Photoreactivity and stability of flavonoid yellows used in cultural heritage

Flavonoid yellows are historical dyes widely found in cultural heritage, in particular those based on the glycosides of luteolin, quercetin and kaempferol. In this research, for the first time, the photodegradation quantum yields (Φ_R) were calculated for luteolin, quercetin and kaempferol as well as their glycosides Lut-7-O-Glc, Que-3-O-Glr, Kaem-3-O-Glc and compared to the reactivity of eriodictyol. The results are discussed within the current state of the art on the mechanisms of degradation for these antioxidant compounds, which are reviewed in the introduction. Φ_R values were obtained in solution irradiating at 313 nm and 366 nm, providing the quantification of the stability of flavonoid yellows. In particular, by irradiating at 366 nm, a very good stability scale is obtained, with Φ_R values for luteolin and the 3-O-glycosides of quercetin and kaempferol on the order of 10⁻⁶. The more reactive quercetin and kaempferol are characterized by Φ_R values of ca. 3 × 10⁻⁵. The relative stability of these yellows can be explained by a photoprotective mechanism based on excited state proton transfer, which is more efficient when there is an OH in the C5 position. In addition, electron transfer, and consequently oxidation, is also enhanced through the OH in position C3 when compared to C5. Solvents effects were also studied and to mimic the environment of the dye in wool, irradiation at 366 nm in a proteinaceous gel was carried out. In this gel, the Φ_R values increased for all the molecules studied, reaching 1 × 10⁻⁴ and 2 × 10⁻⁴ for kaempferol and quercetin, respectively. This drastic enhancement shows that the environment has a major impact on stability. The main degradation products were also studied by irradiation with a xenon source (λ_irr ≥ 300 nm), and characterized by HPLC-DAD-MS and LC-HRMS/MS. These data, on the reaction mechanism, show that the double bond between C2 and C3, in the C ring, is the most reactive point of the molecules, as in all the flavonoids degradation started by solvent attack, followed in some cases by ring opening and the formation of low molecular weight hydroxyl compounds such as benzoic acids, in agreement with the general reaction scheme proposed in literature.