title: On the stability of the bioactive flavonoids quercetin and luteolin under oxygen-free conditions.
abstract: The natural flavonoid compounds quercetin (3,3',4',5,7-pentahydroxyflavone) and luteolin (3',4',5,7-tetrahydroxyflavone) are important bioactive compounds with antioxidative, anti-allergic, and anti-inflammatory properties. However, both are unstable when exposed to atmospheric oxygen, which causes degradation and complicates their analytical determinations. The oxidative change of these flavonoids was observed and followed by UV-visible spectrophotometry, both in aqueous and ethanolic solutions. The distribution of the degradation products in aqueous media was monitored by LC-MS and LC-DAD analysis. The amounts of oxidative reaction products increase with the exposure time. The oxidative degradation reduces the pharmacological efficiency of these antioxidants and renders analytical determination inaccurate. The oxidative changes in flavonoid test solutions can explain the inconsistent dissociation constants reported in the literature. Dissociation constants of quercetin and luteolin were determined both by alkalimetric titration and by UV-visible spectrophotometry under deaerated conditions. The values pK(1) = 5.87 ± 0.14 and pK(2) = 8.48 ± 0.09 for quercetin, and pK(1) = 5.99 ± 0.32 and pK(2) = 8.40 ± 0.42 for luteolin were found.
authors: J. Heyrovský Institute of Physical Chemistry, vvi, Academy of Sciences of the Czech Republic, Prague, Czech Republic.
authors: Sárka
authors: S
authors: Ramešová
authors: Romana
authors: R
authors: Sokolová
authors: Ilaria
authors: I
authors: Degano
authors: Jana
authors: J
authors: Bulíčková
authors: Ján
authors: J
authors: Zabka
authors: Miroslav
authors: M
authors: Gál
mesh_terms: D000975:Antioxidants / Q000032:analysis / Q000737:chemistry; D047311:Luteolin / Q000032:analysis* / Q000737:chemistry; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010100:Oxygen; D011794:Quercetin / Q000032:analysis* / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000975:Antioxidants; D011794:Quercetin; D047311:Luteolin; D010100:Oxygen