Flow injection chemiluminescent method for the successive determination of l-cysteine and l-cystine using photogenerated tris(2,2'-bipyridyl) ruthenium (III)

Flow injection configurations were developed for the individual determination of l-cysteine and l-cystine and for the mixture of both analytes. The method is based on the reaction of l-cysteine with tris(2,2'-bipyridyl) ruthenium (II) and peroxydisulphate under UV irradiation to produce chemiluminescence. Cystine is determined after reduction to cysteine in a copper-coated cadmium reductor mini-column in the flow system. The inclusion of a selection valve in the configuration allows the successive determination of the two analytes. Calibration was linear between 2x10(-6) and 5x10(-4) moll(-1) for cysteine and between 1x10(-6) and 2x10(-4) moll(-1) for cystine. When applied to pharmaceutical formulations, the procedure was free from interferences from common excipients. The results obtained for the assay of commercial formulations compared well with those obtained by an official method and demonstrated good accuracy and precision.     

Studies towards understanding the mechanism of organic reagent enhancement in flame and plasma atomic absorption spectrometry

A series of organic reagents have been tested in atomic absorption measurement for signal enhancement of metal elements. Organic reagents like tetrabutylammonium bromide are demonstrated to enhance the absorption sensitivity to some specific elements such as calcium and chromium. A group of amines were found to have significant enhancement for chromium and calcium measurements. The function of organic reagents in flame and plasma atomic absorption spectrometry (AAS) was investigated in this work with emphases on mechanism of signal enhancement and interference suppression. An alternative mechanism of organic reagent enhancement in flame and plasma AAS has been suggested based on the experimental results obtained in this work. The reduction environments in flame and plasma produced by the organic reagents are considered as major reason for the signal enhancement.     

Interference with ammonium determination by the indophenol-type reaction of salicylate and dichloroisocyanurate

Two widely accepted indophenol-type reactions with ammonium, which use phenol/hypochlorite and salicylate/dichloroisocyanurate as reagents, were compared for possible interferences with other naturally occurring nitrogen-containing compounds. Unlike the first method, the second showed strong interference from all amino acids and peptides tested. As much as 57 mole % of threonine and 49 mole % of serine were degraded to ammonium. Pyrimidines, purines, nucleosides and urea were negative in both reactions. The wide distribution and importance of free and combined amino acids in naturally occurring waters favours the application of the classical phenol/ hypochlorite reaction with ammonium rather than the modified salicylate/dichloroisocyanurate reaction.     

Interference with ammonium determination by the indophenol-type reaction of salicylate and dichloroisocyanurate

Two widely accepted indophenol-type reactions with ammonium, which use phenol/hypochlorite and salicylate/dichloroisocyanurate as reagents, were compared for possible interferences with other naturally occurring nitrogen-containing compounds. Unlike the first method, the second showed strong interference from all amino acids and peptides tested. As much as 57 mole % of threonine and 49 mole % of serine were degraded to ammonium. Pyrimidines, purines, nucleosides and urea were negative in both reactions. The wide distribution and importance of free and combined amino acids in naturally occurring waters favours the application of the classical phenol/ hypochlorite reaction with ammonium rather than the modified salicylate/dichloroisocyanurate reaction. Received: 20 April 1998 / Revised: 28 May 1998 / Accepted: 5 Juni 1998     

Functionalized cadmium sulfide quantum dots as fluorescence probe for silver ion determination

CdS quantum dots (QDs) modified with l-cysteine has been prepared by one step. They are water-soluble and biocompatible. To improve CdS QDs stability and interaction between silver ion and functionalized CdS QDs in aqueous solution, some amounts of fresh l-cysteine were added to functionalized CdS solution. Based on the characteristic fluorescence enhancement of CdS QDs at 545 nm by silver ions in the presence of some amounts of fresh l-cysteine, simultaneously, a gradual red shift of fluorescence emission bands of CdS QDs from 545 to 558 nm was observed. A simple, rapid, sensitive and specific detection method for silver ion was proposed. Under optimum conditions, the fluorescence intensity of CdS QDs is linearly proportional to silver concentration from 2.0 × 10⁻⁸ to 1.0 × 10⁻⁶ mol/L with a detection limit of 5.0 × 10⁻⁹ mol/L. In comparison with single organic fluorophores, functionalized CdS quantum dots are brighter, more stable against photobleaching, and don’t suffer from blinking. Furthermore, owing to the fluorescence enhancement effect of CdS QDs by silver ion, the proposed method showed lower detection blank and higher sensitivity. Possible fluorescence enhancement mechanism was also studied.     

