the chain length predicted, i.e., a progression from random coil (<8 amino acids) to β-pleated sheet (8–13 amino acids) to α-helix (> 13 amino acids). The benzyl ester functions on the BLG grafts are subject to direct modification with amine nucleophiles; studies with butylamine correlate reaction conditions with extent of ester vs peptide cleavage. In the presence of 1-hydroxybenzotriazole, aminolysis of the ester is favored and conversions to Λ-amides up to 75% without peptide cleavage are achieved.     

Microbial and Enzymatic Synthesis of Optically Pure D- and L-3-Trimethylsilyl-alanine by Deracemization of D,L-5-Trimethylsilyl-methyl-hydantion

 The sterospecificities of hydantoinases and N-carbamoyl amino acid amidohydrolases (N-carbamoylases) from different microbial sources were investigated for the stereoselective syntheses of the unnatural silicon-containing amino acids D- and L-3-trimethylsilyl-alanine (3) from the respective racemic hydantoin D,L-1. In a preparative biotransformation, whole resting cells of Agrobacterium sp. IP I 671, immobilized in a Ca-alginate matrix, were used for the synthesis of amino acid D-3 in 88% yield and 95% enantiomeric excess. Since the purified D-N-carbamoylase from Agrobacterium sp. IP I 671 was shown to be 100%D-selective, the enantiomeric purity of 95% of D-3 arising from the transformation with the immobilized cells must be explained by the participation of a further, L-selective N-carbamoylase or, which is more likely, by racemization of the final hydrolysis product by the action of an amino acid racemase. Isolated hydantoinases from Bacillus thermoglucosidasius, Thermus sp., Arthrobacter aurescens DSM 3745, and Arthrobacter crystallopoietes DSM 20117 turned out to be stereospecific for the conversion of the D-form of hydantoin D,L-1. The enantiomerically pure L-form of 3 was also prepared. It was synthesized from racemic N-carbamoyl amino acid D,L-2 by enantiomer-specific hydrolysis of the L-form in presence of L-N-carbamoylase from Arthrobacter aurescens DSM 3747.     

Electroactive films of interpolymer complexes of polyaniline with polyamidosulfonic acids: advantageous features in some possible applications

With the purpose of determining prospects of possible applications of interpolymer complexes of polyaniline (PANI) with poly(amidosulfonic acid)s, we have performed a comparative study of polyaniline films prepared by electrochemical polymerization of aniline in the presence of the polyacids distinguished by different rigidities of the polymer backbone: (1) poly(2-acrylamido-2-methyl-1-propanosulfonic acid) (flexible backbone); (2) poly-p,p′-(2,2′-disulfoacid)-diphenylen-iso-phthalamid (semi-rigid backbone); (3) poly-p,p′-(2,2′-disulfoacid)-diphelylen-tere-phthalamid (rigid backbone); and (4) a copolymer of the latter two acids with monomer feed ratio 1:1 (co-PASA). Spectroelectrochemical studies in the UV–vis–NIR range showed that PANI complexes with rigid-chain polyacids far more effectively modulate absorbance in the Vis–NIR range and can be considered as promising candidates for “smart windows” development. Due to the presence of bulky unmovable polyacid anion, PANI interpolymer complexes (particularly those with the semi-rigid-chain polyacids) possess much wider pH range of electroactivity than common PANI, which is of great importance for biosensor applications. The interpolymer complexes with flexible-chain and semi-rigid-chain polyacids exhibit good optical response to ammonia vapors at conditions of high humidity, which make them promising materials for the development of ammonia optical sensors.     

<Superscript>1</Superscript>H NMR spectra of water contained in solutions of poly(<Emphasis Type="Italic">N</Emphasis>-vinylpyrrolidone) and its modification products in CDCl<Subscript>3</Subscript>

¹H NMR spectra in CDCl₃ of poly(N-vinylpyrrolidone), epoxidized poly(N-vinyl-pyrrolidone), and products derived from the latter by modification with amino acids (glycine, β-alanine, γ-aminobutyric acid, and ε-aminocaproic acid) were examined. The ¹H NMR spectra of the modified polymers contain signals for water protons due to different centers of water sorption. These signals differ in chemical shift and integral intensity and indicate a changed spatial packing of the polymer as the result of its modification. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2377–2380, October, 2005.     

