ANTIOXIDANT ACTIVITY OF SOME NEWLY SYNTHESIZED C2-SYMMETRICAL DIMERS

Hydroxylated biphenyls are widely present in nature and represent an important source of bioactive compounds that has not been well investigated. Structurally they are dimers of phenols where two aromatic rings are C-C single bond bridged. The presence of hydroxylated functionalities in the biphenyl structure provides interesting features in terms of bioavailability, interactions with proteins and antioxidant activities. Continuing our research on hydroxylated biphenyls, a study of some derivatives of natural and natural-like catechols: caffeic acid (CA), hydroxychavicol (HCh) and (E)-4-(3,4-dihydroxyphenyl)but-3-en-2-one (HPh) and their corresponding C2-symmetrical dimers (DCA, DHCh and DHPh) was performed to examine the antioxidant potential of the handpicked monomeric and dimeric compounds.  Chain-breaking antioxidant activities of compounds under study were determined from the kinetic curves of bulk lipid autoxidation. Since dimer of caffeic acid DCA and also DHPh are not lipid soluble substances, their antioxidant activity was tested in presence of DMSO. The effect of concentration (0.1 and 1.0 mM), solvent (in absence and in presence of DMSO), substituents of the phenolic ring (catechol and guaiacyl unit) as well as the effect of side chains is presented. Antioxidant potential of the new catecholic monomers was compared with those of corresponding ortho-methoxyphenols: ferulic acid (FA), eugenol (Eu) and dehydrozingerone (DHZ), previously studied by us.1,2 New order of antioxidant effectivity was obtained: in presence of DMSO: HPh >>CA > DHPh >> DCA and in absence of DMSO: HPh > HCh > CA > Eu > FA > DHZ. DFT calculations at B3LYP/6-31+G** level were carried out in order: 1) to optimize the geometry of neutral species and the phenoxyl radicals that can be generated from the studied compounds; and 2) to calculate the BDEs for the generation of the respective radicals.