The photostabilities of naturally occurring 5-hydroxyflavones, flavonols, their glycosides and their aluminium complexes

The photostabilities of luteolin, in solution, in the presence of aluminium ions, and deposited on a cellulosic substrate have been determined and com pared with those of quercetin and other 5-hydroxyflavonols and their 3-O-gl ycosides. In aqueous methanol solution, luteolin and flavonol 3-glycosides exhibited no degradation over periods of up to 15 h of UV irradiation. However, the f lavonols studied were all found to degrade and their relative photostabilit ies correlate with their redox potentials. Quercetin was the least stable. In the presence of aluminium ions, all the flavonoids, including luteolin, were degraded by UV irradiation. In contrast to the absorption spectra in dilute solution, the reflectance s pectra of both quercetin and luteolin deposited on a cellulosic substrate e xhibited strong absorptions beyond 400 nm. On this substrate these flavonoi ds displayed the characteristic yellow colour associated with flavonoids in some environments. Although the quercetin yellow faded rapidly on exposure to UV radiation, the colour of luteolin darkened. This was due to the form ation of a photoproduct absorbing maximally at 450 nm. The relevance of these observations to cellulosic dyeing and flower coloura tion are discussed. (C) 2000 Elsevier Science S.A. All rights reserved.