Free radical scavenging properties of guaiacol Oligomers: A Combined experimental and quantum study of the guaiacyl-moiety role

Natural polyphenols are known to exhibit a lot of different biological, properties, including antioxidant activity. For some polyphenols these activities are attributed to the presence of a guaiacol moiety. In the present paper we focus on the role of this moiety. For this purpose nine different compounds were enzymatically synthesized from, guaiacol. To elucidate the structure - activity relationship of these polyphenols, DFT-(PCM)B3P86/6- 311+G(2d,3pd)//(PCM)B3P86/6-31+G(d,p) calculations supported the experimental DPPH free radical, scavenging activities. The antioxidant activities were correlated to (i) O-H BDEs (bond dissociation enthalpies), (ii) BDE_D (BDE of a second H atom abstraction from the phenoxyradicals), (iii) spin density, (iv) HOMO (highest occupied molecular orbital) distribution, (v) IPs (ionization potentials), (vi) ΔG and ΔG free energies of HAT (H atom transfer), and (vii) HAT rate constants. BDE_D appeared to be the most important descriptor to understand the free radical scavenging ability of these compounds.