DFT and QTAIM based investigation on the structure and antioxidant behavior of lichen substances Atranorin, Evernic acid and Diffractaic acid

• Published in: Computational biology and chemistry 2019-06, Vol.80, p.66-78
• Main Authors: Shameera Ahamed, T.K., Rajan, Vijisha K., Sabira, K., Muraleedharan, K.
• Format: Article
• Language: English
• Subjects: Antioxidant; Atranorin; DFT; Diffractaic acid; Evernic acid
• ISSN: 1476-9271; 1476-928X
• DOI: 10.1016/j.compbiolchem.2019.03.009

[Display omitted] •DFT based investigation of structural and antioxidant properties of lichen metabolites has been performed.•Selected lichen metabolites are Atranorin (AT) and Evernic acid (EV) and Diffractaic acid (DF).•IHB, Aromaticity, and NCI have been computed with QTAIM gives significant structural characteristics.•Neutral and cationic radical species are stabilized by the spin density delocalization of the unpaired electron.•The HAT and SPLET is the preferred mechanism in gas and water media respectively. In this study, the structural and antioxidant behavior of the three lichen-derived natural compounds such as atranorin (AT), evernic acid (EV) and diffractaic acid (DF) has been investigated in the gas and water phase using both B3LYP and M06-2X functional level of density functional theory (DFT) with two different basis sets 6-31+G (d, p) and 6-311++G (d, p). The intramolecular H–bonds (IHB) strength, aromaticity and noncovalent interactions (NCI) have been computed with the help of the quantum theory of atoms in molecules (QTAIM). This calculation gives major structural characteristics that indirectly influence the antioxidant behavior of the investigated compounds. The spin density (SD) delocalization of the unpaired electron is found to be the main stabilizing factor of neutral and cationic radical species. The main mechanisms, recommended in the literature, for the antioxidant action of polyphenols as radical scavengers such as hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET), were examined. The result shows that the HAT and SPLET mechanism are the most conceivable one for the antioxidant action of this class of compounds in gas and water phase respectively. Preference of SPLET over HAT in water phase is due to the significantly lower value of proton affinity (PA) compared to the bond dissociation enthalpy (BDE) value. This study reveals that O2-H3, O9-H26 and O4-H45 respectively are the most favored site of AT, EV and DF for homolytic as well as heterolytic OH bond breaking.