We have used EPR to study oak bark, bulb onion peel, and their aqueous extracts. We have established that their paramagnetic properties are determined by at least four types of paramagnetic centers: semiquinone radicals, Fe 3+ ions, and two types of Mn 2+ ions. We have observed that metal ions are extracted by water from the plant raw material without any change in their original coordination environment. The aqueous extracts were subjected to oxidative processes: mild oxidation in the dried state, strong oxidation when stored as solutions. We have shown that when the solutions are stored, the oxidation reaction is catalyzed by the metal ions and is accompanied by formation of semiquinone radicals.Keywords: electron paramagnetic resonance spectroscopy, medicinal plants, extracts, phenols and polyphenols, antioxidant activity, semiquinone radicals, Fe 3+ and Mn 2+ ions.Introduction. We know [1, 2] that many medicinal plants and medicinal plant-based extracts have antioxidant activity. This is due to the presence of compounds in the phenol and polyphenol group, in particular flavonoids: hydroxy derivatives of flavanone, flavanonol, cathechins, flavone, and flavonol. The antioxidant activity of these substances is connected with their ability to enter into oxidation-reduction reactions with peroxide radicals, appearing in the body both during natural biochemical processes and under the influence of chemical and physical factors, in particular ultraviolet and ionizing radiation, and also in some diseases, especially cancers and atherosclerosis. Oxidation of phenols and polyphenols is accompanied by formation of low-activity, long-lived radicals of the semiquinone type, inhibiting the chain reaction of free-radical conversions [3,4]. In this case, the paramagnetic centers contained in the medicinal plant or its extract are certainly involved in the oxidation processes and make it possible to obtain important information about the mechanism for the antioxidant activity. However, the nature and conditions for the formation of these paramagnetic centers have not yet been sufficiently studied in most plant materials. The aim of this work was to study the paramagnetic properties of samples selected as model samples: oak bark, bulb onion peel, and their aqueous extracts.Experimental Procedure. We used dry samples of oak bark (from the botanicals producer Biotest NPK [Research and Production Cooperative], Grodno, Belarus) and bulb onion peel, ground and pulverized in an agate mortar to a powdered state, and also dry extracts of oak bark and bulb onion peel obtained from aqueous solutions. For extraction, a sample of the ground material weighing 10 g was placed in a round-bottom flask, 125 mL of distilled water was added, and it was boiled under reflux for 6 h. When extraction was finished, the solution was filtered and then the solvent was removed by evaporation on a water bath. The EPR spectra were recorded at T = 20
