Production and characterization of pullulan from beet molasses using a nonpigmented strain of Aureobasidium pullulans in batch culture

The production of pullulan from beet molasses by a pigment-free strain of Aureobasidium pullulans on shake-flask culture was investigated. Combined pretreatment of molasses with sulfuric acid and activated carbon to remove potential fermentation inhibitors present in molasses resulted in a maximum pullulan concentration of 24 g/L, a biomass dry wt of 14 g/L, a pullulan yield of 52.5%, and a sugar utilization of 92% with optimum fermentation conditions (initial sugar concentration of 50 g/L and initial pH of 7.0). The addition of other nutrients as carbon and nitrogen supplements (olive oil, ammonium sulfate, yeast extract) did not further improve the production of the exopolysaccharides. Structural characterization of the isolated polysaccharides from the fermentation broths by ¹³C-nuclear magnetic resonance spectroscopy and pullulanase digestion combined with size-exclusion chromatography confirmed the identity of pullulan and the homogeneity (>93% dry basis) of the elaborated polysaccharides by the microorganism. Using multiangle laser light scattering and refractive index detectors in conjunction with high-performance size-exclusion chromatography molecular size distributions and estimates of the molecular weight (M _w =2.1−4.1×10⁵), root mean square of the radius of gyration (R _g =30−38 nm), and polydispersity index (M _w /M _n =1.4−2.4) were obtained. The fermentation products of molasses pretreated with sulfuric acid and/or activated carbon were more homogeneous and free of contaminating proteins. In the concentration range of 2.8−10.0 (w/v), the solution’s rheologic behavior of the isolated pullulans was almost Newtonian (within 1 and 1200 s⁻¹ at 20°C); a slight shear thinning was observed at 10.0 (w/v) for the high molecular weight samples. Overall, beet molasses pretreated with sulfuric acid and activated carbon appears as an attractive fermentation medium for the production of pullulan by A. pullulans.     

Enhancement of pullulan production by aureobasidium pullulans in batch culture using olive oil and sucrose as carbon sources

The production of pigment-free pullulan byAureobasidium pullulans, using olive oil and sucrose as carbon (C) sources, in shake flasks, was investigated. Optimum medium composition for pullulan elaboration was 80 g/L sucrose, 25 mL/L olive oil, 5 mL/L Tween-80, 10 g/L glutamic acid, and an initial pH of 5.5. Maximum pullulan concentration (51.5 g/L), productivity (8.6 g/L·d), and yield (80.3%) were achieved under these conditions after 120 h of fermentation. The principal advantage of using olive oil and sucrose simultaneously as C sources was the elimination of the inhibitory effect of high sucrose concentrations (> 60 g/L) on pullulan production by the microorganism. Structural characterization by¹³C-NMR, monosaccharide, and methylation analyses, and pullulanase digestion, combined with size-exclusion chromatography, confirmed the identity of pullulan and the homogeneity of the released polysaccharide in the fermentation broths. There were no significant differences in structure between pullulan samples isolated from either olive oil-supplemented media or olive oil-free media. The molecular size of pullulan from the combined olive oil-sucrose fermentation was slightly lower (1.1 X 10⁶) than that of conventional fermentation with sucrose as a single C source (1.4 X 10⁶). Lowering the initial pH of the medium resulted in increased molecular size for the released polymer, but a lower pullulan yield.     

Separation ofN-carbamoyl aspartate andl-dihydroorotate from serum by high-performance liquid chromatography with fluorimetric detection

