The production and development of many diseases is closely related to oxidative damages caused by free radicals.So it becomes one of investigating hotspots that safe and effective antioxidants in plants are searched.And the spectrophotometry for detecting1

1-diphenyl-2-picryl-hyrazyl radical(DPPH

·

) is a rapid

simple and sensitive method for screening antioxidants.Therefore

the structure-activity relationship of eleven pure and natural flavonoids scavenging DPPH

·

were studied by using the spectrophotometry for detecting DPPH

·

.The results showed that all of the eleven natural flavonoids can scavenge DPPH

·

effectively.The DPPH

·

-scavenging ability in turn was: quercetin

>

heliosin

>

catechin

>

hyperoside

>

rutin

>

kaempferol

>

morin

>

isoquercitroside

>

baicalin

>

nevadensin

>

genistein.Therefore it was discovered that the structure-activity relationship of the eleven natural flavonoids scavenging DPPH

·

was as follows: 1.Bring and/or Aring with adjacent hydroxyl groups could greatly increase their DPPH

·

-scavenging ability;2.The hydroxyl groups in 4′position on the Bring and the hydroxyl group in

6

position on the Aring possessed the high DPPH

·

-sca-venging activity;3.The DPPH

·

-scavenging activities of isoflavonoids were weaker than those of corresponding flavonoids;4.The DPPH

·

-scavenging activities of flavonols were stronger than those of corresponding flavanonols. It suggested that Cring with C

2

-C

3

double bond could enhance its DPPH

·

-scavenging effect; 5.The DPPH

·

scavenging ability of flavonoids was closed related to the positions of their hydroxyl groups; 6.When the hydroxyl group at 3 position on the Cring was glycosylated

its DPPH

·

-scavenging activity was decreased.These results provide a theoretical basis for further exploiting and utilizing flavonoids.