Bulk-modified modified screen-printing carbon electrodes with both lactate oxidase (LOD) and horseradish peroxide (HRP) for the determination of L-lactate in flow injection analysis mode

A screen-printed carbon electrode modified with both HRP and LOD (SPCE–HRP/LOD) has been developed for the determination of l-lactate concentration in real samples. The resulting SPCE–HRP/LOD was prepared in a one-step procedure, and was then optimised as an amperometric biosensor operating at [0, −100] mV versus Ag/AgCl for l-lactate determination in flow injection mode. A significant improvement in the reproducibility (coefficient variation of about 10%) of the preparation of the biosensors was obtained when graphite powder was modified with LOD in the presence of HRP previously oxidised by periodate ion (IO₄⁻). Optimisation studies were performed by examining the effects of LOD loading, periodation step and rate of the binder on analytical performances of SPCE–HRP/LOD. The sensitivity of the optimised SPCE–HRP/LOD to l-lactate was 0.84 nA L μmol⁻¹ in a detection range between 10 and 180 μMol. The possibility of using the developed biosensor to determine l-lactate concentrations in various dairy products was also evaluated.     

Preparation of a metal–ligand fluorescent chemosensor and enantioselective recognition of carboxylate anions in aqueous solution

Two chiral fluorescent chemosensors 1 and 2 were synthesized, and the structure characterized by IR, ¹H NMR, ¹³C NMR, MS spectra and elemental analysis. Their recognition ability was studied in aqueous solution (Tris–HCl buffer pH 7.4, MeOH/H₂O = 1:1) through fluorescence spectra. Receptors 1 and 2 showed a good binding ability to the copper ion. The host 1-Cu²⁺ complex showed a chiral recognition ability to mandelate anions with a preferable binding to l-mandelate than d-mandelate anions. The host 1-Cu²⁺ complex and l- or d-mandelate anions formed 1:1 stoichiometric complex. The binding constant for l-mandelate is 576 M⁻¹, whereas that for d-mandelate is only 38 M⁻¹, which can be distinguished by the different change of fluorescence intensity.     

Design of fluorescent self-assembled multilayers and interfacial sensing for organophosphorus pesticides

This paper details the fabrication of indole (ID) self-assembled multilayers (SAMs) and fluorescence interfacial sensing for organophosphorus (OP) pesticides. Quartz/APES/AuNP/l-Cys/ID film was constructed on l-cysteine modified Quartz/APES/AuNP surface via electrostatic attraction between ID and l-cysteine. Cyclic voltammetry indicates that ID is immobilized successfully on the gold surface. Fluorescence of the Quartz/APES/AuNP/l-Cys/ID film shows sensitive response toward OPs. The fluorescent sensing conditions of the SAMs are optimized that allow linear fluorescence response for methylparathion and monocrotophos over 5.97 × 10⁻⁷ to 3.51 × 10⁻⁶ g L⁻¹ and 3.98 × 10⁻⁶ to 3.47 × 10⁻⁵ g L⁻¹, with detection limit of 6.1 × 10⁻⁸ gL⁻¹ and 3.28 × 10⁻⁶ gL⁻¹, respectively. Compared to bulk phase detection, interfacial fluorescence sensing based on the SAMs technology shows higher sensitivity by at least 2 order of magnitude.     

Separation and determination of l-tyrosine and its metabolites by capillary zone electrophoresis with a wall-jet amperometric detection

A method of capillary electrophoresis with wall-jet amperometric detection (AD) has been developed for separation and determination of l-tyrosine (Tyr) and its metabolites, such as Tyramine (TA), p-hydroxyphenylpyruvic (pHPP), homogentisic acid (HGA) and some dipeptides containing Tyr, such as Tyr-Gly-Gly (YGG), Tyr-Arg (YR) and Tyr-d-Arg (Y-d-R). A carbon disk electrode was used as the working electrode and the optimal detection potential was 1.00 V (versus Ag/AgCl). At 18 kV of applied voltage, the seven compounds were completely separated within 20 min in 110 × 10⁻³ mol/L Na₂HPO₄–NaH₂PO₄ buffer (pH 7.10) containing 3 × 10⁻³ mol/L β-cyclodextrin (β-CD). Good linear relationship was obtained for all analytes and the detection limits of seven analytes were in the range of 0.95–4.25 ng/mL. The proposed method has been applied to examine the metabolic process of l-tyrosine in rabbit's urine.     