Microbial and Enzymatic Synthesis of Optically Pure D- and L-3-Trimethylsilyl-alanine by Deracemization of D,L-5-Trimethylsilyl-methyl-hydantion

Summary.  The sterospecificities of hydantoinases and N-carbamoyl amino acid amidohydrolases (N-carbamoylases) from different microbial sources were investigated for the stereoselective syntheses of the unnatural silicon-containing amino acids D- and L-3-trimethylsilyl-alanine (3) from the respective racemic hydantoin D,L-1. In a preparative biotransformation, whole resting cells of Agrobacterium sp. IP I 671, immobilized in a Ca-alginate matrix, were used for the synthesis of amino acid D-3 in 88% yield and 95% enantiomeric excess. Since the purified D-N-carbamoylase from Agrobacterium sp. IP I 671 was shown to be 100%D-selective, the enantiomeric purity of 95% of D-3 arising from the transformation with the immobilized cells must be explained by the participation of a further, L-selective N-carbamoylase or, which is more likely, by racemization of the final hydrolysis product by the action of an amino acid racemase. Isolated hydantoinases from Bacillus thermoglucosidasius, Thermus sp., Arthrobacter aurescens DSM 3745, and Arthrobacter crystallopoietes DSM 20117 turned out to be stereospecific for the conversion of the D-form of hydantoin D,L-1. The enantiomerically pure L-form of 3 was also prepared. It was synthesized from racemic N-carbamoyl amino acid D,L-2 by enantiomer-specific hydrolysis of the L-form in presence of L-N-carbamoylase from Arthrobacter aurescens DSM 3747. Received December 20, 1999. Accepted January 7, 2000     

Steric stabilization of iron (III) hydroxide sols by various amino acids

The dispersion and coagulation phenomena of iron(III) hydroxide sols were investigated as a function of pH in the absence and presence of amino acids. The amino acids used were glycine,L--alanine,DL--amino-n-butyric acid,L-valine,L-leucine,L- isoleucine,L-glutamic acid andL-arginine. The turbidity measurements of the iron-(III) hydroxide sols, which were prepared by pouring an aqueous iron(III) chloride solution into boiling distilled water, were carried out using a spectrophotometer with an addermixer device and an automatic recording system. The zeta potentials of sol particles were obtained by ultra-microelectrophoresis. The change in turbidity of the sol, as a measure in stability of the sol, increased with increasing pH in the region of pH 2–8, and reached a maximum at the isoelectric point of the particles. The coagulation at the isoelectric point was prevented by adding amino acids, and the stabilization had an optimum point at concentrations which depended upon the kinds of amino acids. The remarkable dispersing effect of amino acids which occurred near the isoelectric point of the particles at the suitable concentration of the ammo acids may be due to the steric protection by amino acid adsorbed. The protective action was explained according to a modified DLVO theory, the modification for London-van der Waals force being applied in order to take the effect of the adsorption layer into account.     

Synthesis of functional poly(amino acids) on cyclophosphazene templates

With the use of hexakis(p-aminophenoxy)cyclotriphosphazene and tetraphenyltetraaminocyclotetraphosphazene whose primary groups initiated the polymerization of L-α-alanine and γ-esters of L-α-glutamic acid n-carboxyanhydrides, poly(amino acids) containing an inorganic central ring have been prepared. Polypeptide chains of various lengths have been obtained by variation of the monomer-to-initiator molar ratio. The molecular masses of the synthesized samples are in good agreement with the calculated parameters, and the polymers show a narrow molecular-mass distribution (M _w /M _n < 1.3). The circular dichroism (CD) spectra and IR studies indicate a predominantly helical conformation of poly(amino acid) chains with the central triphosphazene ring and the disordered structure of poly(amino acid) chains bonded to cyclotetraphosphazene.     

