Βιοφαινόλες και αντιοξειδωτική ικανότητα, σχέση δομής - δραστικότητας

Abstract

Structure-activity relationship studies are a key factor in selecting suitable biophenols as antioxidants depending on the desirable mode of use. Such relationships for AH have not yet thoroughly investigated. In addition, many methodologies testing the antioxidant activity of AH are re-evaluated concerning their suitability to define SAR among biophenols. In this view, and prior to any SAR studies an investigation of methodologies that estimate the radical scavenging activity (DPPH?, ABTS?+) of biophenols was carried out to find whether they can produce meaningful results for SAR studies. The DPPH? test and especially the one based on the kinetic study of the reaction between the AH and the radical, can produce data that are suitable in SAR studies. On the contrary using the ΑBTS?+ test meaningful results cannot be produced since weak antioxidants are found equally or more active than others known to possess strong antioxidant properties. For this reason the DPPH? test was used mainly for the estimation of the radical scavenging activity in the subsequent SAR studies. Most of the studies on antioxidant activity are focused on the degree of hydroxylation on the aromatic ring and less to other structural features. For this reason, the contribution of structural differences in the carbon side chain of certain biophenols, to their antioxidant activity, was investigated experimentally. When it was necessary a computational approach was also carried out by calculating specific molecular descriptors i.e. BDE and IP in order to complement, if possible, experimental findings. The extension of the conjugation to the side carbon chain of p-hydroxy cinnamic acid derivatives such as caffeic and dihydrocaffeic acids was examined using a series of experimental procedures. To reinforce observations on the role of the extended conjugation, a group of related monophenols, namely isoeugenol, dihydroeugenol and eugenol were also included in the study. Extended conjugation is essential for the rapid scavenging of free radicals. Based on the computational approach, the latter can be related to the lower BDE values of caffeic acid and isoeugenol and their better stabilized phenoxy-radical. The presence of catechol moiety seems to mask the influence of the double bond in the radical scavenging activity since the differences observed between caffeic and its dihydro-counterpart were small when compared to those observed for the corresponding monophenols. Extension of conjugation is necessary for effectiveness in dispersed systems, however in the case of bulk oils its contribution remains unclear. The presence of the double bond at the end of the chain (eugenol) has similar effect on the scavenging of free radicals with the complete absence of the bond (dihydroeugenol).