Gallic Acid Radical Generation in Aromatic Plants: A Combined EPR and UV-Vis Spectroscopic Approach

Objectives: In the present work, five aromatic plants: Origanum vulgare L., Rosmarinus officinalis L., Ocimum basilicum L., Salvia officinalis L. and Achillea millefolium L., are examined in order to determine Gallic Acid (GA) radical generation in them by UV-Vis and EPR spectroscopy. Methods: The phenol content of plant extracts was estimated at 280 nm by deconvolution of UV-Vis spectra using a GA calibration curve whilst the radical activity was quantitated by EPR Spectroscopy at 77 Kelvin using the stable free radical DPPH● as a reference. Results: The radical activity ranged from 7 × 1012 spins in O. vulgare L. to 2.2 × 1013 spins in O. basilicum L. of DPPH●/g sample whilst the phenol content as GA ranged from 28.1 mg of GA/g sample in O. vulgare L. to 65.2 mg of GA/g sample in A. millefolium L. Moreover, EPR spectra showed that all samples contain stable radical signals with g-values 2.0046–49 and a line width of 3–5 Gauss. These are characteristic for π-type semiquinone radicals of GA compounds with the unpaired electron partially on the oxygen atom of the phenolic ring radicals. Conclusion: These significant differences between the UV-Vis and EPR measurements reveal that the stabilization of the radical fraction, type GA● in aromatic plants is a combined result of aromatic species and local effects; namely, phenolic groups and aromatic environment.