一种新型钛调整体系: 钛羧酸盐和锂试剂的还原反应

本文首次将一系列α-芳香族酮酸通过与钛酸醇酯[Ti(OR^1)~4]的交换反应制备成钛羧酸盐, 随后将它与锂试剂反应。在此体系中锂试剂发生了一个未见报道的新反应: 将α-芳香族酮酸还原为相应的α-羟基羧酸。进一步研究表明: 锂试剂同时可以发生加成反应, 且因Ti(OR^1)~4, 酮酸和锂试剂的不同, 加成与还原的比例也随之不同。     

l-Cysteine-induced photoluminescence enhancement of CdSe/ZnSe quantum dots in aqueous solution

l-Cysteine molecules dramatically enhance the photoluminescence of colloidal CdSe/ZnSe quantum dots (i.e., ^CTAB/TOPOQD). Based on our spectroscopic studies of temporal variations in QD quantum yields as well as the in situ infrared spectral features of QDs, we propose that adsorption and rearrangement of l-cysteine molecules at the QD–water interface induces the observed unusual enhancement of the photoluminescence quantum yield. Upon addition of l-cysteine to the ^CTAB/TOPOQD solution, the adsorption of l-cysteine to the ^CTAB/TOPOQD colloidal particles is driven by the formation of a kinetically favorable intermediate species, which is formed by the coordination of thiol groups to the QD surface Cd atoms. The above species then reacts further to form a thermodynamically stable QD species, which probably involves coordination of both the amine and thiol groups of l-cysteine on the QD surface. Additional comparison studies using ^MPAQD and other small ligands (i.e., l-alanine, l-serine, and MPA) confirmed our proposed mechanism of l-cysteine adsorption at the ^CTAB/TOPOQD–water interfaces. In addition to these adsorption structures, we also propose that the dramatic enhancement of QY observed in this study is probably induced by the rearrangement and structural organization of l-cysteine and CTAB molecules at the QD–water interface, which improves the homogeneity and self-organization of the interfacial molecules.     

Mechanisms involved in stannane generation by aqueous tetrahydroborate(III): Role of acidity and l-cysteine

The role played by acidity (0.01–5 mol L^− 1 HNO₃) and l-cysteine (0.1–0.2 mol L^− 1) in the formation of stannane by reaction of Sn(IV) solution with aqueous tetrahydroborate(III) (0.05–0.2 mol L^− 1), has been investigated by continuous flow hydride generation coupled with atomic absorption spectrometry using a miniature argon–hydrogen diffusion flame as the atomizer. Different mixing sequences and reaction times of the reagents were useful in the identification of those processes which contribute to the generation of stannane in different reaction conditions, both in the absence and in the presence of l-cysteine. The lack of stannane generation at high acidities is due to the formation of Sn substrates and hydridoboron species which are unreactive. The capture of the stannane in solution, following its ionization to SnH₃⁺ from already formed stannane, does not play any role. While the presence of l-cysteine, does not affect the generation efficiency at lower acidities, it expands the optimum range of acidities for stannane generation to higher values. This effect can be addressed to both the buffering capacity of l-cysteine and to the formation of Sn-(l-cysteine) complexes, while the formation of (l-cysteine)–borane complexes do not play a significant role. Formation of Sn-(l-cysteine) complexes also appears to be useful for stabilization of tin solution at low acidities values.     

