Determination Of L-cysteine Research Articles

Nafion/lead nitroprusside nanoparticles modified carbon ceramic electrode as a novel amperometric sensor for l-cysteine

This work describes the electrochemical and electrocatalytic properties of carbon ceramic electrode (CCE) modified with lead nitroprusside (PbNP) nanoparticles as a new electrocatalyst material. The structure of deposited film on the CCE was characterized by energy dispersive X-ray (EDX), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM). The cyclic voltammogram (CV) of the PbNP modified CCE showed two well-defined redox couples due to [Fe(CN) 5NO] 3−/[Fe(CN) 5NO] 2− and Pb IV/Pb II redox reactions. The modified electrode showed electrocatalytic activity toward the oxidation of l-cysteine and was used as an amperometric sensor. Also, to reduce the fouling effect of l-cysteine and its oxidation products on the modified electrode, a thin film of Nafion was coated on the electrode surface. The sensor response was linearly changed with l-cysteine concentration in the range of 1 × 10 −6 to 6.72 × 10 −5 mol L −1 with a detection limit (signal/noise ratio [ S/ N] = 3) of 0.46 μM. The sensor sensitivity was 0.17 μA (μM) −1, and some important advantages such as simple preparation, fast response, good stability, interference-free signals, antifouling properties, and reproducibility of the sensor for amperometric determination of l-cysteine were achieved.

Nanostructured electropolymerized film of 5-amino-2-mercapto-1,3,4-thiadiazole on glassy carbon electrode for the selective determination of l-cysteine

We report the electropolymerization of 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) in 0.1 M H 2SO 4 on glassy carbon electrode (GCE) and utilization of the resulting polymer film for the selective determination of l-cysteine (CY) at physiological pH for the first time. The electropolymerized film was characterized by cyclic voltammetry (CV), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM image shows a homogeneous film containing spherical structure with a thickness of ∼25 nm for p-AMT deposited by 15 cycles. The binding energies at 163.5, 400.2 and 398.8 eV in the XPS corroborate that the p-AMT was linked by S–S, HN–NH and N N groups. The p-AMT film successfully separates the voltammetric signals of CY and ascorbic acid (AA) with a potential difference of 480 mV which is higher than the previous reports.

Flow‐injection determination of cysteine in pharmaceuticals based on luminol–persulphate chemiluminescence detection

A flow injection (FI) method is reported for the determination of l-cysteine, based on its enhancement on chemiluminescence (CL) emission of luminol oxidized by sodium persulphate in alkaline solution. The calibration graph was linear over the range 1.0 x 10(-9)-5.0 x 10(-7) mol/L (r(2) = 0.9992), with relative standard deviations (RSDs) in the range 1.1-2.3% (n = 4). The limit of detection (3 sigma blank) was 5.0 x 10(-10) mol/L with a sample throughput of 120/h. The method was applied to pharmaceuticals and the results obtained were in reasonable agreement with the amount labelled. The proposed method was also applied to cysteine in synthetic amino acid mixtures. Calibration graphs of N-acetylcysteine and glutathione over the range 1.0-50 x 10(-8) and 0.5-7.5 x 10(-7) mol/L were also established (r(2) = 0.998 and 0.9986) with RSDs in the range 1.0-2.0% (n = 4), and the limits of detection (3 sigma blank) were 5.0 x 10(-9) and 1.0 x 10(-8) mol/L, respectively.

Electrocatalytic oxidation and sensitive determination of l-cysteine at a poly(aminoquinone)-carbon nanotubes hybrid film modified glassy carbon electrode

Poly(aminoquinone) (PAQ) was synthesized through iodate-oxidation/Michael addition reaction of 1,2-dihydroxybenzene with 4,4′-methylenedianiline and characterized with SEM, FT-IR and UV-Vis techniques. The PAQ-multiwalled carbon nanotubes (MWCNTs) hybrid film modified glassy carbon (GC) electrode showed well-defined redox peaks of the quinone groups of the polymer, which effectively mediated the oxidation of l-cysteine (CySH) in a pH 7.0 phosphate buffer, with an overpotential decrease by ca. 0.26 V (versus bare GC). The PAQ-MWCNTs-Nafion/GC electrode was used for sensitive biosensing to CySH, with a limit of detection of 75 nmol L−1.