Enthalpic interactions of some α-amino acids with glycol in aqueous solutions at 298.15 K

The enthalpies of mixing of six kinds of aqueous amino acid solutions (Glycine, l-alanine, l-valine, l-serine, l-threonine and l-proline) and aqueous glycol solution and their respective enthalpies of dilution have been determined at 298.15 K using flow microcalorimetry. The experimental data have been analyzed according to the McMillan–Mayer formalism to obtain the heterotactic enthalpic interaction coefficients (h_xy). h_xy coefficients have been discussed from the points of view of solute–solute interactions.     

Simultaneous reactive extraction separation of amino acids from water with D2EHPA in hollow fiber contactors

Reactive extraction separation of binary amino acids from water using a microporous hollow fiber has been studied, in which the acidic extractant di(2-ethylhexyl)phosphoric acid (D2EHPA) was selected as an active carrier dissolved in kerosene. l-Phenylalanine (Phe) was extracted from an aqueous solution through the shell side of module to the organic phase through the lumen of fiber in the extraction module, in which l-Phe was then back-extracted to stripping phase in stripping module. Experiments were conducted as a function of the initial feed concentration of equimolar Phe and l-aspartic acid (l-Asp) (5 mol/m³), feed pH (3–5), the carrier concentration (0.1–0.5 mol/dm³), and stripping acidity (0.1–2 mol/dm³). The effect of process variables on the separation factor of Phe/Asp and the possible transport resistances including aqueous-layer diffusion, membrane diffusion, organic-layer, and interfacial chemical reaction were quantitatively studied and discussed. The high separation factor (β) of Phe/Asp was obtained to be 18.5 at feed pH 5 and 2 mol/dm³ of strip solution (HCl). The extraction and stripping processes appear to rely on pH dependence of the distribution coefficient of amino acids in reactive extraction system. The separation factor (β) was enhanced in hollow fiber membrane (HFM) process compared with conventional solvent process, which was a result of the counter transport of hydrogen ions.     

Resonance Rayleigh scattering and resonance non-linear scattering method for the determination of aminoglycoside antibiotics with water solubility CdS quantum dots as probe

In pH 6.6 Britton–Robinson buffer medium, the CdS quantum dots capped by thioglycolic acid could react with aminoglycoside (AGs) antibiotics such as neomycin sulfate (NEO) and streptomycin sulfate (STP) to form the large aggregates by virtue of electrostatic attraction and the hydrophobic force, which resulted in a great enhancement of resonance Rayleigh scattering (RRS) and resonance non-linear scattering such as second-order scattering (SOS) and frequency doubling scattering (FDS). The maximum scattering peak was located at 310 nm for RRS, 568 nm for SOS and 390 nm for FDS, respectively. The enhancements of scattering intensity (ΔI) were directly proportional to the concentration of AGs in a certain ranges. A new method for the determination of trace NEO and STP using CdS quantum dots probe was developed. The detection limits (3σ) were 1.7 ng mL⁻¹ (NEO) and 4.4 ng mL⁻¹ (STP) by RRS method, were 5.2 ng mL⁻¹ (NEO) and 20.9 ng mL⁻¹ (STP) by SOS method and were 4.4 ng mL⁻¹ (NEO) and 25.7 ng mL⁻¹ (STP) by FDS method, respectively. The sensitivity of RRS method was the highest. The optimum conditions and influence factors were investigated. In addition, the reaction mechanism was discussed.     

Heterogeneous interaction between zwitterions of some l-α-amino acids and ethanol molecule in water at 298.15 K

Dissolution enthalpies of l-α-aminobutyric acid, l-α-isoleucine, l-α-serine, l-α-threonine and l-α-cysteine in water and aqueous ethanol solutions have been measured by calorimetry at a temperature of 298.15 K. The obtained results were used to calculate the enthalpic heterogeneous pair interaction coefficients between zwitterions of amino acids and a molecule of ethanol in water. These values were interpreted in the terms of the hydrophobic or hydrophilic effects of the side chains of amino acids on their interactions with a polar molecule of ethanol in water.     

