Mn(II)-sodium dodecyl sulphate complex mimic enzyme-catalyzed fluorescence quenching of Pyronine B by hydrogen peroxide

Mn(II)-sodium dodecyl sulphate complex (Mn(II)-SDS) is used to mimic the active group of peroxidase. The catalytic characteristic of this mimic enzyme catalyst in the oxidation reaction of fluorescence substrate, tetraethyldiaminoxanthyl chloride (Pyronine B (PB)), with hydrogen peroxide has been studied. The experimental results show that Mn(II)-SDS complex has similar catalytic activity that of peroxidase. The steady-state catalytic rate depends upon mimic enzyme and substrate concentrations, and the Michaelis-Menten parameters K_m, V_max and K_cat are 7.6×10⁻⁶ M, 7.9×10⁻⁷ M s⁻¹ and 7.9 s⁻¹, respectively. The catalytic activity of Mn(II)-SDS complex is compared with those of HRP and Hemin. Though the catalytic activity of Mn(II)-SDS complex is 15.9% of that of HRP, it can catalyze the oxidation reaction of PB with hydrogen peroxide lead to fluorescence quenching of PB. Under optimum conditions, linear relationship between fluorescence quenching F₀/F and concentration of H₂O₂ is in the range of (0.0-3.6) × 10⁻⁷ M. The detection limit is determined to be 3.0×10⁻⁹ M. By coupling this mimic catalytic reaction with the catalytic reaction of glucose oxidase (GOD), glucose can be detected. Linear relationship between F₀/F and concentration of glucose is in the range of (0.0-1.4) × 10⁻⁷ M. The detection limit is determined to be 4.2×10⁻⁹ M. This method is applied to the determination of glucose in human serum and the results are in good agreement with the phenol-4-aminoantipyrine (4-AAP).     

Kinetic determination of cysteine and thiosulphate by inhibition of Hg(II) catalyzed ligand substitution reaction

The sulphur containing inhibitors (I), cysteine (Cys) and sodium thiosulphate (THS), have been found to inhibit Hg(II) catalyzed exchange of cyanide in hexacyanoferrate(II) by nitroso-R-salt (NRS). The inhibitory effect of both the ligands are attributed to their binding tendencies with Hg(II) leading to the formation of catalyst-inhibitor (C-I) complex. The reactions have been followed spectrophotometrically in aqueous medium at 720 nm by noting the increase in absorbance of the green colour product, [Fe(CN)₅NRS]³⁻ at pH 6.50 ± 0.02, temp 25.0 ± 0.1 °C and ionic strength (μ) 0.1 M (KNO₃). A most plausible mechanistic scheme involving the role of analytes (inhibitors) has been proposed. The values of equilibrium constants for complex formation between catalyst-inhibitor (K_CI), catalyst-substrate (K_S) and Mechaelis-Menton constant (K_m) have been computed from the kinetic data. The linear calibration curves have been established between absorbance and inhibitor concentrations under specified conditions. Cys and THS have been determined in the range 1-5 × 10^− 7 M and 4.9-16.9 × 10^− 7 M respectively. The detection limits have been computed to be 1 × 10^− 7 M and 4.9 × 10^− 7 M for Cys and THS, respectively.     

Combined cation-exchange and extraction chromatographic method of pre-concentration and concomitant separation of Cu(II) with high molecular mass liquid cation exchanger after its online detection