Solid state electrochemical of the erbium hexacyanoferrate-modified carbon ceramic electrode and its electrocatalytic oxidation of l-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.

A novel potentiometric biosensor for selective l-cysteine determination using l-cysteine-desulfhydrase producing Trichosporon jirovecii yeast cells coupled with sulfide electrode

Trichosporon jirovecii yeast cells are used for the first time as a source of l-cysteine desulfhydrase enzyme (EC 4.4.1.1) and incorporated in a biosensor for determining l-cysteine. The cells are grown under cadmium stress conditions to increase the expression level of the enzyme. The intact cells are immobilized on the membrane of a solid-state Ag 2S electrode to provide a simple l-cysteine responsive biosensor. Upon immersion of the sensor in l-cysteine containing solutions, l-cysteine undergoes enzymatic hydrolysis into pyruvate, ammonia and sulfide ion. The rate of sulfide ion formation is potentiometrically measured as a function of l-cysteine concentration. Under optimized conditions (phosphate buffer pH 7, temperature 37 ± 1 °C and actual weight of immobilized yeast cells 100 mg), a linear relationship between l-cysteine concentration and the initial rate of sulfide liberation (d E/d t) is obtained. The sensor response covers the concentration range of 0.2–150 mg L −1 (1.7–1250 μmol L −1) l-cysteine. Validation of the assay method according to the quality control/quality assurance standards (precision, accuracy, between-day variability, within-day reproducibility, range of measurements and lower limit of detection) reveals remarkable performance characteristics of the proposed biosensor. The sensor is satisfactorily utilized for determination of l-cysteine in some pharmaceutical formulations. The lower limit of detection is ∼1 μmol L −1 and the accuracy and precision of the method are 97.5% and ±1.1%, respectively. Structurally similar sulfur containing compounds such as glutathione, cystine, methionine, and d-cysteine do no interfere.

Vapour phase Fourier transform infrared spectrometric determination of l-cysteine and l-cystine

A novel and selective procedure for the determination of l-cysteine and l-cystine based on vapour-generation Fourier transform infrared spectrometry is described. Potassium iodate solution was injected into a glass vessel containing l-cysteine and/or l-cystine. The evolved CO was swept by a stream of nitrogen to an infrared gas cell. The vapour phase FTIR spectra were continuously recorded, as a function of time, between 2240 and 2000 cm −1, which includes the CO absorption band. The maximum absorbance at 2170 cm −1 was selected as a measurement criterion. The calibration curve was linear over the range 6–300 mg L −1. The method provided a limit of detection of 2 mg L −1 of l-cysteine, a throughput of 12 samples h −1 and an R.S.D. of 1.76% for five independent analyses of a 75 mg L −1 l-cysteine solution. For the measurement of l-cysteine and l-cystine separately, after measuring total concentration of l-cysteine and l-cystine, l-cysteine was masked with p-benzoquinone at a pH of 3 and individual l-cystine was determined. The amount of l-cysteine was obtained by difference. The method was applied to the determination of l-cysteine and l-cystine in pharmaceutical and urine samples. Results obtained for real samples compared well with those obtained by a reference spectrometric method.

L-Cysteine determination in pharmaceutical formulations using a biosensor based on laccase from Aspergillus oryzae

A new graphite electrode modified with laccase from Aspergillus oryzae was developed for the determination of l-cysteine in pharmaceutical formulations. The biosensor was characterized with respect to phenolic compounds (caffeic acid, catechol, dopamine, hydroquinone, l-DOPA and methyl-l-DOPA), inhibition effects (ascorbic acid, benzoic acid, l-cysteine, sodium sulfite, thiourea and metal ions: Ag+, Zn2+, Hg2+), pH dependence, linear range, sensitivity and Michaelis–Menten constant. Measurements were performed in the presence of hydroquinone and l-cysteine in 0.1 mol L−1 phosphate buffer (pH 7.0) at an applied potential of −0.08 V versus Ag/AgCl. The recovery of l-cysteine from three samples ranged from 89.7 to 103.4%, the relative standard deviation was less than 1.0% for a solution containing 4.95 × 10−4 mol L−1 hydroquinone (n = 8) and the lifetime of the biosensor was 9 months (at least 950 determinations). Results obtained for the determination of l-cysteine in pharmaceutical formulations using the proposed biosensor and those obtained by a standard method are in agreement at the 95% confidence level.