Low-frequency vibrational modes of dl-homocysteic acid and related compounds

In this paper several polycrystalline molecules with sulfonate groups and some of their metal complexes, including dl-homocysteic acid (DLH) and its Sr- and Cu-complexes, pyridine-3-sulphonic acid and its Co- and Ni-complexes, sulfanilic acid and l-cysteic acid were investigated using THz time-domain methods at room temperature. The results of THz absorption spectra show that the molecules have characteristic bands in the region of 0.2–2.7 THz (6–90 cm⁻¹). THz technique can be used to distinguish different molecules with sulfonate groups and to determine the bonding of metal ions and the changes of hydrogen bond networks. In the THz region DLH has three bands: 1.61, 1.93 and 2.02 THz; and 0.85, 1.23 and 1.73 THz for Sr-DLH complex, 1.94 THz for Cu-DLH complex, respectively. The absorption bands of pyridine-3-sulphonic acid are located at 0.81, 1.66 and 2.34 THz; the bands at 0.96, 1.70 and 2.38 THz for its Co-complex, 0.76, 1.26 and 1.87 THz for its Ni-complex. Sulphanilic acid has three bands: 0.97, 1.46 and 2.05 THz; and the absorption bands of l-cysteic acid are at 0.82, 1.62, 1.87 and 2.07 THz, respectively. The THz absorption spectra after complexation are different from the ligands, which indicate the bonding of metal ions and the changes of hydrogen bond networks. M–O and other vibrations appear in the FIR region for those metal–ligand complexes. The bands in the THz region were assigned to the rocking, torsion, rotation, wagging and other modes of different groups in the molecules. Preliminary assignments of the bands were carried out using Gaussian program calculation.     

A novel fluorescent assay for edaravone with aqueous functional CdSe quantum dots

Aqueous thiol-capped CdSe QDs with a narrow, symmetric emission were prepared under a low temperature. Based on the fluorescence enhancement of thiol-stabilized CdSe quantum dots (QDs) caused by edaravone, a simple, rapid and specific quantitative method was proposed to the edaravone determination. The concentration dependence of fluorescence intensity followed the binding of edaravone to surface of the thiol-capped CdSe QDs was effectively described by a modified Langmuir-type binding isotherm. Factors affecting the fluorescence detection for edaravone with thiol-stabilized CdSe QDs were studied, such as the effect of pH, reaction time, the concentration of CdSe QDs and so on. Under the optimal conditions, the calibration plot of C/(I − I₀) with concentration of edaravone was linear in the range of (1.45–17.42) μg/mL (0.008–0.1 μmol/L) with correlation coefficient of 0.998. The limit of detection (LOD) (3σ/κ) was 0.15 μg/mL (0.0009 μmol/mL). Possible interaction mechanism was discussed.     

L-Cysteine-coated CdSe/CdS core-shell quantum dots as selective fluorescence probe for copper(II) determination

Quantum dots (QDs) or semiconductor nanocrystals have been receiving great interest in the last few years. In this paper, L-cysteine-coated CdSe/CdS core-shell QDs (λ_em = 585 nm) have been prepared, which have excellent water-solubility. The full width at half maximum (FWHM) of the photoluminescence of these nanocrystals is very narrow (about 30 nm), and the quantum yield (QY) is 15% relative to Rhodamine 6G in ethanol (QY = 95%). With excess free L-cysteine in the solution, the fluorescence intensity of L-cysteine-coated CdSe/CdS QDs showed improved stability. It was found that the fluorescence of L-cysteine-capped CdSe/CdS QDs could be quenched only by copper (II) ions and was insensitive to other physiologically important cations, such as Ca²⁺, Mg²⁺, Zn²⁺, Al³⁺, Fe³⁺, Mn²⁺ and Ni²⁺ etc. Based on this finding, the quantitative analysis of Cu²⁺ with L-cysteine-capped CdSe/CdS QDs has been established. The linear range was from 1.0 × 10^− 8 to 2.0 × 10^− 7 mol L^− 1 and the limit of detection (LOD) was 3.0 × 10^− 9 mol L^− 1 (S/N = 3). The proposed method has first been applied to the determination of Cu²⁺ in vegetable samples with recoveries of 99.6–105.8%.     