A selective method has been developed for the extraction chromatographic trace level separation of Cu(II) with Versatic 10 (liquid cation exchanger) coated on silanised silica gel (SSG-V10). Cu(II) has been extracted from 0.1 M acetate buffer at the range of pH 4.0–5.5. The effects of foreign ions, pH, flow-rate, stripping agents on extraction and elution have been investigated. Exchange capacity of the prepared exchanger at different temperatures with respect to Cu(II) has been determined. The extraction equilibrium constant (K_ex) and different standard thermodynamic parameters have also been calculated by temperature variation method. Positive value of ΔH (7.98 kJ mol⁻¹) and ΔS (0.1916 kJ mol⁻¹) and negative value of ΔG (−49.16 kJ mol⁻¹) indicated that the process was endothermic, entropy gaining and spontaneous. Preconcentration factor was optimized at 74.7 ± 0.2 and the desorption constants K_desorption¹(1.4 × 10⁻²) and K_desorption²(9.8 × 10⁻²) were determined. The effect of pH on R_f values in ion exchange paper chromatography has been investigated. In order to investigate the sorption isotherm, two equilibrium models, the Freundlich and Langmuir isotherms, were analyzed. Cu(II) has been separated from synthetic binary and multi-component mixtures containing various metal ions associated with it in ores and alloy samples. The method effectively permits sequential separation of Cu(II) from synthetic quaternary mixture containing its congeners Bi(III), Sn(II), Hg(II) and Cu(II), Cd(II), Pb(II) of same analytical group. The method was found effective for the selective detection, removal and recovery of Cu(II) from industrial waste and standard alloy samples following its preconcentration on the column. A plausible mechanism for the extraction of Cu(II) has been suggested.     

Adsorption of a protein-porphyrin complex at a liquid-liquid interface studied by total internal reflection synchronous fluorescence spectroscopy

Interfacial analysis has attracted more and more attention owing to its fundamental and biological importance. Total internal reflection fluorescence (TIRF) spectroscopy is a useful method to study interfacial properties. The synchronous scanning fluorescence technique provides a selective tool to analyze a specific component in a complex system. The interaction and adsorption of bovine serum albumin (BSA) and meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) at toluene-water interface were studied successfully by the coupling technique of total internal reflection synchronous fluorescence (TIRSF). New methods are provided for the determination of the critical micelle concentration (cmc), apparent adsorption equilibrium constant (K_ad) and maximum amount of adsorption (f_max) at the liquid-liquid interface. The results indicated that BSA could adsorb onto the toluene-water interface as a complex of BSA-TPPS in a ratio of 1:1 ratio based on Langmuir adsorption isothermal model. The cmc, apparent K_ad and f_max for BSA at pH 3.1 were determined to be 1.0 × 10⁻⁴ mol L⁻¹, 1.15 × 10⁵ L mol⁻¹ and 1.14 × 10⁻⁹ mol cm⁻², respectively.     

Tyrosinase immobilized magnetic nanobeads for the amperometric assay of enzyme inhibitors: application to the skin whitening agents

The immobilization of tyrosinase onto glutaraldehyde activated streptavidine magnetic particles and subsequent retention onto a magnetized carbon paste electrode for the amperometric assay of tyrosinase inhibitors is described. Tyrosine was used as substrate as it is the first substrate in the melanogenesis process. The sensing mode is based on monitoring the decrease of the amperometric signal corresponding to the electrochemical reduction of dopaquinone enzymatically generated. This current decrease is due to the presence of inhibitors acting directly on the enzyme or inhibitors acting on the product of the enzymatic reaction, i.e. dopaquinone. The methodology is designed for the evaluation of the inhibitory potency of the most frequently used active substances in cosmetic marketed products against hyperpigmentation such as kojic acid, azelaic acid and benzoic acid. These compounds bind to the tyrosinase active center. Ascorbic acid is also investigated as it interrupts the synthesis pathway of melanin by reducing the melanin intermediate dopaquinone back to l-dopa. By comparing the obtained IC₅₀, under the same experimental conditions, the order of their inhibitory potency was: kojic acid (IC₅₀ = 3.7 × 10⁻⁶ M, K_i = 8.6 × 10⁻⁷ M), ascorbic acid (IC₅₀ = 1.2 × 10⁻⁵ M), benzoic acid (IC₅₀ = 7.2 × 10⁻⁵ M, K_i = 2.0 × 10⁻⁵ M) and azelaic acid (IC₅₀ = 1.3 × 10⁻⁴ M, K_i = 4.2 × 10⁻⁵ M) in close agreement with literature spectrophotometric inhibition data using the soluble tyrosinase.     

Cucurbit[7]uril stabilization of a diarylmethane carbocation in aqueous solution

The stability of the 4,4′-bis(dimethylamino)diphenylmethane carbocation is significantly enhanced in aqueous solution by its inclusion in the cucurbit[7]uril host cavity. The formation of the host-guest complex (K_CB[7] = 2.0 × 10⁴ M⁻¹) shifts the carbinol-carbocation equilibrium, maximizing the formation of the intensely blue carbocation to 90% at pH 5.2.     

