Flavonoids are assumed to have beneficial effects due to their antioxidant properties. The catechol moiety present in numerous flavonoids is oxidized during the antioxidative reaction yielding a quinone. Quinones are toxic due to their ability to react, e.g. with thiol groups. The 3,5,7-trihydroxy-4H-chromen-4-one group is another antioxidant pharmacophor in certain flavonoids. During the antioxidative reaction this group is also oxidized. The aim of the present study is to determine the thiol reactivity of this oxidized group. Galangin is a flavonoid that only contains the 3,5,7-trihydroxy-4H-chromen-4-one group as the antioxidant pharmacophor. Incubation of galangin with horseradish peroxidase/H(2)O(2) leads to an oxidation product which after addition of glutathione is instantaneously converted to an adduct. Based on these results it is expected that--similar to the catechol containing antioxidants--the 3,5,7-trihydroxy-4H-chromen-4-one containing antioxidants shift the damage provoked by oxidative stress from lipid peroxidation to thiol arylation. This should be considered in application of these types of antioxidants.