Sensitive spectrophotometric assay for 3-hydroxy-substituted flavonoids, based on their binding with molybdenum, antimony, or bismuth

A sensitive spectrophotometric assay has been developed for flavonoids base d on their binding with molybdenum, antimony, or bismuth. Acetylation of th e hydroxyl group of flavonoids abolished metal binding, thus suggesting a d irect role of the hydroxyl groups. From a comparison of several related fla vonoids differing in the position of hydroxyl substitutions, the hydroxyl g roup at position 3 was found to be an important requirement for the formati on of a yellow complex. This flavonoid metal complex showed that a specific and significant bathochromic shift in the visible spectrum of the native f lavonoid and the corresponding lambda(max) value was used for the colorimet ric assays with different metal salts. The molybdenum complex was found to yield higher absorbance compared to antimony and bismuth complexes of vario us flavonoids. The present method offers a sensitive assay in the 5-25 nM r ange for these flavonoids and gave comparable results with HPLC quantitativ e determination.