A Comprehensive Study of the Use of Cu(I)/4,4'-Dicarboxy-2,2'-biquinoline Complexes to Measure the Total Reducing Capacity: Application in Herbal Extracts.

A method for the determination of total reducing capacity (TRC) based on the reduction of Cu(II) to Cu(I) by antioxidants in a buffered solution (pH 7.0) containing 4,4'-dicarboxy-2,2'-biquinoline acid (BCA) was developed. Absorbance values at 558 nm characteristic of the Cu(I)/BCA complexes formed were used to determine the TRC of aqueous extracts of twelve Brazilian plants. The TRC values obtained with the suggested method correlated well with values obtained using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method (r² = 0.959). They were also compared with the total polyphenol content (using the Folin-Ciocalteu reagent) and the good agreement (r² = 0.919) indicates that the polyphenols should be responsible for this reducing capacity. The method proposed here (and successfully applied in plant extracts) can be used to measure the TRC of aqueous samples derived from other plants (e.g., teas, juices, beers and wines) and even in biological samples (e.g., serum, urine and follicular fluid). To achieve a structure-activity relationship of the proposed reaction, the reduction capability of 25 standard antioxidants (phenolic derivatives, flavonoids, stilbenoids, vitamins, etc.) was individually evaluated and the apparent molar absorptivity values (at 558 nm) obtained were compared and discussed.