Efficient electrocatalysis of l-cysteine oxidation at carbon ionic liquid electrode

The electrochemistry of l-cysteine (CySH) in neutral aqueous media was investigated using carbon ionic liquid electrode (CILE). Comparative experiments were carried out using glassy carbon electrodes. At CILE, highly reproducible and well-defined cyclic voltammograms were obtained for l-cysteine with a peak potential of 0.49 V vs Ag/AgCl, showing that CILE manifests a good electrocatalytic activity toward oxidation of l-cysteine. A linear dynamic range of 2–210 μM with an experimental detection limit of 2 μM was obtained. The method was successfully applied to the determination of l-cysteine in a sample of soya milk. Cysteine oxidation at CILE does not result in deactivation of the electrode surface. Mechanistic studies showed that, at CILE, the overall CySH oxidation is controlled by the oxidation of the CyS − electroactive species.

Electrocatalytic Oxidation and Highly Selective Voltammetric Determination of l-Cysteine at the Surface of a 1-[4-(Ferrocenyl ethynyl)phenyl]-1-ethanone Modified Carbon Paste Electrode

A carbon paste electrode (CPE) chemically modified with 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone (4-FEPEMCPE) was employed to study the electrocatalytic oxidation of L-cysteine using cyclic voltammetry, differential pulse voltammetry and double potential step chronoamperometry as diagnostic techniques. The diffusion coefficient (D = 7.863 x 10(-6) cm2 s(-1)) of L-cysteine was also estimated using chronoamperometry. The electron-transfer coefficient, alpha (= 0.40), for L-cysteine at the surface of 4-FEPEMCPE was determined using cyclic voltammetry technique. It was found that under an optimum pH (= 7.00), the oxidation of L-cysteine at the surface of such an electrode occurred at a potential of about 350 mV less positive than that of an unmodified CPE. The catalytic oxidation peak currents represented a linear dependence on the L-cysteine concentration. Linear analytical curves were obtained in the ranges of 9.0 x 10(-5) - 4.9 x 10(-3) M and 2.0 x 10(-5) - 2.8 x 10(-3) M of L-cysteine with correlation coefficients of 0.9981 and 0.9982 in cyclic voltammetry and differential pulse voltammetry, respectively. The detection limits (2 sigma) were determined to be 9.9 x 10(-6) M and 5 x 10(-6) M with cyclic voltammetry and differential pulse voltammetry, respectively. The influences of twenty other amino acids, such as glutamine, L-glutamic acid, L-glysine, L-histidine, L-isoleucine, L-leucine, L-arginine hydrochloride, L-aspargine, L-aspartic acid, S-carboxy methyl-L-cysteine, L-methionine, L-phenyl alanine, L-proline, L-serine, L-threonine, L-cystine, cysteamine and gluthathione, on the current response of the sensor were examined. The obtained results did not show any influence on the analytical signal of L-cysteine by these amino acids (except for cysteamine). The method was also used for the selective determination of L-cysteine in patient-blood plasma and some pharmaceutical preparations by using standard addition method.

Sensing of l-cysteine at glassy carbon electrode using Nile blue A as a mediator

The oxidation of l-cysteine has been studied at a glassy carbon electrode by electrocatalytic effects of Nile blue A (NBA) as a mediator using cyclic, linear sweep voltammetry and chronoamperometry as diagnostic techniques. The results showed that the catalytic current of NBA depended on the concentration of l-cysteine. Although l-cysteine itself showed a very poor electrochemical response at the glassy carbon electrode, the response could be greatly enhanced by using NBA as a mediator, which enabled a sensitive determination of l-cysteine. This sensor could be used for determination of l-cysteine in the range of 10.0 × 10 −6 to 2.5 × 10 −4 M with a limit of detection of 1.3 μM. The rate constant was estimated for catalytic reaction of NBA with l-cysteine using chronoamperometry. The influence of potential interfering substances on the peak current was studied and the results showed that the method was highly selective for l-cysteine determination. The proposed method was used for the determination of l-cysteine in synthetic and real samples.