Summary A high-performance liquid chromatographic method, with 9-anthryldiazomethane as derivatizing agent, has been developed for the simultaneous determination ofN-carbamoyl aspartate andl-dihydroorotate in serum. Sample preparation for 1 mL serum was by simple liquid-liquid extraction and then derivatization. The compounds were separated on a Luna C18(2) column by use of a gradient prepared from acetonitrile and 10 mM sodium acetate buffer, pH 6.0, and fluorimetric detection was performed at excitation and emission wavelengths of 365 nm and 412 nm, respectively. The response was found to be linearly dependent on concentration between 0.8 and 60 μg mL⁻¹ forl-dihydrooratate and between 0.9 and 90 μg mL⁻¹ forN-carbamoyl aspartate; the mean recovery rates were 50 and 51%, respectively. The limits of detection and quantification were 0.33 μg mL⁻¹ and 0.6 μg mL⁻¹, respectively, forl-dihydroorotate and 0.4 μg mL⁻¹ and 0.7 μg mL⁻¹ forN-carbamoyl aspartate. This method can be used to assess accumulation ofN-carbamoyl aspartate andl-dihydroorotate in body fluids in situations where cellular pyrimidine de novo synthesis is impaired.     

Biological responses of suspension cultures of Taxus chinensis var. mairei to shear stresses in the short term

The biological responses of Taxus chinensis var. mairei suspension cultures to a range of shear rates, were investigated in a Couette-type shear reactor. It was found that the shear rate below 458 s⁻¹ enhanced the primary metabolism, increasing mitochondrial activity and protein expression and inhibiting the activity of phenylalanine ammonia lyase (PAL), an enzyme relevant to the secondary metabolism. However, the shear rate over 719 s⁻¹ damaged taxus cells to some extent, decreasing the mitochondrial activity, increasing the membrane permeability, and even causing cell hypersensitive responses. As a result, PAL and intracellular peroxidase, an enzyme in cells relevant to reactive oxygen species scavenging, were activated and extracellular phenolics were accumulated. Additionally, shear-induced accumulation of free radicals and alkalization of the culture medium were observed. The results might shed light on a better understanding of the mechanism of plant cells responses to shearstress through the signal transduction pathway.     

Determination and Pharmacokinetic Study of Calycosin-7-O-β-d-glucoside in Rat Plasma After Intravenous Administration of Aidi Lyophilizer

Aidi injection is a clinical medicine used in China for the treatment of cancer. Calycosin-7-O-β-d-glucoside is the main effective components of the formulas. In this study, a high performance liquid chromatographic (LC) method was developed to quantify calycosin-7-O-β-d-glucoside in rat plasma using a liquid–liquid extraction and ultraviolet (UV) absorbance detection. LC analysis was performed on a Diamonsil C₁₈ column (200 × 4.6 mm i.d., 5 μm particle size) with isocratic mobile phase consisting of acetonitrile–0.05% phosphoric acid (19.5:80.5, v/v) of a flow rate of 1.0 mL min⁻¹. The linear range was 0.11–17.6 μg mL⁻¹ and the low quantification limit was 0.11 μg mL⁻¹ (S/N = 10). The intra- and inter-day relative standard deviations (RSD) in the measurement of quality control (QC) samples 0.11, 0.22, 1.32 and 8.80 μg mL⁻¹ ranged from 4.1 to 6.3 and 4.3 to 6.2%, respectively. The accuracy was from −6.7 to 4.3% in terms of relative error (RE). Calycosin-7-O-β-d-glucoside was stable in storage at −20 °C for 2 weeks and stable after three freeze–thaw cycles in rat plasma. This method was validated for specificity, accuracy, precision and was successfully applied to pharmacokinetic study of calycosin-7-O-β-d-glucoside in rat plasma after intravenous administration of Aidi lyophilizer.     

Overproduction of Glucoamylase by a Deregulated Mutant of a Thermophilic Mould <Emphasis Type="Italic">Thermomucor indicae-seudaticae</Emphasis>