Quantitative study of stereospecific binding of monoclonal antibody to anti-benzo(a)pyrene diol epoxide-N-dG adducts by capillary electrophoresis immunoassay

The stereospecific binding of monoclonal antibody (mAb) 8E11 to anti-benzo(a)pyrene diol epoxide (BPDE)-dG adducts in single nucleoside, long oligonucleotide, and genomic DNA were quantitatively evaluated using noncompetitive and competitive capillary electrophoresis (CE) immunoassays. Two single-stranded TMR-BPDE-90mers containing a single anti-BPDE-dG adduct with defined stereochemistry and a fluorescent label at 5′-end were used as fluorescent probes for competitive CE immunoassay. To quantitatively evaluate the binding affinity through competitive CE immunoassays, a series of equations were derived according to the binding stoichiometry. The binding of mAb 8E11 to trans-(+)-anti-BPDE-dG displays strongest affinity (K_b: 3.57 × 10⁸ M⁻¹) among all four investigated anti-BPDE-dG mononucleoside adducts, and the cis-(−)-anti-BPDE-dG displays lowest affinity (K_b: 1.14 ×10⁷ M⁻¹). The binding of monoclonal antibody (mAb) 8E11 to BPDE-dG adducts in long DNA (90mer) preferentially forms the complex with a stoichiometry of 1:1, and that mAb 8E11 displays a slightly higher affinity with trans-(+)-anti-BPDE-90mers (K_b: 6.36 ± 0.54 × 10⁸ M⁻¹) than trans-(−)-anti-BPDE-90mers (K_b: 4.52 ± 0.52 × 10⁸ M⁻¹). The mAb 8E11 also displays high affinity with BPDE-dG adducts in genomic DNA (K_b: 3.74 × 10⁸ M⁻¹), indicating its promising applications for sensitive immuno-detection of BPDE-DNA adducts in genomic DNA.     

Determination of melamine in different milk batches using a novel chemosensor based on the luminescence quenching of Ru(II) carbonyl complex

A novel, simple, sensitive and precise spectrofluorimetric method was developed for measuring the melamine concentration in different milk batch samples. The method was based upon measuring the quenching of the luminescence intensity of the produced yellow colored ruthenium^(II) carbonyl complex of the general formula [Ru(CO)₂(L)] (where L = anion of tetradentate Schiff base). The Ru^(II) complex exhibited characteristic luminescence band in the visible region. The remarkable quenching of the luminescence intensity of [Ru(CO)₂(L)] complex by various concentrations of melamine was successfully used as a chemosensor for the assessment of melamine in different milk samples at λ_ex = 400 nm and pH 7.4 in DMSO with a linear dynamic range 1.0 × 10⁻⁶ to 3.0 × 10⁻⁹ mol L⁻¹ and lower detection limit (LOD) and quantification detection limit (QOD) of 3.3 × 10⁻¹⁰ and 1.0 × 10⁻⁹ mol L⁻¹, respectively.     

Zinc(II) complexes of 1,3-bis(2-pyridylmethylthio)propane: Anion dependency, crystal structure and DNA binding study

Zinc(II) complexes of the formula [Zn(L)(X)₂] (where X = Cl⁻, N₃⁻, NCO⁻ and SCN⁻ (1a-d, respectively)) and {[Zn(L)(ClO₄)(H₂O)](ClO₄)}_n (2), were isolated in the pure form on the reaction of 1,3-bis(2-pyridylmethylthio)propane (L) with different zinc(II) salts. All the complexes were characterized by physicochemical and spectroscopic tools. The X-ray crystallographic analyses of the complexes 1d and 2 showed that the former is mononuclear while complex 2 is a 1D coordination polymer, {[Zn(L)(ClO₄)(H₂O)](ClO₄)}_n, due to a different coordination mode of the tetradentate ligand L. The zinc(II) ions present an octahedral coordination geometry in both compounds, which is more distorted in the mononuclear complex 1d. The study indicates that the counter anion of the zinc(II) salt used as reactant leads to a different type of complex when isolated as a crystalline material. A spectroscopic study of the interaction of complex, 2 with calf thymus-DNA (CT-DNA) in Tris-HCl buffer showed a significant non-intercalative interaction with a binding constant (K_b) of 4.7 × 10⁴ M⁻¹, and the linear Stern-Volmer quenching constant (K_sv) and the binding sites (n) were found to be 1.3 × 10³ and 0.92 respectively, calculated from ethidium bromide (EB) fluorescence displacement experiments.     

