Thioamides as radical scavenging compounds: Methods for screening antioxidant activity and detection

Abstract

Heteroaromatic thiols and thiones attracted the attention of chemists, pharmacologists and biochemists because of participation in the interception of free radicals. For the first time offered independent and reliable methods for evaluating of the antioxidant activity of thioamides-derivatives of pyridine, quinoline, imidazole, triazole, tetrazole, pyrimidine, pyrrolidine and 7-mercapto-4-methylcoumarin -based on kinetic parameters of the thioamide reaction with chromogenic radical (rate constant, M(-1)min(-1) and time to decrease concentration of test free radical by 50%, TEC50, min) or thermodynamics of the thioamides reaction with molecular iodine (extent of thioamide conversion, %). To compare the antioxidant activity of thioamides and widely used standard-antioxidant Trolox (6-Hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid) we have proposed to use a value of relative antioxidant activity constant. As it was established, the kinetics of interaction between the chromogenic radical and thioamides in the presence of an excess of 2,2'-diphenyl-1-picrylhydrazyl (DPPH) is described by the kinetics of the pseudo first order with respect to the reacting components. A kinetic-spectrophotometric method for the quantification of heteroaromatic thioamides is elaborated and was tested in the analysis of urine. Thioamides were detected at concentrations of 1.53μgml(-1), RSD=4.6% (2-mercaptoimidazole, V), 2.08μgml(-1), RSD=1.8% (1-methylimidazoline-2-thione, VI), 1.45μgml(-1), RSD=4.3% (2-mercaptopyridine, IX).The proposed procedures show good precision and accuracy of the results obtained.

Keywords: Antioxidant activity; Kinetic method for the quantification; Kinetic-spectrophotometric method; Molecular iodine; The degree of reaction conversion; Thioamides.