Thiol oxidation at 2-mercaptopyrimidine-appended cobalt phthalocyanine modified glassy carbon electrodes

The electrocatalytic activity of cobalt tetra-2-mercaptopyrimidylphthalocyanine-modified glassy carbon electrode (CoTMPyrPc-GCE) toward gluthathione, l-cysteine and 2-mercaptoethanol has been investigated. CoTMPyrPc-GCE provides a significant improvement on the reported oxidation potential of l-cysteine in pH 4.0 phosphate buffered solution. The oxidation peak potential (E_p), for l-cysteine, was 0.15 V – a relatively lowered oxidation potential compared to reported phthalocyanine complexes. The electrode prepared by drop-dry/thermal annealing method was very stable and sensitive over a long period of time. The stability has been attributed to the thermal annealing and the structure of the mercaptopyrimidine on the periphery of the CoPc.     

Carbon- and nitrogen-centered radicals produced from l-lysine by radiation-induced oxidation: A pulse radiolysis study

Radical species generated from the reactions of a basic amino acid, l-lysine (Lys), with hydroxyl radicals (OH) and sulfate radical anion () have been detected by the method of pulse radiolysis. On the basis of electron transfer reactivities toward tetranitromethane (TNM), it was demonstrated that reducing carbon-centered radicals are generated as a result of hydrogen abstraction from CH₂ of Lys with a G-value of 1.9 × 10⁻⁷ mol J⁻¹. On the other hand, direct oxidation of l-Lys by formed a transient species with different spectroscopic properties, most likely, the ε-N-centered Lys radical.     

Antitumor triterpene saponins from Anemone flaccida

A new triterpenoid saponin, 3-O-[（6′-butyryl）-β-D-glucopyranosyl]-28-O-[α-L-rhamnopyranosyl-（1→4）-β-D-glucopyranosyl-（1→6）-β-D-glucopyranosyl] oleanolic acid, as well as three known triterpenoid saponins were isolated from the rhizomes of Anemone flaccida. Their structures were elucidated by spectroscopic methods. These compounds showed significant antitumor activities.     

Free radical scavenging and radioprotective effects of carnosine and anserine

Two histidine-containing natural dipeptides, carnosine and anserine (β-alanyl-1-methyl-l-histidine), have been examined for their antioxidant and radioprotective abilities. Pulse radiolysis studies indicated the antioxidative properties of carnosine and anserine aqueous solutions at different pH. The rate constants for the reaction OH radical with carnosine at neutral pH were determined to be 5.3×10⁹ M⁻¹ s⁻¹ at 300 nm, and 4.1×10⁹ M⁻¹ s⁻¹ at 400 nm, respectively. Carnosine and anserine also protected plasmid pUC18 DNA from X-ray radiation-induced strand breaks as evidenced from the studies by agarose gel electrophoresis. Carnosine showed higher protective efficiency under the experimental conditions. Our data demonstrated that carnosine and anserine may play an important role in the maintenance of the antioxidant system.     

Enthalpies of solution of l-threonine in a (water + alcohol) mixture at 298.15 K

The enthalpies of solution of l-threonine in the (water + methanol), (water + ethanol), (water + n-propanol), and (water + i-propanol) mixtures, with an alcohol content up to 0.4 mol fractions, have been measured calorimetrically at T = 298.15 K. The standard enthalpies of solution and transfer of l-threonine from water to an aqueous alcohol have been calculated. The effect of the structure properties of the mixed solvent on the specified enthalpy characteristics of l-threonine is discussed. The enthalpy coefficients of pairwise interactions between amino acid and alcohol molecules have been computed. It has been found that these coefficients become increasingly positive in the methanol, ethanol, n-propanol, and i-propanol consequence. A comparative analysis of the thermodynamic characteristics of dissolution of l-threonine and some other amino acids (glycine, l-alanine and l-valine) in the mixtures studied has been made.     

Theoretical studies of l-ascorbic acid (vitamin C) and selected oxidised, anionic and free-radical forms

Calculations were carried out at the restricted and unrestricted B3LYP/6-311++G(d,p) and B3LYP/EPR-II levels for two conformers (1 and 2) of l-ascorbic acid and their respective oxidation products to di- and monodehydroascorbates. For the didehydroascorbates, corresponding to Conformer 1, free radical properties are compared with previously published calculations in the gaseous and aqueous solution states and with experimental EPR values. Calculated molecular structures, EPR and vibrational spectral and energetic properties are reported including some proposed changes to previous EPR assignments. Conformer 2 of l-ascorbic acid is predicted to have lower energy than Conformer 1, under the method and basis sets used, by between 11 and 26 kJ mol⁻¹ and is stabilised by internal hydrogen bonding. For particular monodehydroascorbates, formed by loss of H from the chain CH(OH) group, calculated properties of conformers of five neutral mono-radicals and the corresponding five anionic di-radicals are reported. Relaxed potential energy surface (PES) scans were carried out for two proton transfer processes and relative energies of stable minima and barriers between them determined. The physiological significance of the protective oxidation of l-ascorbic acid by free radicals is briefly described.     

