Reactions of the radical cation derived from 2,2 '-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS(.+)) with amino acids. Kinetics and mechanism

Stable free radicals derived from 2,2'-azinobis(3-ethylbenzothiazoline-6-su lfonic acid) (ABTS(.+)) have been extensively employed to monitor the antio xidant capacity of biological fluids and beverages. Besides reacting with t ypical antioxidants (such as phenols or thiols) these radicals react with a variety of hydrogen or electron donors. The present work reports on the ki netics and mechanism of these radical reactions with several amino acids. R eaction rates notably increase when the pH of the media increases and, when measured under similar conditions, follows the order cysteine > > tryptophan > tyrosine > histidine > cystine The kinetics of the process is interpreted in terms of a mechanism comprisi ng an initial pH dependent reversible step, followed by secondary reactions of the substrate derived radical with itself or with another ABTS(.+); thi s simple three-step mechanism leads to very complex kinetic expressions. Th e specific rate constants of several of the elementary steps were determine d by working under a wide range of substrate, radical, and ABTS concentrati ons. The values obtained for the initial interaction between the ABTS deriv ed radical and the substrate range from 0.5 M-1 s(-1) to 1.9 + 10(6) M-1 s( -1) for histidine and cysteine, respectively.