Thermomucor indicae-seudaticae, a glucoamylase-producing thermophilic mould, was mutagenised using nitrous acid and gamma (⁶⁰Co) irradiation in a sequential manner to isolate deregulated mutants for enhanced production of glucoamylase. The mutants were isolated on Emerson YpSs agar containing a non-metabolisable glucose analogue 2-deoxy-d-glucose (2-DG) for selection. The preliminary screening for glucoamylase production using starch–iodine plate assay followed by quantitative confirmation in submerged fermentation permitted the isolation of several variants showing varying levels of derepression and glucoamylase secretion. The mutant strain T. indicae-seudaticae CR19 was able to grow in the presence of 0.5 g l⁻¹ 2-DG and produced 1.8-fold higher glucoamylase. As with the parent strain, glucoamylase production by T. indicae-seudaticae CR19 in 250-ml Erlenmeyer flasks attained a peak in 48 h of fermentation, showing higher glucoamylase productivity (0.67 U ml⁻¹ h⁻¹) than the former (0.375 U ml⁻¹ h⁻¹). A large-scale cultivation in 5-l laboratory bioreactor confirmed similar fermentation profiles, though the glucoamylase production peak was attained within 36 h attributable to the better control of process parameters. Although the mutant grew slightly slow in the presence of 2-DG and exhibited less sporulation, it showed faster growth on normal Emerson medium with a higher specific growth rate (0.138 h⁻¹) compared to the parent strain (0.123 h⁻¹). The glucoamylase produced by both strains was optimally active at 60 °C and pH 7.0 and displayed broad substrate specificity by cleaving α-1,4- and α-1,6-glycosidic linkages in starch, amylopectin, amylose and pullulan. Improved productivity and higher specific growth rate make T. indicae-seudaticae CR19 a useful strain for glucoamylase production.     

Photochromic transformations in solutions of 8,8′-diquinolyl disulfide and di(mercaptoquinolinato)nickel(<Emphasis Type="SmallCaps">II</Emphasis>)

The nature of intermediate species and their reactions were studied by laser pulse photolysis for a photochromic system consisting of 8,8′-diquinolyl disulfide (RSSR) and a planar Ni^II complex di(mercaptoquinolinato)nickel(II) (Ni(SR)₂) in toluene and benzene solutions. Under exposure to laser radiation, disulfide RSSR dissociates to two RS^· radicals, whose spectrum has an intense absorption band with a maximum at λ = 400 nm (ε = 8400 L mol⁻¹ cm⁻¹). The radicals disappear by recombination (2k _rec = 4.6 · 10⁹ L mol⁻¹ s⁻¹). In the presence of the Ni(SR)₂ complex, coordination of the radical (k _coord = 4.4 · 10⁹ L mol⁻¹ s⁻¹) competes with recombination to form a radical complex RS^· Ni(SR)₂ having an intense absorption band with a maximum at 460 nm (ε = 16 600 L mol⁻¹ cm⁻¹). This species decays in the second-order reaction (2k = 4.6 · 10⁴ L mol⁻¹ s⁻¹). Since the photochromic system returns to the initial state, the reaction of two radical complexes is assumed to produce radical recombination and reduction of the disulfide and Ni(SR)₂ complex. Analysis of the kinetic data showed that some RS^· radicals decay in the microsecond time interval due to the reaction with the RS^· Ni(SR)₂ radical complex (k = 3.1 · 10⁹ L mol⁻¹ s⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2291–2300, October, 2005.     

Application of a Validated, Stability-Indicating LC Method to Stress Degradation Studies of Ramipril and Moexipril.HCl

A stability-indicating reversed-phase liquid chromatographic (RPLC) method has been established for analysis of ramipril (RAM) and moexipril hydrochloride (MOEX.HCl) in the presence of the degradation products generated in studies of forced decomposition. The drug substances were subjected to stress by hydrolysis (0.1 m NaOH and 0.1 m HCl), oxidation (30% H₂O₂), photolysis (254 nm), and thermal treatment (80 °C). The drugs were degraded under basic and acidic conditions and by thermal treatment but were stable under other stress conditions investigated. Successful separation of the drugs from the degradation products was achieved on a cyanopropyl column with 40:60 (v/v) aqueous 0.01 m ammonium acetate buffer (pH 6)–methanol as mobile phase at a flow rate of 1 mL min⁻¹. Detection was by UV absorption at 210 nm. Response was a linear function of concentration over the range 5–50 μg mL⁻¹ (r > 0.9995), with limits of detection and quantitation (LOD and LOQ) of 0.04 and 0.09 μg mL⁻¹, respectively, for RAM and 0.014 and 0.32 μg mL⁻¹, respectively, for moexipril. The method was validated for specificity, selectivity, solution stability, accuracy, and precision. Statistical analysis proved the method enabled reproducible and selective quantification of RAM and MOEX as the bulk drug and in pharmaceutical preparations. Because the method effectively separates the drugs from their degradation products, it can be used as stability-indicating.     

