Acidichromism and ionochromism of luteolin and apigenin, the main components of the naturally occurring yellow weld: A spectrophotometric and fluorimetric study

Luteolin and apigenin, extracted from Reseda luteola L., were spectrophotometrically and fluorimetrically studied. The spectra were investigated as a function of pH in methanol/water solutions ( 1/2, v/v) in the 2 -12 pH range. The absorption spectra markedly shifted to the red by increasing the pH. Three acid-base dissociation steps were detected for luteolin ( pK(a)= 6.9; 8.6; 10.3) and two for apigenin ( pK(a)= 6.6; 9.3). Fluorescence emission was very weak or undetectable ( Phi(F) < 10(-4)) in acidic solution, but increased in intensity with increasing the pH. Both molecules exhibited a great propensity towards complex formation with metal ions, with association constants on the order of 10(5) - 10(7) for the first complexation step; in the presence of excess Al3+ ions, multiple equilibria were detected. A marked fluorescence enhancement was observed upon complexation with Al3+ ions ( Phi(F) similar to 1 1 for luteolin and similar to 10(-2) for apigenin).