A method for rapid screening of interactions of pharmacologically active compounds with albumin

We determine the association constants for ligand–protein complex formation using the flow injection method. We carry out the measurements at high flow rates (F = 1 mL min⁻¹) of a carrier phase. Therefore, determination of the association constant takes only a few minutes. Injection of 1 nM of the ligand (10 μL of 1 μM concentration of the ligand solution) is sufficient for a single measurement. This method is tested and verified for a number of complexes of selected drugs (cefaclor, etodolac, sulindac) with albumin (BSA). We obtain K = 4.45 × 10³ M⁻¹ for cefaclor, K = 1.00 × 10⁵ M⁻¹ for etodolac and K = 1.03 × 10⁵ M⁻¹ for sulindac in agreement with the literature data. We also determine the association constants of 20 newly synthesized 3β- and 3α-aminotropane derivatives with potential antipsychotic activity – ligands of 5-HT_1A, 5-HT_2A and D₂ receptors with the albumin. Results of the studies reported here indicate that potential antipsychotic drugs bind weakly to the transporter protein (BSA) with K ≈ 10²–10³ M⁻¹. Our method allows measuring K in a wide range of values (10²–10⁹ M⁻¹). This range depends only on the solubility of the ligand and sensitivity of the detector.     

Mechanisms involved in chemical vapor generation by aqueous tetrahydroborate(III) derivatization : Role of hexacyanoferrate(III) in plumbane generation

The role played by K₃Fe(CN)₆ (0.08 or 1.5 g l^− 1) in producing strong enhancement factors in the generation efficiency of plumbane in the reaction of NaBH₄ (10 or 40 g l^− 1) with Pb(II) (50 μg l^− 1) in 0.1 M HCl solution, was investigated by using continuous flow chemical vapor generation coupled with atomic fluorescence spectrometry (CF-CVG-AFS). Different mixing sequences and reaction times of reagents were tested using different chemifold setups. Part of CF-CVG-AFS experiments were performed using the on-line, delayed addition of Pb(II) to a K₃Fe(CN)₆ + NaBH₄ reaction mixture. Kinetic calculations estimating the concentration of K₃Fe(CN)₆ remaining in the K₃Fe(CN)₆ + NaBH₄ reaction mixture before it merged with Pb(II) solution were also performed. Batch experiments measuring the amount of hydrogen evolved (pressure of H₂ vs time) and pH variation during K₃Fe(CN)₆ + NaBH₄ + HCl reaction were performed in order to have a correct estimation of the concentration of K₃Fe(CN)₆ remaining in the reaction system. The comparison of CF-CVG-AFS experiments with kinetic calculations indicates that strong enhancement factors of plumbane generation can be obtained without any interaction of K₃Fe(CN)₆ with Pb(II). The key role of K₃Fe(CN)₆ is recognized in its reaction with NaBH₄ to give “special” borane complex intermediates, which are highly effective in the generation of plumbane from Pb(II).     

Trace determination of thiosulphate and thioglycolic acid using novel inhibitory kinetic spectrophotometric method

Sulphur containing compounds such as sodium thiosulphate (STS) and thioglycolic acid (TGA) inhibit the rate of cyanide substitution by nitroso-R-salt (NRS) in hexacyanoruthenate(II) catalysed by Hg(II) ions due to their strong binding tendencies with Hg(II) catalyst. This inhibitory effect of sodium thiosulphate and thioglycolic acid is used as the basis for their determination at micro levels. The reaction was followed spectrophotometrically at 525 nm (λ_max of [Ru(CN)₅NRS]³⁻ complex) under optimised reaction conditions at 8.75 × 10^− 5 M [Ru(CN)₆⁴⁻], 3.50 × 10^− 4 M [NRS], pH 7.00 ± 0.02, ionic strength (µ) 0.1 M (KCl) and temp 45.0 ±0.1 °C. The modified mechanistic scheme is proposed to understand the inhibition caused by sulphur containing compounds (STS and TGA) on Hg(II) catalysed substitution of cyanide by NRS in [Ru(CN)₆]⁴⁻. The range of analytical concentration of inhibitor depends upon two factors; the amount of Hg(II) catalyst present in the indicator reaction and the stability of the Hg(II)-inhibitor complex under consideration. Under optimum conditions STS and TGA have been determined in the range of 0.98-7.0 × 10^− 6 M and 0.30-7.0 × 10^− 6 M. The detection limits for STS and TGA were found to be 3.0 × 10^− 7 M and 1.0 × 10^− 7 M respectively.     

