Study of Electrochemical Oxidation Mechanism of the Tryptophan Metabolite Kynurenine on Glassy Carbon Electrode

Tryptophan (TRP) in the human body is generally metabolized by two different pathways, to serotonin or via kynurenine (KYN), where the majority is consumed through the latter. Studies relate that the imbalance between these two pathways is associated with different types of diseases. This work aims to investigate for the first time the redox properties of KYN in aqueous electrolytes on glassy carbon electrode (GCE), using electrochemical techniques, cyclic voltammetry (CV), differential pulse voltammetry (DPV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS). The electrooxidation of KYN was compared with the anodic behavior of aniline and kynurenic acid. The KYN oxidation mechanism was proposed and occurs in the 2-aminobenzoyl group from a main step with the withdrawal of one electron and the formation of an intermediate cation radical (KYN^+•). The KYN^+• follows a dimerization and polymerization reaction, forming different electroactive products (polyKYNs) that are adsorbed on the GCE surface. The EIS data indicated that the adsorbed polyKYNs films on GCEs in a strongly acidic medium (pH = 0.3) are conductive and in a physiological medium they are resistive, hindering new subsequent reactions. All redox reactions identified were dependent on an acid–base equilibrium, since they were strongly influenced by the pH of the medium, occurring more easily in alkaline media. The diffusion coefficient of KYN was determined in phosphate buffer pH = 7.0 as 1.59 × 10⁻⁶ cm² s⁻¹. The voltammetric responses of DP were also explored here for the development of a sensitive electroanalytical method for detection and quantification of KYN. For the development of the method, analytical parameters were studied, such as work concentration range, linearity, limit of detection (LOD) and quantification (LOQ), repeatability, reproducibility, and selectivity to possible interferences. A method using DPV and GCE was developed for determination of KYN in acidic medium (pH = 0.30) with a LOD of 0.43 μmol L⁻¹.