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 (pKa=6.9; 8.6; 10.3) and two for apigenin (pKa=6.6; 9.3). Fluorescence emission was very weak or undetectable (ΦF<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 105–107 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 (ΦF∼1 for luteolin and ∼10−2 for apigenin).
Acidichromism and Ionochromism of Luteolin and Apigenin, the Main Components of the Naturally Occurring Yellow Weld: a Spectrophotometric and Fluorimetric Study
FAVARO, Giovanna;CLEMENTI, CATIA;ROMANI, Aldo;
2007
Abstract
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 (pKa=6.9; 8.6; 10.3) and two for apigenin (pKa=6.6; 9.3). Fluorescence emission was very weak or undetectable (ΦF<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 105–107 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 (ΦF∼1 for luteolin and ∼10−2 for apigenin).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