Spectrophotometric determination of titanium with o-carboxyphenylfluorone in cationic micellar media, and its equilibrium and kinetic studies

Spectrophotometric determination of titanium(IV) was accomplished with o-carboxyphenylfluorone (OCPF) in the presence of hexadecyltrimethyl ammonium chloride (HTAC) under strongly acidic media. In the determination of titanium(IV), Beer's law was obeyed in the range of 24-340 ng mL⁻¹ with an effective molar absorption coefficient (at 530 nm) and relative standard deviation of 2.24 × 10⁵ dm³ mol⁻¹ cm⁻¹ and 0.64% (n = 8), respectively. The severe interference of iron ions was easily eliminated by the addition of ethylenediaminetetraacetic acid (EDTA); the effects of other foreign substances were low. Equilibrium and kinetic studies under analytical conditions were investigated to quantitatively evaluate the reaction mechanism. The obtained orange complex is considered to be Ti(OCPF)₄. Its stability log K_f and rate constant K_obs are 16.88 and 1.65 × 10⁻² s⁻¹, respectively. It is suggested that the color of the complex is related to the species of OCPF in the solution.     

A new Cu(II)-5-(4-sulphophenylazo)-8-aminoquinoline complex used for copper determination in presence of gold and silver in water and mineral samples

In this work, a characterization of reagent chromophere 5-(4-sulphophenylazo)-8-aminoquinoline [SPA] by IR and ¹H RMN was carried out and a pK_a value of 3.55 ± 0.03 was found as well. An 1:2 stoichiometry for the Cu(II)-SPA complex was determined at pH 9 by Job and molar ratio methods. A value of 1.4 × 10¹⁴ for the stability constant was also found. Based on the formation of this complex a new method for the copper determination in presence of gold and silver was developed by derivative spectrophotometry using a previous preconcentration on solid phase. In this method, the analytical measures were executed directly in the solid phase containing the complex. The Cu(II) reacts with the reagent chromophere SPA previously retained in the anionic exchange DEAE Sephadex A25. In this determination, the first derivative at 605 nm was used. The quantification range was between (3.2 ± 0.3 × 10⁻¹) × 10⁻⁸ and (94.4 ± 0.9) × 10⁻⁸ mol L⁻¹ (3.2 ± 0.3 × 10⁻¹) × 10⁻⁸ , and (94.4 ± 0.9) × 10⁻⁸ mol L⁻¹. The repeatability expressed as RSD was between 1.1 and 2.0%. The method was applied successfully for the copper determination in mineral residuals and natural water samples. The results were consistent with those provided by ICP-mass spectrometry.     

New solid phase extractors for selective separation and preconcentration of mercury (II) based on silica gel immobilized aliphatic amines 2-thiophenecarboxaldehyde Schiff’s bases

