Roles of the Phenol OHs for the Reducing Ability of Antioxidant Acylphloroglucinols. A DFT Study

Abstract

Acylphloroglucinols (ACPLs) are a large class of compounds structurally derived from 1,3,5-trihydroxybenzene and characterised by the presence of a CRO group. Many ACPLs of natural origin exhibit biological activities. The antioxidant activity is particularly significant for ACPLs with substituents containing additional O atoms and additional C=C double bonds. Previous works have investigated most ACPLs with reported antioxidant activity, modelling their reducing ability through the consideration of complexes of their molecules with a Cu²⁺ ion. Phenol OHs are known to be the major responsible of the antioxidant activity of hydroxybenzenes. The current work considers selected antioxidant ACPLs and investigates the effect of the removal of one or more phenol OHs on the molecules’ ability to coordinate and reduce a Cu²⁺ ion. Calculations were performed at the DFT/B3LYP level with the 6-31+G(d,p) basis set for the C, O and H atoms and the LANL2DZ pseudopotential for the Cu²⁺ ion. For all the structures considered, the best characteristics of the complex correspond to the cases when the ion binds to two sites simultaneously. Besides obvious roles of the phenol OHs as binding sites, the results indicate influences of these OHs on the molecule-ion affinity of the complexes, on the strength of the intramolecular hydrogen bonds in the molecule, and on the distribution of the spin of the unpaired electron in the molecular ion resulting from complexation.