Structure–antioxidant activity relationship study of natural hydroxybenzaldehydes using in vitro assays

Structure–activity relationships (SARs) were sought among protocatechuic aldehyde 1, syringaldehyde 2, vanillin 3, p-hydroxybenzaldehyde 4 and salicylaldehyde, 5 using various in vitro antioxidant assays (crocin bleaching assay, ABTS, DPPH, Rancimat and liposomes). These aldehydes are known to contribute to the evolution of flavor during aging of alcoholic beverages in wood barrels and as natural preservatives. Their acid counterparts (1′–5′) were also studied for comparison. The non-substituted hydroxybenzaldehydes (4, 5) and the respective acids showed negligible activity under the CBA, DPPH and the accelerated bulk oil oxidation conditions, so that no SAR could be drawn. The only compound that exhibited high antioxidant activity, equal or even higher than 1′, in all assay environments was 1. Syringaldehyde was found to possess exceptionally high activity in comparison to 2′, only under the CBA conditions. Moreover, 1 and 2 were found to be strong inhibitors of bulk oil or of lecithin liposome oxidation. Our findings add to knowledge about the antioxidant activity of the particular aldehydes apart from their flavoring and antibacterial properties.