Flavonoid biocides: Structure/activity relations of flavonoid phytoalexin analogues

Two sets of semisynthetic flavonoid phytoalexin analogues were synthesized and their lipophilicity ( R M ) correlated with their antifungal and antibacterial activity. Two natural phytoalexins, pisatin and maackiain, were also tested under these same conditions, and their R M 's determined. The observed structure/activity relationships suggest that these analogues, and the phytoalexins, function primarily as uncouplers of oxidative phosphorylation. The relative acidity and number of hydroxyl groups per molecule appear to be the main factors affecting the antifungal activity of flavonoids. with the bacterium, Streptococcus faecium, there is not the same simple correlation between lipophilicity and antibacterial activity for the two sets of analogues, implying they have different modes of action.