Electrocatalytic Characteristics of Ferrocenecarboxylic Acid Modified Carbon Paste Electrode in the Oxidation and Determination ofL-Cysteine

The electrochemical behavior of L-cysteine studied at the surface of ferrocenecarboxylic acid modified carbon paste electrode (FCMCPE) in aqueous media using cyclic voltammetry and double step potential chronoamperometry. It has been found that under optimum condition (pH 7.00) in cyclic voltammetry, the oxidation of L-cysteine is occurs at a potential about 580 mV less positive than that an unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and catalytic reaction rate constant, K′h were also determined using electrochemical approaches. The electrocatalytic oxidation peak current of L-cysteine showed a linear dependent on the L-cysteine concentration and linear calibration curves were obtained in the ranges of 10−5 M–10−3 M and 4.1×10−8 M–3.7×10−5 M of L-cysteine concentration with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods respectively. The detection limits (2δ) were determined as 2.4×10−6 M and 2.5×10−8 M by CV and DPV methods. This method was also examined for determination of L-cysteine in some samples, such as Soya protein powder, serum of human blood by using recovery and standard addition methods.

Electrocatalytic and voltammetric determination of sulfhydryl compounds through iron nitroprusside modified graphite paste electrode

Iron nitroprusside Fe(II)NP was incorporated into a carbon paste electrode and the electrochemical studies were performed with cyclic voltammetry. The cyclic voltammogram of Fe(II)NP exhibits two redox couple with formal potential (E0')1 = 0.24 e (E0')2 = 0.85 V vs SCE attributed to Fe(II)/Fe(II) and Fe(II)(CN)5NO / Fe(III)(CN)5NO, respectively. The redox couple with (E0')2 = 0.85 V presents an electrocatalytic response for sulfhydryl compounds. The electrocatalytic oxidation of sulfhydryl compounds by the mediator has been used for the determination of L-cysteine and N-acetylcysteine. The modified graphite paste electrode gives a linear range from 9.2 ×10-4 <FONT FACE=Symbol>-</FONT> 2.0 ×10-2; 9.6 × 10-4 <FONT FACE=Symbol>-</FONT> 1.4 × 10-2mol L-1 for the determination of L-cysteine and N-acetylcysteine, respectively, with detection limit of 1.9 ×10-4 mol L-1; 1.5 × 10-4 mol L-1 and relative standard desviations ± 5% and 1.5 × 10-3 mol L-1 ± 4% (n=3). The amperometric sensitivities are 0.024 and 0.027 muA/mumol L-1 for L-cysteine and N-acetylcysteine, respectively. The application of this electrode was tested and a commercial pharmaceutical product (Fluimucil) has been determined.

Highly sensitive spectrofluorimetric determination of l-cysteine based on inhibition of hemoglobin

A highly sensitive spectrofluorimetric method for the determination of l-cysteine based on its inhibitory action on hemoglobin (Hb) activity was developed. The optimal conditions were studied. A linear calibration graph was obtained over the range 2.00×10 −7–1.50×10 −6 mol l −1 for l-cysteine. The relative standard derivation was 1.30% at l-cysteine concentration of 8.00×10 −7 mol l −1 ( n=11). Further experimental results revealed that the inhibition of l-cysteine on this system was of the competitive type. The method was applied to determine l-cysteine in protein hydrolysate and cystine electrolyte samples with satisfactory results.