Resolution of an intense sweetener mixture by use of a flow injection sensor with on-line solid-phase extraction

An integrated solid-phase spectrophotometry–FIA method is proposed for simultaneous determination of the mixture of saccharin (1,2-benzisothiazol-3(2H)-one-1,1-dioxide; E-954) (SA) and aspartame (N-l-α-aspartyl-l-phenylalanine-1-methyl ester; E-951) (AS). The procedure is based on on-line preconcentration of AS on a C₁₈ silica gel minicolumn and separation from SA, followed by measurement, at λ=210 nm, of the absorbance of SA which is transiently retained on the adsorbent Sephadex G-25 placed in the flow-through cell of a monochannel FIA setup using pH 3.0 orthophosphoric acid–dihydrogen phosphate buffer, 3.75×10^–3 mol L⁻¹, as carrier. Subsequent desorption of AS with methanol enables its determination at λ=205 nm. With a sampling frequency of 10 h⁻¹, the applicable concentration range, the detection limit, and the relative standard deviation were from 1.0 to 200.0 μg mL⁻¹, 0.30 μg mL⁻¹, and 1.0% (80 μg mL⁻¹, n=10), respectively, for SA and from 10.0 to 200.0 μg mL⁻¹, 1.4 μg mL⁻¹, and 1.6% (100 μg mL⁻¹, n=10) for AS. The method was used to determine the amounts of aspartame and saccharin in sweets and drinks. Recovery was always between 99 and 101%. The method enabled satisfactory determination of blends of SA and AS in low-calorie and dietary products and the results were compared with those from an HPLC reference method.     

A kinetic, spectral and theoretical investigation on the role of oxygen in the radiolytic oxidation of a sorbityl cyclic acetal

The oxidation process of the cyclic acetal sorbitylfurfural (SF) has been thoroughly examined from the kinetic, spectroscopic and theoretical point of view. Oxidation has been initiated by the radiolitically produced OH radical in the presence of variable oxygen amounts. Two competing reaction pathways are evidenced which lead to quite different products, although they do not affect the acetal ring integrity. The peroxidation of the hydroxylated furanic ring (k ₄=(6.1±0.9)×10⁸ M⁻¹ s⁻¹) maintains the ring structurevia HO₂^• elimination (k ₆=(1.9±0.4)×10⁵ s⁻¹). Unlike that, the peroxidation of the pseudo-allylic radical (k ₅=(1.9±0.9)×10⁹ M⁻¹ s⁻¹), formedvia β-cleavage, fixes the destructured intermediate, leading to a tetroxide, which slowly decomposes through a Russell mechanism (k ₈=(2.3±0.6)×10² s⁻¹). It is confirmed that the steady state concentration of the tetroxide is very low, which suggests a molar absorption coefficient for it around 1.2×10⁴ M⁻¹ cm⁻¹ at 265 nm. The end products of the latter pathway have been characterized as carboxylic and butenald-sorbitol derivatives. The kinetic and spectral data of every step of the process have been fitted by the above outlined mechanism. The energetics of the mechanism has been detailed byab initio computations as well, carrying further substantiation to it. Semi-empirical calculations were also employed to describe the spectral properties of each intermediate.     