DNA binding and oxidative cleavage activity of ternary (l-proline)copper(II) complexes of heterocyclic bases

Ternary copper(II) complexes [Cu(l-pro)(B)(H₂O)](NO₃) (1, 2) where l-pro = l-proline, B is a N,N-donor heterocyclic base, viz. 2,2′-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), are synthesized, characterized, and their DNA binding and cleavage activity studied. The bpy complex (1) is structurally characterized by single-crystal X-ray crystallography. The complexes show the presence of a distorted square-pyramidal (4 + 1) CuN₃O₂ coordination geometry. Complex [Cu(l-pro)(bpy)(H₂O)](NO₃) (1) crystallizes in the triclinic space group P1 with unit cell parameters: a = 7.082(3) Å, b = 10.483(5) Å, c = 11.581(5) Å, α = 89.700(7)°, β = 83.488(8)°, γ = 84.109(8)° and V = 849.7(7) Å³. The one-electron paramagnetic complexes display a d–d band near 600 nm in water and show a cyclic voltammetric response due to Cu(II)/Cu(I) couple near 0.1 V (versus SCE) in Tris–HCl buffer–0.1 M KCl. Binding interactions of the complexes with calf thymus (CT) DNA have been investigated by emission, absorption, viscosity and DNA thermal denaturation studies. The phen complex displays significant binding propensity to the CT DNA giving an order: 2 (phen)  1 (bpy). The bpy complex does not show any apparent binding to the DNA and hence poor cleavage efficiency. Complex 2 shows efficient oxidative cleavage of SC-DNA in the presence of 3-mercaptopropionic acid (MPA) involving hydroxyl radical species as evidenced from the control data showing inhibition of DNA cleavage in the presence of DMSO and catalase.     

Syntheses of l-glucosamine donors for 1,2-trans-glycosylation reactions

Two new l-glucosamine donors, that is pent-4-enyl 3,4,6-tri-O-acetyl-2-allyloxycarbonylamino-2-deoxy-β-l-glucopyranoside 16 and ethyl 3,4,6-tri-O-acetyl-2-allyloxycarbonylamino-2-deoxy-1-thio-β-l-glucopyranoside 21 were prepared in 12 steps from l-arabinose. The reaction pathway uses 3,4,6-tri-O-acetyl-l-glucal 5, and then 3,4,6-tri-O-acetyl-2-deoxy-2-iodo-α-l-mannopyranosyl azide 8 as intermediates. The latter, together with donors 16 and 21, were used for preparing l-glucosamine neoglycolipids.     

Determination of ultra trace amounts of protein by 4-chlorosulfo-(2′-hyaroxylphenylazo)-rhodanine-Ti(IV) complex [ClSARP-Ti(IV)] as the fluorescence spectral probe in AOT microemulsion

Experiments indicated that protein can enhance the fluorescence of the 4-chlorosulfo-(2′-hydroxylophenylazo)-rhodanine-Ti(IV) complex [ClSARP-Ti(IV)] in the presence of bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT) microemulsion. Based on this, a sensitive and reproducible fluorometric method for the determination of micro amount protein was developed. The calibration curves of four proteins were given. Under the optimum experimental conditions, the enhanced fluorescence intensity of the system was in proportional to the concentration of protein in the range of 0.1–11 μg mL⁻¹ for bovine serum albumin (BSA), 1.0–10 μg mL⁻¹ for human serum albumin (HSA), 1.0–50 μg mL⁻¹ for ovalbumin (Ova) and 2.5–18 μg mL⁻¹ for γ-globulins (γ-G). Their detection limits were 0.070, 0.071, 0.33 and 0.22 μg mL⁻¹, respectively. The ClSARP-Ti(IV) complex as a spectral probe can be used to the determination of protein in milk powder and oatmeal yielding with satisfactory results. Therefore, the proposed method is one of the most sensitive methods available. In addition, the interaction mechanism of this system is studied by multi-techniques.