Analysis of the Polyphenols Content in Medicinal Plants Based on the Reduction of Cu(II)/Bicinchoninic Complexes

A spectrophotometric method is proposed for the determination of the polyphenols content in aqueous extracts of plants. The method is based on the reduction of Cu(II) to Cu(l) by polyphenols, in the presence of bicinchoninic acid in a buffered medium (ammonium acetate, pH 7.0) with the formation of Cu(I)/BCA complexes. A calibration curve of absorbance (at 558 nm) vs tannic acid concentration is linear (r = 0.995; n = 7) with tannic acid from 0.1 to 0.7 mu mol L-1. The limit of detection and relative standard deviation were 40 nmol L-1 (99% confidence level) and 3.8% (0.4 mu mol L-1 tannic acid, n = 7), respectively. For the aqueous extracts of Hamamelis virginiana L., Maytenus ilicifolia Mart. ex Reissek, Hydrocotyle bonariensis Lam, Annona muricata L., Myrciaria cauliflora (Mart.) O. Berg., Caesearia sylvestris Sw., Schinus terebinthifolia (Raddi), and Stryphnodendron adstringens (Mart.) Coville, the total polyphenol contents, expressed as tannic acid, were 3.5, 1.3, 2.0, 3.1, 15.4, 3.1, 9.1, and 6.9%, respectively.