LC-MS-MS Determination of Cyclo{(2S)-2-amino-8-[(aminocarbonyl)hydrazono] decanoyl-1-l-tryptophyl-l-isoleucyl-(2R)-2-piperidinecarbonyl} a Novel Histone Deacetylase Inhibitor in Rat Serum

A rapid and sensitive liquid chromatography-tandem mass spectrometry assay was developed for the determination of a novel histone deacetylase inhibitor, cyclo{(2S)-2-amino-8-[(aminocarbonyl)hydrazono]decanoyl-1-l-tryptophyl-l-isoleucyl-(2R)-2-piperidinecarbonyl} (SD-2007), in rat serum. The mobile phase consisted of acetonitrile and ammonium formate (10 mM) (85:15 v/v), and the flow rate was 0.25 mL min⁻¹. Chromatographic separations were achieved by isocratic elution on a C₁₈ column. Multiple reaction monitoring was based on the transition of m/z = 681.8 → 83.6 for SD-2007 and 372.1 → 176.1 for trazodone (internal standard). A linearity was observed over a concentration range from 2 to 1,000 ng mL⁻¹ (r ² > 0.999), with the lower limit of quantification at 2 ng mL⁻¹ with 100 μL of rat serum. The mean intra- and inter-day assay accuracy ranged from 98.5–109.7% to 95.2–102.7%, respectively, and the mean intra- and inter-day precision was between 4.3–11.3% and 2.9–13.3%. The developed assay was applied to a pharmacokinetic study of SD-2007 in rats after intravenous injection (dose 4 mg kg⁻¹).     

Electrochemical behaviors of metol on ionic-liquid-modified carbon paste electrode and its analytical application

The electrochemical behaviors of metol on an ionic liquid N-butylpyridinium hexafluorophosphate modified carbon paste electrode (IL-CPE) were studied in this paper. The results indicated that a pair of well-defined quasi-reversible redox peaks of metol appeared with the decrease of overpotential and the increase of redox peak current, which was the characteristics of electrocatalytic oxidation. The electrocatalytic mechanism was discussed and the electrochemical parameters were calculated with results of the charge-transfer coefficient (α) as 0.45, the electrode reaction rate constant (k _s) as 4.02 × 10⁻³ s⁻¹, and the diffusion coefficient (D) as 6.35 × 10⁻⁵ cm²/s. Under the optimal conditions, the anodic peak current was linear with the metol concentration in the range of 5.0 × 10⁻⁶ ∼ 1.0 × 10⁻³ mol/L (n = 11, γ = 0.994) and the detection limit was estimated as 2.33 × 10⁻⁶ mol/L (3σ). The proposed method was successfully applied to determination of metol content in synthetic samples and photographic solutions.     

Electrochemical behaviour of pyridoxine hydrochloride (vitamin B6) at carbon paste electrode modified with crown ethers

The electrochemical behaviour of pyridoxine hydrochloride (pyridoxine HCl) at the plain carbon paste electrode and the electrode modified with oxa crown ether has been studied using voltammetric and impedance measurements. The macrocycles used as modifiers were 18-crown-6, dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6 and dibenzo-24-crown-8, out of which DB18C6 gave better response for pyridoxine HCl. Tris buffer (pH 10.3) was chosen as an appropriate medium among the several supporting electrolytes of varying pH studied. The characterization of the DB18C6-modified electrode (CME-DB18C6) using kinetic parameters such as number of electrons (n) and electron transfer coefficient (α) is studied by cyclic voltammetry. Electrochemical impedance spectroscopic measurements obtained confirm the current enhancement over the modified electrode. Analytical applications of this electrode have been studied for the determination of pyridoxine HCl. A sensitive linear working range of 0.6 to 100 μg cm⁻³ with a detection limit of 0.4 μg cm⁻³ by differential pulse voltammetry was observed for pyridoxine HCl on CME-DB18C6. However, on decreasing the scan rate to 5 mV s⁻¹, the detection limit lowered to 0.2 μg cm⁻³. Interference from some vitamins like thiamine hydrochloride, riboflavin, nicotinamide, para-aminobenzoic acid, cyanocobalamin, folic acid and d-biotin and amino acid l-tryptophan was studied, and simultaneously, riboflavin, thiamine hydrochloride and pyridoxine HCl were determined over the modified electrode, CME-DB18C6. The modified electrode is successfully used for the determination of pyridoxine HCl in multivitamin pharmaceutical preparations.     

