Adsorption of simple flavonoids: Heterogeneous isomerization of flavanone in 2'-hydroxychalcone

Adsorption experiments with flavanone, flavone, 3-hydroxyflavone, and 2'-hydroxychalcone dissolved in cyclohexane were carried out by using hydroxyapatite and alumina as adsorbents. The data fitted Langmuir's isotherms, allowing us to determine the strength of adsorption for each substrate. The results are explained on the basis of the reactivity indexes and energetic magnitudes of the flavonoids calculated by the AM1 semiempirical molecular orbital method using optimized molecular geometries. Flavonoid adsorption is generally weaker on hydroxyapatite than on alumina. For hydroxyapatite, the polarity of the molecules is a determining factor in the degree of adsorption of compounds. The carbonylic oxygen atom seems to be responsible for these adsorptive properties. On the other hand, in the case of alumina the adsorption is due mostly to interaction forces involving H bonds. On alumina the conversion of flavanone to 2'-hydroxychalcone was detected, Under the operation conditions employed, the percentage conversion of flavanone to 2'-hydroxychalcone was about 50%. A four-step reaction mechanism that involves enolization, adsorption, chemical reaction, and desorption is proposed. (C) 1996 Academic Press, Inc.