Antioxidant Activity and Polyphenol Content of Some Brazilian Medicinal Plants Exploiting the Formation of the Fe(II)/2,2′-bipyridine Complexes

Abstract

Aqueous plant extracts contain polyphenols that reduce Fe³+ to Fe²+, which in the presence of excess 2,2′-bipyridine (bipy) yield the Fe(bipy)₃²⁺ complex (max = 521 nm). This reaction was used to quantify the total polyphenol content (TPC) of nineteen Brazilian medicinal species using pyrogallic acid (PA) as a standard. A calibration curve of absorbance values at 521 nm values vs. PA was linear from 0.378 to 3.03 mg L-¹ (r = 0.998; n = 8) with a limit of detection 0.16 mg L-¹. The TPC quantification using the proposed reaction was compared with values obtained with the Folin-Ciocalteu reagent, yielding good agreement (r = 0.965). For the same species the TPC obtained with the proposed method also resulted in good correlation (r = 0.773) with the antioxidant capacity values quantified using the 2,2-diphenyl-1-picrylhydrazyl method. It was also found that Fe(bipy)₃²⁺ complexes can be used to calculate the reducing capability of polyphenols. The polyphenol structure, and the number and position of the hydroxyl group attached to the benzene ring modify this reductive capacity.