Coulometric Karl Fischer titration of trace water in diaphragm-free cells

Factors influencing the accuracy and precision for diaphragm-free Karl Fischer coulometric determinations of low μg-amounts of water have been studied using the Metrohm 756 (pulsed current) coulometer and eight different types of commercial coulometric reagents and some modifications of these. As in the case of diaphragm-free coulometric titration of large amounts of water, the positive errors, due to the formation of oxidizable reduction products (of sulfur dioxide) in the cathode reaction (besides hydrogen), were found to be minimized by the use of highest possible pulse current (in the range 100–400 mA) in combination with the fastest possible titration rate. Most accurate (102–103%) and precise results (typical relative standard deviation 1.8%) were obtained for reagents containing very large concentrations of imidazole in combination with the presence of modifiers like hexanol, chloroform and propylene glycol (i.e. the HYDRA-POINT reagents). Similar results were obtained when this type of reagent was mixed 60/40 with xylene according to the ASTM recommendation for water determinations in petroleum products like crude and lubricating oils. Addition of decanol to this type of reagent mixture was found to reduce the influence from the oxidative reduction products significantly. A reduction of the error from 3.6% relative to 1.6% was achieved by addition of 9% (v/v) of decanol to a 60/40 reagent mixture of HYDRA-POINT Coulometric Gen (containing hexanol as modifier) and xylene. For larger concentrations of decanol the pulse current had to be lowered to 100 mA and this might explain why no further improvement was observed. An additional attempt to minimize the interference by lowering the concentration of sulfur dioxide in the reagents gave no significant effect. However, by means of a home-built computer-controlled coulometric instrumentation based on continuous instead of pulsed current (including a large cathodic current density) it was possible to achieve recovery rates close to 100% for the best reagents investigated. The reason for this improvement is discussed.     

Wasserbestimmung in Phenolen durch Karl-Fischer-Titration

Summary Karl Fischer titration can be applied to many phenols without any interference, though some are oxidized during titration. One-component reagents are to be preferred because they are less sensitive to interferences. Only pyrogallol and aminophenols are oxidized in this case. With two-component reagents dihydroxybenzenes and naphthols are oxidized, too. With coulometric titration interferences are more frequent. The oxidation rate depends on the pH and can be suppressed or minimised by adding salicylic acid to the Karl Fischer reagent.     

Enantioselective screen-printed amperometric biosensor for the determination of D-amino acids

D-amino acids are generally considered to be important markers of bacterial contamination of food products. A screen-printed amperometric biosensor for the detection of D-amino acids has been constructed by the immobilization of D-amino acid oxidase on a graphite working electrode of a screen-printed strip modified with Prussian Blue and Nafion layers. Enzyme immobilization was then carried out by cross-linking of a mixture of the enzyme and bovine serum albumin with glutaraldehyde. As a result of the mediator addition and because of the multi-layer construction of the biosensor, including a polymer layer to avoid the interferences, the limit of the detection of the developed biosensor was two orders of magnitude improved in comparison to other screen-printed biosensors, as far as the determination of amino acids is concerned. Additional modification of the graphite electrode with carbon nanotubes led to a significant enhancement of the signal magnitude. A fast linear response of the developed biosensor was subsequently observed in static measurements for D-alanine in the concentration range from 5 to 200 microM. Excellent enantioselectivity towards D-amino acids was discovered. During the experiment, D-amino acids were detected in fruit juices and some milk samples. The complex matrix of natural milk samples had no influence on the response of the biosensor. The results were in good agreement with those obtained by capillary electrophoresis measurements.     

The spectrophotometric determination of rhenium

Some 50 analytical reagents were briefly checked for use in the spectrophotometric determination of rhenium. Seven reagents were checked in detail and interferences were examined. The most promising reagents appeared to be I-phenyl-2-thiourea and I,5-diphcnylcarbohydrazide.     

Sensitive derivatization reagents for thiol compounds in high-performance liquid chromatography with electrochemical detection

Three N-substituted maleimides were tested as derivatizing reagents; N-(4-anilinophenyl) maleimide (APM) was the most favorable in terms of sensitivity and reactivity. N-Acetyl-l-cysrteine, glutathione, l-cysteine and d-penicillamine were readily converted into the adducts with APM. Picogram levels of these thiol compounds were separated and quantified.     

Benzil and Benzoin: General Spray Reagents for the Visualization of Organic Compounds on Thin Layer Chromatograms

Benzil and Benzoin spray reagents have been adopted to detect some organic compounds on thin layer chromatograms (TLC). These reagents were found to visualize sugars, amino acids, dicarboxylic acids and some natural products on thin layer plates. Colors produced with these reagents were shades of the spectrum and characteristic of individual or class of the organic and natural products.     

Direct access to new β-d-galactofuranoconjugates: application to the synthesis of galactofuranosyl-l-cysteine and l-serine

Galactofuranose post-translational modifications, although quite rare, were detected in some biomolecules produced by parasites. While hexopyranosides were already linked to various peptides and proteins, few hexofuranosides have been artificially conjugated to amino acids. We thus report herein a robust glycosylation methodology to obtain S-alkyl, O-serine and S-cysteine-β-d-galactofuranosides starting from readily available galactofuranose donors. O-Acetyl, thioimidoyl and acetimidoyl donors were compared in terms of yields and selectivity when reacted with mercaptans, l-cysteine and l-serine. Acetimidates turned out to be the best notably for amino acids glycosylation.     

