Paramagnetic centers in oak and onion extracts

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, Fe³⁺ ions, and two types of Mn²⁺ 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. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 440–445, May–June, 2009.