Pseudo-poly(amino acid)s: study on construction and characterization of novel chiral and thermally stable nanostructured poly(ester-imide)s containing different trimellitylimido-amino acid-based diacids and pyromellitoyl-tyrosine-based diol

A new class of chiral and potentially biodegradable poly(ester-imide)s (PEI)s as pseudo-poly(amino acid)s (PAA)s bearing natural amino acids in the main chain was synthesized. In this investigation, N,N′-(pyromellitoyl)-bis-(L-tyrosine dimethyl ester) as a biodegradable optically active diphenol and synthesized trimellitic anhydride-derived dicarboxylic acids containing different natural amino acids such as S-valine, L-methionine, L-leucine, L-isoleucine, and L-phenylalanine were used for direct polyesterification. With the aim of tosyl chloride/pyridine/N,N′-dimethylformamide system as a condensing agent, the new optically active PEIs were obtained in good yields and moderate inherent viscosity up to 0.42 dL/g. The obtained polymers were characterized with FT-IR, ¹H-NMR, X-ray diffraction (XRD), field emission scanning electron microscopy, elemental, and thermogravimetric analysis techniques. These polymers show high solubility in organic solvents, such as N,N′-dimethyl acetamide, N-methyl-2-pyrrolidone, and sulfuric acid at room temperature, and are insoluble in solvents, such as methylene chloride, cyclohexane, and water. Morphology probes showed these pseudo-poly(amino acid)s were noncrystalline and nanostructured polymers. On the basis of thermogravimetric analysis data, such PAAs are thermally stable and can be classified as self-extinguishing polymers. In addition due to the existence of amino acids in the polymer backbones these pseudo-PAAs not only are optically active but also are expected to be biodegradable and therefore could be classified under eco-friendly polymers.     

Determination of chromium(VI) and total chromium in water by in-electrode coulometric titration in a porous glassy carbon electrode

Chromium(VI) is determined through its direct electrochemical reduction in the bulk of a porous glassy carbon electrode. An electrode filled with the acidified sample and Cr(VI) is reduced by means of a constant current whereas the potential of the electrode is monitored. The limits of detection and quantification were found to be 1.9 and 6.0 μg · L⁻¹, resp. The linear range, repeatability and reproducibility were found to be 5–500 μg · L⁻¹, 1.2, and 1.8%, resp. The influence of Fe(III), Ca(II), Mg(II), sulphates, nitrates, humic acids and surfactants was investigated. Total chromium was measured after chemical oxidation of Cr(III) to chromate by permanganate. The method was applied to analyses of water samples.     

Amino acids in aqueous solution. Effect of molecular structure and temperature on thermodynamics of dissolution

The enthalpies of dissolution of glycine (Gly) and L-α-alanine (Ala) in water at 288.15–318.15 K were measured. The results were compared with the earlier obtained data for L-α-phenylalanine (Phe) and L-α-histidine (His). The standard enthalpies of dissolution (Δ_soln H ₀) and differences (ΔC _p ⁰ ) between the limiting partial molar heat capacity of the amino acids in solution and the heat capacity of the amino acids in the crystalline state were calculated in the temperature interval 273–373 K. Changes in the entropy of dissolution (ΔΔ_soln S ₀) and reduced Gibbs energy [Δ (Δ_soln G ₀/T)] in the temperature interval from 273 to 373 K were determined from the known thermodynamic relationships. The ΔC _p ⁰ value is negative for hydrophilic glycine and positive for other amino acids. The ΔΔ_soln S ⁰ values increase with an increase in the hydrophobicity of the amino acids. The Δ(Δ_soln G ₀/T) values become more negative in the order Ala, Phe, Gly, His. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 711–714, April, 2007.     

