Acidichromism and Ionochromism of Luteolin and Apigenin,the Main Components of the Naturally Occurring Yellow Weld: A Spectrophotometric and Fluorimetric Study |
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Authors: | G Favaro C Clementi A Romani V Vickackaite |
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Institution: | (1) Department of Chemistry, University of Perugia, 06123 Perugia, Italy;(2) Department of Analytical and Environmental Chemistry, Vilnius University, 03225 Vilnius, Lithuania |
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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 (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 (Φ
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 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 (Φ
F ∼ 1 for luteolin and ∼10−2 for apigenin). |
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Keywords: | Luteolin Apigenin UV-visible spectrophotometry UV-visible fluorimetry |
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