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 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).
Favaro
G
G
Department of Chemistry, University of Perugia, Perugia, Italy. favaro@unipg.it
Clementi
C
C
Romani
A
A
Vickackaite
V
V
D000535:Aluminum; D047310:Apigenin / Q000737:chemistry*; D002412:Cations; D005456:Fluorescent Dyes / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D047311:Luteolin / Q000737:chemistry*; D010860:Pigments, Biological / Q000737:chemistry*; D031951:Resedaceae / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry; D013784:Textiles
D002412:Cations; D005456:Fluorescent Dyes; D010860:Pigments, Biological; D047310:Apigenin; D000535:Aluminum; D047311:Luteolin
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