Spectrophotometric method for quantification of antioxidant capacity of tropical fruits based on the formation of Fe(II)/2,2'-bipyridyl complex

The suggested method is based on the reduction of Fe(III) to Fe(II) by antioxidant compounds (acetate buffer; pH 4.6) containing 2,2'-bipyridine (bipy, Fig. 1) in aqueous solution. Absorbance values recorded at 521 nm characteristic of the orange-reddish Fe(bipy)32+ complex formed were used to determine the TAC (expressed as equivalents of ascorbic acid) of ten unconventional Brazilian fruits. These TCA results were compared with those obtained with the free radical ABTS•+ reagent[2] and also with the Folin-Ciocalteu (FC) index. For the analyzed fruit samples (Table 1) the TAC values obtained with the proposed method (Fe(bipy)32+) showed very correlation well with those obtained using the ABTS•+ (r2 = 0.818) indicating that both methods can be used in the TAC quantification in this type of sample. Both TAC values (Fe(bipy)32+ and ABTS●+) also showed a positive correlation with the Folin-Ciocalteu index (r2 = 0.818 and r2 = 0.849, respectively) suggesting that the compounds oxidized by the Folin-Ciocalteu reagent[3] are responsible for the antioxidant capacity found in these samples. The suggested method is simple, easy to perform, reproducible and can be used in routine analysis because reagents are not expensive and equipment and materials are commonly available in most laboratories.