Thermochemistry of antioxidant action of isoflavones and their deprotonated forms in aqueous solution: hydrogen or electron transfer?
Monika Biela *, Andrea Kleinová, Erik Klein
Institute of Physical Chemistry and Chemical Physics, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovakia
E-mail: * monika.biela@stuba.sk
Abstract: Isoflavones possessing several weakly acidic hydroxyl groups can undergo successive deprotonations in aqueous solution. Therefore, their antioxidant properties cannot be ascribed only to the neutral forms, but also to corresponding phenoxide anions. It was already confirmed that isoflavones prefer the formation of dianions in aqueous solution. For eight isoflavones and their preferred (poly)deprotonated forms, we have decided to study the thermochemistry of hydrogen atom transfer and electron abstraction in terms of corresponding reaction enthalpies, i.e., O–H bond dissociation enthalpies and ionization potentials. Our results clearly indicate that the increase in negative charge causes significant drops in ionization potentials and bond dissociation enthalpies. On the other hand, proton affinities show opposite trend. For dianions, this disables to find a generally valid trend – corresponding reaction enthalpies strongly depend on the structure of isoflavone, especially on the number/positions of OH groups.
Keywords: Bond dissociation enthalpy, Consecutive deprotonations, Electron transfer, Phenoxide anion
Acta Chimica Slovaca, Vol. 15, No. 1, 2022, pp. 29—35, DOI: 10.2478/acs-2022-0004