2-Thiophenecarboxaldhyde is chemically bonded to silica gel surface immobilized monoamine, ethylenediamine and diethylenetriamine by a simple Schiff’s base reaction to produce three new SP-extractors, phases (I-III). The selectivity properties of these phases toward Hg(II) uptake as well as eight other metal ions: Ca(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Pb(II) were extensively studied and evaluated as a function of pH of metal ion solution and equilibrium shaking time by the batch equilibrium technique. The data obtained clearly indicate that the new SP-extractors have the highest affinity for retention of Hg(II) ion. Their Hg(II) uptake in mmol g⁻¹ and distribution coefficient as log K_d values are always higher than the uptake of any other metal ion along the range of pH used (pH 1.0-10.0). The uptake of Hg(II) using phase I was 2.0 mmol g⁻¹ (log K_d 6.6) at pH 1.0 and 2.0. 1.8 mmol g⁻¹ (log K_d 4.25), 1.6 mmol g⁻¹ (log K_d 3.90) and 1.08 mmol g⁻¹ (log K_d 3.37) at pH 3.0, 5.0 and 8.0, respectively. Selective separation of Hg(II) from the other eight coexisting metal ions under investigation was achieved successfully using phase I at pH 2.0 either under static or dynamic conditions. Hg(II) was completely retained while Ca(II), Co(II) and Cd(II) ions were not retained. Ni(II), Cu(II), Zn(II), Pb(II) and Fe(III) showed very low percentage retention values to be 0.74, 0.97, 3.5 and 6.3%, respectively. Moreover, the high recovery values (95.5 ± 0.5, 95.8 ± 0.5 and 99.0% ± 1.0) of percolating two liters of doubly distilled water, drinking tap water and Nile river water spiked with 5 ng/l of Hg(II) over 100 mg of phase I packed in a minicolumn and used as a thin layer enrichment bed demonstrate the accuracy and validity of the new SP-extractors for preconcentration of the ultratrace amount of spiked Hg(II) prior to the determination by borohydride generation atomic absorption spectrometry (AAS) with no matrix interference. The detection limit (3σ) for Hg(II) based on enrichment factor 1000 was 4.75 pg/ml. The precision (R.S.D.) obtained for different amounts of mercury was in the range 0.52-1.01% (N = 3) at the 25-100 ng/l level.     

Electrochemical,magnetic, catalytic and DNA cleavage studies of binuclear copper(II) complexes derived from pendant substituted tetraaza macrobicyclic compartmental ligands

A series of macrobicyclic unsymmetrical binuclear copper(II) complexes of compartmental ligands were synthesized from the Schiff base condensation of 1,8[N,N′-bis{(3-formyl-2-hydroxy-5-methyl)benzyl}]-1,4,8,11- tetraaza-5,5,7,12,12,14-hexa methylcyclotetradecane with diamines like 1,2-diamino ethane, 1,3-diamino propane, 1,4-diaminobutane, 1,2-diaminobenzene and 1,8-diaminonaphthalene. The complexes were characterized by elemental and spectral analysis. Electrochemical studies of the copper(II) complexes show two irreversible one-electron reduction processes around E¹_pc = −0.70 to −1.10 V and E²_pc = −0.98 to −1.36 V. ESR spectra of the binuclear copper(II) complexes show a broad signal at g = 2.10 and μ_eff values in the range 1.46–1.59 BM, which convey the presence of antiferromagnetic coupling. Cryomagnetic investigation of the binuclear complexes [Cu₂L³(ClO₄)](ClO₄) and [Cu₂L⁴(ClO₄)](ClO₄) show that the observed −2J values are 144 and 216 cm⁻¹, respectively. The observed initial rate (V_in) for the catalytic hydrolysis of p-nitrophenyl phosphate by the binuclear copper(II) complexes were in the range 1.8 × 10⁻⁵ to 2.1 × 10⁻⁵ Ms⁻¹. The initial rate (V_in) for the catalytic oxidation of catechol to o-quinone by the binuclear copper(II) complexes were in the range 2.7 × 10⁻⁵ to 3.5 × 10⁻⁵ Ms⁻¹. The copper(II) complexes have been found to promote cleavage of plasmid pBR 322 DNA from the supercoiled form I to the open circular form II.     

Kinetic study of the oxidative addition reaction between methyl iodide and [Rh(FcCOCHCOCF3)(CO)(PPh3)]: Structure of [Rh(FcCOCHCOCF3)(CO)(PPh3)(CH3)(I)]

