The structure-activity relationship of flavonoids as inhibitors of xanthine oxidase and as scavengers of the superoxide radical, produced by the action of the enzyme xanthine oxidase, was investigated. The hydroxyl groups at C-5 and C-7 and the double bond between C-2 and C-3 were essential for a high inhibitory activity on xanthine oxidase. Flavones showed slightly higher inhibitory activity than flavonols. All flavonoid derivatives except isorhamnetin (30) were less active than the original compounds. For a high superoxide scavenging activity on the other hand, a hydroxyl group at C-3' in ring B and at C-3 were essential. According to their effect on xanthine oxidase and as superoxide scavengers, the flavonoids could be classified into six groups: superoxide scavengers without inhibitory activity on xanthine oxidase (category A), xanthine oxidase inhibitors without any additional superoxide scavenging activity (category B), xanthine oxidase inhibitors with an additional superoxide scavenging activity (category C), xanthine oxidase inhibitors with an additional pro-oxidant effect on the production of superoxide (category D), flavonoids with a marginal effect on xanthine oxidase but with a prooxidant effect on the production of superoxide (category E), and finally, flavonoids with no effect on xanthine oxidase or superoxide (category F).

The superoxide scavenging activities of 12 flavonoids were measured. The superoxide anions were generated by a hypoxanthine-xanthine oxidase system and measured by the nitrite method. The results showed that the scavenging ability enhanced with an increasing number of hydroxyl groups in ring B. Substitution at C3 position with a hydroxyl group increased the activity. Compared to a methoxyl group or a glycoside in this position, a free hydroxyl group showed the highest activity. A saturated C2-C3 bond showed a higher activity than a unsaturated bond. The absence of a carbonyl group at C4 position increased the activity.

A bench-scale study was performed to investigate formation and speciation of halonitromethanes (HNMs) in raw and treated waters obtained from a drinking water treatment plant. HNM species were measured after chlorination, ozonation-chlorination, chloramination, and ozonation-chloramination, and compared with trihalomethanes (THMs). Pre-ozonation before chlorination resulted in enhanced HNM formation, producing trihalogenated HNMs as major species. Formation of THMs showed very a different formation pattern from HNM formation such that it was much higher in the raw than the treated water, and decreased after pre-ozonation. These findings indicated that precursors and formation pathways for HNMs and THMs are not the same. Increases in pH and bromide concentrations increased HNM and THM formation during ozonation-chlorination and THM formation during chlorination. The bromide effect shifted the formation of HNMs and THMs toward brominated species, with its impact being greater in the treated than raw water. On the other hand, there was no pH or bromide effect on HNM formation during chlorination. The presence of nitrite increased HNM formation under both chlorination and ozonation-chlorination conditions, but it had no influence on THM formation. HNM formation during ozonation-chloramination remained about 1 microg/L level even at the highest bromide and nitrite concentrations. Monochloramination alone did not form any measurable HNMs. The results indicated that the use of chloramine can be an effective way to minimize HNM formation at typical drinking water treatment conditions.

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