Electrochemical oxidation of l-cysteine catalyzed by 10-ethylphenothiazine

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.

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.

Photochemically catalyzed reaction of ochratoxin A with D- and L-cysteine

The photolysis (>300 nm) of ochratoxin A (OTA, N-[[(3R)-5-chloro-8-hydroxy-3-methyl-1-oxo-7-isochromanyl]carbonyl]-3-phenyl-L-alanine, 1) in the presence of excess (2 and 12 molar equiv) cysteine (CySH) has been investigated and found to yield sulfur adducts 5 and 6 that are characterized by liquid chromatography-mass spectrometry and 1H-NMR spectroscopy. The adduct 5 was ascribed to the Michael addition conjugate resulting from covalent attachment of CySH to the ochratoxin quinone (4) generated by photooxidation of OTA. This species was also formed by photolysis of a synthetic sample of the hydroquinone of OTA (ochratoxin hydroquinone, 3) in the presence of 12 equiv L-CySH. The conjugate 5 derived from photolysis of 3 with L-CySH was used for 1H-NMR analysis. The sulfur adduct 6 was the major species detected from covalent attachment of CySH to photoactivated OTA, and it resulted from direct displacement of the OTA Cl atom by CySH. The implications of the cysteinyl adducts to the in vivo toxicity of OTA are discussed, with particular emphasis given to conjugate 5, as products from the photooxidative pathway may be of relevance to the nephrotoxic properties of OTA.

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.