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

• Published in: Journal of fluorescence Vol. 17; no. 6; pp. 707 - 714
• Main Authors: Favaro, G, Clementi, C, Romani, A, Vickackaite, V
• Format: Journal Article
• Language: English
• Published: Netherlands 01.11.2007
• Subjects: Aluminum; Apigenin - chemistry; Cations; Fluorescent Dyes - chemistry; Hydrogen-Ion Concentration; Luteolin - chemistry; Pigments, Biological - chemistry; Resedaceae - chemistry; Spectrometry, Fluorescence; Spectrophotometry; Textiles
• ISSN: 1053-0509; 1573-4994
• DOI: 10.1007/s10895-007-0222-0

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 Al(3+) ions, multiple equilibria were detected. A marked fluorescence enhancement was observed upon complexation with Al(3+) ions (Phi (F) approximately 1 for luteolin and approximately 10(-2) for apigenin).