Antioxidant Potential in Lipophilic and Hydrophilic Extracts from Medicinal Herbs (Salvia officinalis and Echinacea angustifolia). A Comparison Between Assays Based on Electron Paramagnetic Resonance and Spectrophotometry

Abstract

Recently, detection of antioxidant activity of plant extracts has caught the attention of researchers. Determination of antioxidant activity is usually performed using stable free radicals. A number of assays is available for screening the antioxidant activity of food constituents, but few of these methods are applicable to lipophilic substances. EPR is the analytical technique that directly measures free radicals making possible the analysis of turbid or highly colored samples. The aim of this study was to determine how the detection technique, plant species and the different growing conditions influenced the antioxidative activity of aqueous and lipid extracts from leaves. Sage (Salvia officinalis L.) and coneflower (Echinacea angustifolia DC.) grown in hydroponics and in soil, were analyzed. Radical decay kinetics, due to plant extracts, were monitored by an EPR spectrometer, equipped with a data acquisition system and a software package designed for analysis and simulation of spectra. Parameters obtained by the fitting of the curves allowed the calculation of the antioxidant potential. The radicals Fremy’s salt (hydrophilic) and 1,1-diphenyl-2-picrylhydrazyl (DPPH, lipophilic) were employed. Comparisons with data obtained by a spectrophotometric detection with the radical cation ABTS⁺ [2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)] were also made. Fremy’s salt and DPPH gave 1st and 2nd order kinetics, respectively. EPR can give more reliable measurements of the antioxidant potential than spectrophotometric assay. The reduced sensitivity of spectrophotometric detection was showed both for sage and coneflower extracts. Antioxidant potential of sage resulted extraordinary high compared to coneflower and a different ratio of aqueous to lipid antioxidant potential in the two species could be monitored. Antioxidant potentials were also higher for coneflower grown in hydroponics than in soil. In addition to the technique adopted, the detection of antioxidant potential was influenced by the growing conditions of plants and hydroponics showed to be able to increase antioxidant potential of both coneflower lipophilic and hydrophilic extracts. By registration of EPR kinetics we can get reliable measurements of the antioxidant potential of plant extracts, avoiding the artifacts due to the appearance of other radicals resulting from oxidative processes.