Amino acids are molecules containing an amine group, a carboxylic acid group and a side chain that varies between different amino acids. These molecules are particularly important in biochemistry. Amino acids play central roles both as building blocks of proteins and as intermediates in metabolism. Cysteine plays a key role in stabilizing extracellular proteins. Tryptophan, an essential amino acid is the largest of the amino acids. Glutathione can also protect the cell against hypoxia, toxicity, mutagenicty or transformation by radiation and many carcinogens. In this work, we describe simultaneous determination of two amino acid, L-cysteine and tryptophan, using a carbon nanotubes paste electrode (CNPE) modified with p-aminophenol as a mediator in aqueous solution at a pH=6.0. Cyclic voltammetry (CV), differential pulse voltammetry (DPV), double potential step chronoamperometry and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of L-cysteine at a chemically modified electrode. The results showed an efficient electrocatalytic activity of the electrode for the oxidation of L-cyseine, which leads to a reduction in its overpotential more than 0.55 V. Using differential pulse voltammetry, L-cysteine and tryptophan in a mixture can each be measured independently from each other with a potential difference of 0.60 V. The results showed that tryptophan cannot catalysis at a surface of the modified carbon nanotubes paste electrode. The peaks current was linearly depend on L-cysteine and tryptophen concentrations in the concentration range of 0.5–100.0 μmol L^–1 L-cysteine and 10.0–300.0 μmol L^–1 tryptophan. The detection limits for L-cysteine and tryptophan were 0.3 μmol L^–1 and 5.7 μmol L^–1, respectively. The diffusion coefficient and the rate of electro-oxidation of L-cysteine at the surface of p-aminophenol-modified carbon nanotubes paste electrode was determined 6.20×10⁻⁴ cm² s⁻¹ and 4.36×10³ M^-1 s^-1 respectively for the experimental conditions. The relative standard deviation (RSD%) for ten replicated analysis of 10.0 μM L-cysteine was found 2.4%. This method was also used for the determination of L-cysteine and tryptophan in urine, tablet, water river, serom and plasma samples by the standard addition method. Cyclic voltammetry (CV), square wave voltammetry (SWV), double potential step chronoamperometry and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of gluthathione (GSH) at a chemically modified electrode prepared by incorporating p-aminophenol (p-AP) into multi-wall carbon nanotubes paste matrix. The result showed that p-aminophenol multi wall carbon nanotube paste electrode (p-APMWCNTPE) had high electrocatalytic activity for glutathione. It has been shown by these methods that p-AP can catalyze the oxidation of glutathione in aqueous buffer solution (pH=5.0) and produces a sharp oxidation peak current at about +0.21 vs. Ag/AgCl reference electrode. The square wave voltammetric peak current of GSH increased linearly with the corresponding GSH concentration in the two linear ranges of 0.2 – 4.3 μmol L^–1 and 4.3-100.0 μM with a detection limit of 0.09 μmol L^–1.The catalytic rate constant for oxidation of GSH at the p-APMWCNTPE was also determined and found to be about 9.30×10³ M^-1 s^-1. The diffusion coefficient of GSH was also estimated as 6.00×10⁻⁴ cm² s⁻¹ for the experimental conditions, using chronoamperometry. The relative standard deviation for ten replicated analysis of 10.0 μM GSH was found 2.1%. This method was also examined as a selective, simple and precise new method for voltammetric. This method was used for the Determination of glutathione in real sample such as a hemolysed erythrocyte and urine. The results indicate that p-APMCNTPE has good stability and reproducibility. The data obtained clearly showed that the combination of carbon nanotubes and p-aminophenol definitely improve the characteristics of L-cysteine and glutathione oxidation.

Key Words

L-cysteine, Tryptophan, Glutathione, Multiwall carbon nanotubes paste electrode, p-Aminophenol.