An experimental approach to structure-activity relationships of caffeic and dihydrocaffeic acids and related monophenols

The importance of a catechol moiety to the radical-scavenging activity of p-hydroxycinnamic derivatives is widely accepted, whereas the role of the carbon side chain remains controversial. Extension of the conjugation to this chain is a molecular feature that requires some attention. Differences in the activity of caffeic and dihydrocaffeic acids were examined using a series of experimental procedures: assays using free radicals [1,1-diphenyl-2-picrylhydrazyl (DPPH•) and 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS•+)]; assays based on redox reactions; and accelerated tests in bulk oils and dispersed systems. To reinforce observations on the role of the side chain, a group of related monophenols, differing in one double bond and/or its position in the chain, was also examined. Extended conjugation was essential for the rapid scavenging of free radicals and for effectiveness in dispersed systems (both liposomes and emulsions). In the case of bulk oils, the contribution of conjugation remained unclear. Information based on redox reactions was not useful for the aim of our study. The experimental conditions may mask or enhance the effect of the chain characteristics on the activity, so the size and the order of activity may be influenced. Thus, structure-activity relationships should be drawn with caution.