Probing structural properties and antioxidant activity mechanisms for eleocarpanthraquinone

In this work, we present a computational investigation on the structure and energetics of eleocarpanthraquinone, a newly isolated polyphenolic anthrone-antraquinone. Properties such as bond lengths, angles, atomic charges, bond dissociation enthalpies (BDEs), and ionization potential (IP) were determined through the use of density functional theory (DFT). The B3LYP and M06-2X exchange-correlation functionals were employed along with the 6-31+G(d,p), 6-31+ +G(d,p), and 6-311+G(d,p) basis sets for performing computations in the gas-phase, water, methanol, and ethanol. The conformation presenting all the hydroxyl groups undergoing hydrogen-bond interactions with neighboring oxygen atoms (conformation 5) was assigned as the most stable structure while its counterpart presenting no hydrogen-bond interaction was found to be 36.45 kcal/mol less stable than conformation 5 in the potential energy surface probed at the B3LYP/6-311+G(d,p) level of theory in the gas-phase, for instance. More importantly, the lowest O-H bond dissociation enthalpy was determined to be 93.80 kcal/mol at the B3LYP/6-311+G(d,p) level of theory in water against the 146.58 kcal/mol regarding the IP computed at the same approach, suggesting the hydrogen atom transfer mechanism as being preferred over the single electron transfer mechanism in regards to the antioxidant potential for the case of eleocarpanthraquinone; the same conclusion was drawn from the outcomes of all the other approaches used.