The kinetics of oxidative addition of CH₃I to [Rh(FcCOCHCOCF₃)(CO)(PPh₃)], where Fc = ferrocenyl and (FcCOCHCOCF₃)⁻ = fctfa = ferrocenoylacetonato, have been studied utilizing UV/Vis, IR, ¹H and ³¹P NMR techniques. Three definite sets of reactions involving isomers of at least two distinctly different classes of Rh^III-alkyl and two different classes of Rh^III-acyl species were observed. Rate constants for this reaction in CHCl₃ at 25 °C, applicable to the reaction sequence below, were determined as k₁ = 0.00611(1) dm³ mol⁻¹ s⁻¹, k₋₁ = 0.0005(1) s⁻¹, k₃ = 0.00017(2) s⁻¹ and k₄ = 0.0000044(1) s⁻¹ while k₋₃ ? k₃ and k₋₄ ? k₄ but both ≠0. The indeterminable equilibrium K₂ was fast enough to be maintained during Rh^I depletion in the first set of reactions and during the Rh^IIIalkyl2 formation in the second set of reactions. From a ¹H and ³¹P NMR study in CDCl₃, K_c1 was found to be 0.68, K_c2 = 2.57, K_c3 = 1.00, K_c4 = 4.56 and K_c5 = 1.65.     

Determination of levodopa by capillary electrophoresis with chemiluminescence detection

A rapid and simple method using capillary electrophoresis (CE) with chemiluminescence (CL) detection was developed for the determination of levodopa. This method was based on enhance effect of levodopa on the CL reaction between luminol and potassium hexacyanoferrate(III) (K₃[Fe(CN)₆]) in alkaline aqueous solution. CL detection employed a lab-built reaction flow cell and a photon counter. The optimized conditions for the CL detection were 1.0 × 10⁻⁵ M luminol added to the CE running buffer and 5.0 × 10⁻⁵ M K₃[Fe(CN)₆] in 0.6 M NaOH solution introduced postcolumn. Under the optimal conditions, a linear range from 5.0 × 10⁻⁸ to 2.5 × 10⁻⁶ M (r = 9991), and a detection limit of 2.0 × 10⁻⁸ M (signal/noise = 3) for levodopa were achieved. The precision (R.S.D.) on peak area (at 5.0 × 10⁻⁷ M of levodopa, n = 11) was 4.1%. The applicability of the method for the analysis of pharmaceutical and human plasma samples was examined.     

Isothermal crystallization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Isothermal crystallization behavior of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was investigated by means of differential scanning calorimetry and polarized optical microscopy (POM). The Avrami analysis can be used successfully to describe the isothermal crystallization kinetics of PHBV, which indicates that the Avrami exponent n=3 is good for all the temperatures investigated. The spherulitic growth rate, G, was determined by POM. The result shows that the G has a maximum value at about 353 K. Using the equilibrium melting temperature (448 K) determined by the Flory equation for melting point depression together with U∗=1500 cal mol⁻¹, T_∞=30 K and T_g=278 K, the nucleation parameter K_g was determined, which was found to be 3.14 ± 0.07 × 10⁵ (K²), lower than that for pure PHB. The surface-free energy σ=2.55×10⁻² J m⁻² and σ_e=2.70±0.06×10⁻² J m⁻² were estimated and the work of chain-folding (q=12.5±0.2 kJ mol⁻¹) was derived from σ_e, and found to be lower than that for PHB. This implies that the chains of PHBV are more flexible than that of PHB.     

Q-mode curve resolution of UV-vis spectra for structural transformation studies of anthocyanins in acidic solutions

Chemometric analysis of ultraviolet-visible (UV-vis) spectra for pH values 1.0, 3.3, 5.3, and 6.9 was used to investigate the kinetics and the structural transformations of anthocyanins in extracts of calyces of hibiscus flowers of the Hibiscus acetosella Welw. ex Finicius for the first time. Six different species were detected: the quinoidal base (A), the flavylium cation (AH⁺), the pseudobase or carbinol pseudobase (B), cis-chalcone (C_C), trans-chalcone (C_t), and ionized cis-chalcone (⁻CC). Four equilibrium constant values were calculated using relative concentrations, hydration, pK_h = 2.60 ± 0.01, tautomeric, K_T = 0.14 ± 0.01, acid-base, pK_a = 4.24 ± 0.04, and ionization of the cis-chalcone, pKCC=8.74±1.5×1⁰−2. The calculated protonation rate of the tautomers is KH+=0.08±7.6×10⁻³. These constants are in excellent agreement with those measured previously in salt form. From a kinetic viewpoint, the situation encountered is interesting since the reported investigation is limited to visible light absorption in acid medium. These models have not been reported in the literature.