Response Surface Optimization of the Critical Medium Components for Pullulan Production by <Emphasis Type="Italic">Aureobasidium pullulans</Emphasis> FB-1

Culture conditions for pullulan production by Aureobasidium pullulans were optimized using response surface methodology at shake flask level without pH control. In the present investigation, a five-level with five-factor central composite rotatable design of experiments was employed to optimize the levels of five factors significantly affecting the pullulan production, biomass production, and sugar utilization in submerged cultivation. The selected factors included concentration of sucrose, ammonium sulphate, yeast extract, dipotassium hydrogen phosphate, and sodium chloride. Using this methodology, the optimal values for concentration of sucrose, ammonium sulphate, yeast extract, dipotassium hydrogen phosphate, and sodium chloride were 5.31%, 0.11%, 0.07%, 0.05%, and 0.15% (w/v), respectively. This optimized medium has projected a theoretically production of pullulan of 4.44%, biomass yield of 1.03%, and sugar utilization of 97.12%. The multiple correlation coefficient ‘R’ was 0.9976, 0.9761 and 0.9919 for pullulan production, biomass production, and sugar utilization, respectively. The value of R being very close to one justifies an excellent correlation between the predicted and the experimental data.     

Kinetics and mechanism of the acid-catalysed hydrolysis of the carbonatotetraimidazolecobalt(III) ion

Summary The kinetics of the acid-catalysed hydrolysis of the [(imidazole)₄Co(CO₃)]⁺ ion was found to follow the rate law -dln[complex]/dt = k ₁ K[H⁺](1 + K[H ⁺]) in the 25–45 °C range, [H⁺] 0.05–1.0 m range and I = 1.0m. The reaction sequence consists of a rapid protonation equilibrium followed by the one-end dissociation of the coordinated carbonato ligand (rate-determining step) and subsequent fast release of the monodentate carbonato ligand. The rate parameter values, k ₁ and ITK, at 25 °C are 6.48 × 10⁻³s⁻¹ and 0.31m ⁻¹, respectively, and activation parameters for k ₁ are ΔH ₁ ^≠ = 86.1 ± 1.2kJ mol⁻¹ and ΔS ₁ ^≠ = 2.1 ± 6.3 J mol⁻¹K⁻¹. The hydrolysis rate increases with increase in ionic strength. The different ways of dealing with the data fit are presented and discussed. The kinetic results are compared with those for the similar cobalt(III) complexes.     

Simultaneous RP-LC Determination of Losartan Potassium, Ramipril, and Hydrochlorothiazide in Pharmaceutical Preparations

A simple, rapid, and precise reversed-phase high-performance liquid chromatographic method has been developed for simultaneous determination of losartan potassium, ramipril, and hydrochlorothiazide. The three drugs were separated on a 150 mm × 4.6 mm i.d., 5 μm particle, Cosmosil C₁₈ column. The mobile phase was 0.025 m sodium perchlorate–acetonitrile, 62:38 (v/v), containing 0.1% heptanesulphonic acid, pH adjusted to 2.85 with orthophosphoric acid, at a flow rate of 1.0 mL min⁻¹. UV detection was performed at 215 nm. The method was validated for linearity, accuracy, precision, and limit of quantitation. Linearity, accuracy, and precision were acceptable in the ranges 35–65 μg mL⁻¹ for losartan, 1.75–3.25 μg mL⁻¹ for ramipril, and 8.75–16.25 μg mL⁻¹ for hydrochlorothiazide.     

Synthesis and biological activity of a cyclic hexapeptide from Dianthus superbus

Plant-originated cyclopolypeptide (XIII) was synthesized by coupling of dipeptide Boc-l-asn(bzh)-l-phe-OH and tetrapeptide gly-l-leu-l-ala-l-tyr-OMe followed by cyclization of a linear hexapeptide segment. Structure elucidation of XIII was done on basis of detailed spectral analysis including FTIR, ¹H NMR, ¹³C NMR, FAB MS and elemental analysis. From the results of pharmacological screening, it was concluded that XIII possesses high cytotoxic activity against DLA and EAC cell lines with CTC₅₀ values of 15.1 μM and 18.6 μM, and potent antimicrobial activity against pathogenic fungi C. albicans with MIC of 6 μg mL⁻¹. Moreover, XIII possesses moderate anthelmintic activity against earthworms M. konkanensis, P. corethruses, and Eudrilus sp. at 2 mg mL⁻¹ dose level.     