Heat capacities of solid copoly(amino acid)s

Recently heat capacities C_p of poly(amino acid)s of all naturally occurring amino acids have been determined. In a second step the heat capacities of four copoly(amino acid) s are studied in this research. Poly(L -lysine · HBr-alanine), poly(L -Lysine · HBr-phenylalanine), poly(sodium-L -glutamate-tyrosine), and poly(L -proline-glycine-proline) heat capacities are measured by differential scanning calorimetry in the temperature range 230–390 K. This is followed by an analysis using approximate group vibrations and fitting the C_p contributions of the skeletal vibrations of the corresponding homopolymers to a two-parameter Tarasov function. Good agreement is found between experiment and calculation. Predictions of heat capacities based on homopoly(amino acid)s are thus expected to be possible for all polypeptides, and enthalpies, entropies, and Gibbs functions for the solid state can be derived.     

High-performance liquid chromatographic separation of unusual amino acid enantiomers derivatized with (1S,2S)-1,3-diacetoxy-1-(4-nitrophenyl)-2-propyl-isothiocyanate

Summary A new chiral derivatizing agent (CDA), (1S,2S)-1,3-diacetoxy-1-(4-nitrophenyl)-2-propylisothiocyanate, (S,S)-DANI, was applied to the separation of the enantiomers of unusual amino acids containing two chiral centers. Different β-methyl-α-amino acids (β-MePhe, β-MeTyr and β-MeTrp) and β-amino acids with cycloalkane skeletons (2-aminocyclopentanecarboxylic acid and 2-aminocyclohexanecarboxylic acid) were derivatized and the thiourea derivatives produced were separated by reversed-phase high-performance liquid chromatography. The applicability of this new CDA in the separation of unusual amino acids is demonstrated. The four stereoisomers of the investigated amino acids (except β-MePhe) could be separated in one chromatographic run. Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

A study of the temperature effect on the IR spectra of crystalline amino acids,dipeptids, and polyamino acids. VI. <Emphasis Type="Italic">L</Emphasis>-alanine and <Emphasis Type="Italic">DL</Emphasis>-alanine

The results of IR and single crystal X-ray diffraction studies on the dynamics of molecular groups and structural changes in L-alanine and DL-alanine (NH₃⁺-CH(CH₃)-COO⁻) with temperature variation are given. An analysis of changes in the 4000–600 cm⁻¹ frequency range of the IR spectra with temperature variation reveals the occurrence of the anomaly for the ∼974 cm⁻¹ band in DL-alanine, which is similar to the anomaly for the 955 cm-1 band, previously described for L-alanine. The X-ray diffraction data for L and DL-alanine show that no dramatic changes in the unit cell parameters, conformations of amino acid molecules themselves, and hydrogen bond lengths occur with temperature variation, which would indicate the structural phase transition. Changes in the IR spectra of L-alanine and DL-alanine with temperature variation are compared to the changes in the vibrational spectra of other amino acids on cooling.     

Determination of anti-oxidant capacity and content of phenols,phenolic acids,and flavonols in Indian and European gooseberry

Phenolic acids and derivatives of quercetin in Indian (amla) and European gooseberry were determined by high-performance liquid chromatography with diode array detector. The calibration curves were constructed using phenolic compounds standards (the coefficient of determination (R ²) was 0.9990–0.9997 for phenolic acids and 0.9989–0.9994 for flavonols, respectively). The lowest detection limit was 0.28 mg L⁻¹ and 0.35 mg L⁻¹ for hyperoside and gallic acid, respectively, whereas the highest was 1.80 mg L⁻¹ and 7.98 mg L⁻¹ for quercetin and chlorogenic acid, respectively. The quantification limits calculated were 0.85–24.04 mg L⁻¹ for hyperoside and chlorogenic acid, respectively. The predominant phenolic acid in amla and gooseberry is gallic acid: (5.37 ± 0.04) mg per 100 g of dry mass (d.m.) and (3.21 ± 0.03) mg per 100 g of d.m., respectively. The next one was caffeic acid, 0.65–1.22 mg per 100 g of d.m., followed by p-coumaric acid, 0.84–1.17 mg per 100 g of d.m. Out of the flavonols, rutin is predominant: (3.11 ± 0.13) mg per 100 g of d.m. and (2.12 ± 0.03) mg per 100 g of d.m., respectively. Anti-oxidant activity was also determined.