Speciation of arsenic by hydride generation-atomic absorption spectrometry (HG-AAS) in hydrochloric acid reaction medium

The effects on the absorbance signals obtained using HG-AAS of variations in concentrations of the reaction medium (hydrochloric acid), the reducing agent [sodium tetrahydroborate(III); NaBH(4)], the pre-reducing agent (l-cysteine), and the contact time (between l-cysteine and arsenic-containing solutions) for the arsines generated from solutions of arsenite, arsenate, monomethylarsonic acid (MMA), and dimethylarsenic acid (DMA), have been investigated to find a method for analysis of the four arsenic species in environmental samples. Signals were found to be greatly enhanced in low acid concentration in both the absence (0.03-0.60 M HCl) and the presence of l-cysteine (0.001-0.03 M HCl), however with l-cysteine present, higher signals were obtained. Total arsenic content and speciation of DMA, As(III), MMA, and As(V) in mixtures containing the four arsenic species, as well as some environmental samples have been obtained using the following conditions: (i) total arsenic: 0.01 M acid, 2% NaBH(4), 5% l-cysteine, and contact time<10 min; (ii) DMA: 1.0 M acid, 0.3-0.6% NaBH(4), 4.0% l-cysteine, and contact time <5 min; (iii) As(III): 4-6 M acid and 0.05% NaBH(4) in the absence of l-cysteine; (iv) MMA: 4.0 M acid, 0.03% NaBH(4), 0.4% l-cysteine, and contact time of 30 min; (v) As(V): by difference. Detection limits (ppb) for analysis of total arsenic, DMA, As(III), and MMA were found to be 1.1 (n=7), 0.5 (n=5), 0.6 (n=7), and 1.8 (n=4), respectively. Good percentage recoveries (102-114%) of added spikes were obtained for all analyses.     

Combinatorial chemistry: libraries from libraries, the art of the diversity-oriented transformation of resin-bound peptides and chiral polyamides to low molecular weight acyclic and heterocyclic compounds

Combinatorial chemistry has deeply impacted the drug discovery process by accelerating the synthesis and screening of large numbers of compounds having therapeutic and/or diagnostic potential. These techniques offer unique enhancement in the potential identification of new and/or therapeutic candidates. Our efforts over the past 10 years in the design and diversity-oriented synthesis of low molecular weight acyclic and heterocyclic combinatorial libraries derived from amino acids, peptides, and/or peptidomimetics are described. Employing a "toolbox" of various chemical transformations, including alkylation, oxidation, reduction, acylation, and the use of a variety of multifunctional reagents, the "libraries from libraries" concept has enabled the continued development of an ever-expanding, structurally varied series of organic chemical libraries.     

Sequential injection analysis system for on-line monitoring of l-cysteine concentration in biological processes

A sequential injection analysis (SIA) system was developed to monitor the concentration of l-cysteine in biological processes on-line. It is based on the redox reaction of l-cysteine with iron(III) in the presence of 1,10-phenanthroline (phen) and the detection of the red-iron(II)-phen complex with a spectrophotometry. The system was fully automated using software written in the LabVIEW™ development environment. A number of system variables such as the flow rate of the carrier buffer solution, the volume ratio of the sample to the reagents, and the reaction coil length, etc., were evaluated to increase the sensitivity and performance of the SIA system. Under partially optimized operating conditions the performance of the SIA system was linear up to a concentration of l-cysteine of 1 mM (R² = 0.998) with a detection limit of 0.005 mM and a sample frequency of 15 hr⁻¹. The SIA system was employed to monitor the concentration of l-cysteine on-line in a continuously stirred reactor and a fermentation process of Saccharomyces cerevisiae. The on-line monitored data were in good agreement with the off-line data measured by a HPLC with a fluorescence detector (n = 15, R² = 09899).     

Visual detection of cysteine and homocysteine

The determination of cysteine and homocysteine levels is of great current interest for the monitoring of desease states. A new colorimetric method for the simultaneous detection of l-cysteine and l-homocysteine has been developed. A fluorescein derivative reacts with the above amino acids, producing their respective thiazolidines resulting in color changes. Interference from other amino acids and proteins is minimal.