The electrochemical polymerization of <Emphasis Type="Italic">o</Emphasis>-phenylenediamine on <Emphasis Type="SmallCaps">l</Emphasis>-tyrosine functionalized glassy carbon electrode and its application

The polymerization of o-phenylenediamine (OPD) on l-tyrosine (Tyr) functionalized glassy carbon electrode (GCE) and its electro-catalytic oxidation towards ascorbic acid (AA) had been studied in this report. l-Tyrosine was first covalently grafted on GCE surface via electrochemical oxidation, which was followed by the electrochemical polymerization of OPD on the l-tyrosine functionalized GCE. Then, the poly(o-phenylenediamine)/l-tyrosine composite film modified GCE (POPD-Tyr/GCE) was obtained. X-ray photo-electron spectroscopy (XPS), field emission scanning electron microscope (SEM), and electrochemical techniques have been used to characterize the grafting of l-tyrosine and the polymerization and morphology of OPD film on GCE surface. Due to the doping of the carboxylic functionalities in l-tyrosine molecules, the POPD film showed good redox activity in neutral medium, and thus, the POPD-Tyr/GCE exhibited excellent electrocatalytic response to AA in 0.1 mol l⁻¹ phosphate buffer solution (PBS, pH 6.8). The anode peak potential of AA shifted from 0.58 V at GCE to 0.35 V at POPD-Tyr/GCE with a greatly enhanced current response. A linear calibration graph was obtained over the AA concentration range of 2.5 × 10⁻⁴–1.5 × 10^–3 mol l⁻¹ with a correlation coefficient of 0.9998. The detection limit (3δ) for AA was 9.2 × 10⁻⁵ mol l⁻¹. The modified electrode showed good stability and reproducibility and had been used for the determination of AA content in vitamin C tablet with satisfactory results.     

Solid state electrochemical of the erbium hexacyanoferrate-modified carbon ceramic electrode and its electrocatalytic oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-cysteine

A kind of erbium hexacyanoferrate (ErHCF)-modified carbon ceramic electrodes (CCEs) fabricated by mechanically attaching ErHCF samples to the surface of CCEs derived from sol–gel technique was proposed. The resulting modified electrodes exhibit well-defined redox responses with the formal potential of +0.215 V [vs saturated calomel electrode (SCE)] at a scan rate of 20 mV s⁻¹ in 0.5 M KCl (pH 7) solution. The voltammetric characteristics of the ErHCF-modified CCEs were investigated by voltammetry. Attractively, the ErHCF-modified CCEs presented good electrocatalytic activity with a marked decrease in the overvoltage about 400 mV for l-cysteine oxidation. The calibration plot for l-cysteine determination was linear at 5.0 × 10⁻⁶–1.3 × 10⁻⁴ M with a linear regression equation of I(A) = 0.558 + 0.148c (μM) (R ² = 0.9989, n = 20), and the detection limit was 2 × 10⁻⁶ M (S/N = 3). At last, the ErHCF-modified CCEs were used for amperometric detection of l-cysteine in real samples.     

Studies of the reaction of the hydroxyl radical with the oxalate ion in an acidic aqueous solution by pulse radiolysis

The reaction of the · OH radical with the oxalate ion in an acidic aqueous solution was studied by pulse radiolysis. The rate constant for the reaction of formation of the radical HOOC-COO·(λ_max = 250 nm, ɛ = 1800 L mol⁻¹ cm⁻¹) is (5.0±0.5)·107 L mol⁻¹ s⁻¹. In the reaction with the hydrogen ion (k = 1.1·10⁷ L mol⁻¹ s⁻¹), the radical HOOC-COO· is transformed into a nonidentified radical designated arbitrarily as H⁺(HOOC-COO)· (λ_max = 260 nm, ɛ = 4000 L mol⁻¹ cm⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1165–1167, June, 2008.