Development of a spectrophotometric method for quantification of antioxidant capacity based on the formation of free radical derived from n- (4-methoxyphenyl)-1,4-phenylene

Abstract

N-(4-methoxyphenyl)-1,4-phenylenediamine, abbreviated MPPD, also known as Variamine Blue B, is a p-phenylenediamine used as visual indicator in the complexometric titration of Fe(III) with EDTA (pH 2.0).It has been observed the colorless MPPD can be oxidized to its respective free radical MPPD*+ (purple) in buffered acid medium (pH 4,0; acetate).In presence of reducing agents (e.g. ascorbic acid) the colored radical is consumed, so this reaction was proposed for the development of spectrophotometric method for the quantification of the antioxidant capacity. Initially, the physical-chemical parameters which influence the formation and stabilization of the MPPD*+ will be evaluated. Then the consumption of the MPPD*+ according to the type and chemical structure of a given antioxidant (e.g. phenolic acids, flavonoids and carotenes) will be evaluated using standard solutions of these compounds. After finding the best experimental conditions, the MPPD*+ solution will be used to quantify the antioxidant capacity of samples rich in polyphenolic compounds such as herbal teas and extracts. These results will be compared with the values obtained using the reference method which is based on the consumption of ABTS*+ free radical. In addition, the results of the antioxidant capacity obtained with both methods (proposed and reference) for the same samples will be compared with the values of total polyphenol content (Folin-Ciocalteu reagent), since to the polyphenolic compounds are attributed a great antioxidant activity.