Electrochemiluminescent Determination of l-Cysteine with a Flow-Injection Analysis System

A flow-injection electrochemiluminescent method for L-cysteine determination has been developed based on its enhancement of the electrochemiluminecence of luminol at a glassy carbon electrode. This method is simple and sensitive for cysteine determination. Under the selected experimental parameters, the linear range for cysteine concentration was 1.0 x 10(-6) - 5.0 x 10(-5) mol/l, and the detection limit was 0.67 micromol/l (S/N = 3). The relative standard deviation for 11 measurements of 1.0 x 10(-5) mol/l cysteine was 4.5%. The proposed method has been applied to the detection of cysteine in pharmaceutical injections with satisfactory results.

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 2×10 −6 and 5×10 −4 mol l −1 for cysteine and between 1×10 −6 and 2×10 −4 mol l −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.

Voltammetric behavior of L-cysteine in the presence of CPB at a silver electrode.

The voltammetric behavior of L-cysteine at a silver electrode is described. L-Cysteine can be anodically accumulated at a silver electrode surface as a sparingly soluble silver salt; at more negative potentials, the insoluble compound is stripped cathodically yielding a small current peak at about -1.10 V (vs. SCE). In the presence of cetyl pyridine bromide (CPB), the stripping peak shifts slightly to a more negative potential, and the peak height increases significantly. Thus, the peak becomes more useful for the determination of L-cysteine. In contrast to other surfactants, CPB can improve the accumulation and stripping of L-cysteine obviously. The voltammetric behavior of cysteamine, 3-mercaptopropionic acid and homocysteine is discussed as well.

Voltammetric determination of L-cysteine at conductive diamond electrodes.

Boron-doped diamond (BDD) electrodes were used to examine L-cysteine (CySH) oxidation in alkaline media. The results of the voltammetric and polarization measurements showed that at BDD electrodes the overall CySH oxidation reaction is controlled by the initial electrochemical step, i.e., the oxidation of the CyS- electroactive species. The same conclusion was supported by the results of a study of pH effects. Conversely, at glassy carbon (GC) electrodes, the same reaction is controlled by the desorption of the reaction products. These results account for the poor response for CySH determination at GC compared to BDD. It was found that BDD exhibits excellent behavior for CySH determination, clearly outperforming GC. The results demonstrate that measurement of the peak current for CySH oxidation can be used as a basis for simple method for determining CySH in the micromolar concentration range by the use of BDD electrodes.

Quick Cuvette Test for Thiol Compounds

ABSTRACT Plastic cuvettes coated chemically with Prussian Blue films were investigated as quick disposable spectrophotometric test for mercapto-compounds. The determination is based on discoloration of the coating film due to its reduction by analyte. The changes of film absorbance were measured at 720 nm. It was found that the cuvettes can be used for selective determination of cysteamine, β-mercaptoethanol and L-cysteine in millimolar ranges of concentration. The test is useful for determination of L-cysteine and N-acetylcysteine in pharmaceutical products.

L-Cysteine determination using a polyphenol oxidase-based inhibition flow injection procedure

The use of crude extract of sweet potato root ( Ipomoea batatas L. Lam.) as a source of polyphenol oxidase (PPO; tyrosinase; EC 1.14.18.1) for the determination of L-cysteine in pharmaceutical products is reported. This enzyme catalyses the oxidation of catechol to o-quinone with a strong absorption at 410 nm. When L-cysteine solution is inserted into the flow injection (FI) system, its inhibitory effect on the PPO activity was proportional to the L-cysteine concentration. The analytical curve of absorbance versus concentration of L-cysteine obtained was A = 0.851 − 963.16[ L-cysteine]; r = 0.9987 in the L-cysteine concentration range from 6.0 × 10 −5 to 8.0 × 10 −4 mol l −1 with a detection limit of 4.4 × 10 −6 mol l −1. Recovery of L-cysteine from three samples ranged from 98.8 to 101.4% and the relative standard deviations (Sr) were less than 1.2% for solutions containing 2.0 × 10 −4 and 4.0 × 10 −4 mol l −1 of L-cysteine ( n = 10) with stable baselines. The throughput was 26 determinations per hour and the results obtained for L-cysteine by the proposed FI procedure were in good agreement with those obtained using a pharmacopeial procedure and within an acceptable range of error.