The structure-activity relationships between flavonoids and their tracheal relaxant action are little known. In the present study, 26 natural and synthetic flavonoids, divided into the five classes of flavones, flavonols, flavanones, isoflavones, and chalcones, were tested, and their IC50 values were determined. The IC50 values of these five classes indicated that flavones were more potent than flavonols. Flavones were also more potent than flavanones suggesting that the presence of a double bond between C-2 and C-3 is important. However, flavones were similar to isoflavones in potency. Chalcones, a class with an open C-ring, appeared to be the least potent among these five classes. Introduction of a hydroxy group at position C-6 of flavones increases their relaxant activities. So does adding a hydroxy group at position C-7 of flavones. It appears that the optimal number of hydroxy groups introduced to the A-ring of flavones is one. As more hydroxy groups are introduced to positions at C-5, C-6, and/or C-7 of flavones, the IC50 values increase. It seems that flavones or flavonols with a pyrogallol moiety either in the A- or B-ring, respectively, have no activity. It appears that flavonols with ortho-hydroxy groups in the B-ring are more potent than those with meta-hydroxy groups. The activity of 6-hydroxyflavone disappears if the C-6 hydroxy group of the A-ring is methoxylated. If the C-4′ hydroxy group of the B ring is methoxylated, the relaxant effect of these flavones is also attenuated or disappears. Therefore, the hydroxy group on either the A- or B-ring of flavones and flavonols being methylated resulted in lower potency of the tracheal relaxant effects. However, when all hydroxy groups on both the A- and B-rings of flavones or flavonols are methoxylated this results in higher potency. Therefore, the influence of methoxylation in flavones may be similar to that in flavonols. However, if the C-3 hydroxy group on the C-ring of flavonols, but not flavones which lack this hydroxy group, is methoxylated, the relaxant effects may increase. Glycosylation of the hydroxy group at position C-7 of flavones or flavanones attenuates the relaxant effects.
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