Tautomerism of flavonol glucosides: relevance to plant UV protection and flower colour

Abstract

The fluorescence emission spectra of the naturally occurring flavonoids, quercetin-7-glucoside, a 3′,4′-dihydroxyflavonol, and kaempferol-7-glucoside, a 4′-hydroxyflavonol, have been determined as a function of concentration in aqueous solutions. These spectra indicate that the extent of keto–enol phototautomerism in both flavonoids is greatest at high concentrations: a situation which favours molecules aggregation/dimerization. Such behaviour is consistent with phototautomerism being facilitated by a concerted, intermolecular transfer of protons between the partners in the flavonoid dimer. This excited state tautomerism dissipates absorbed energy harmlessly and as such provides a possible mechanism by which these molecules may function in the protection of plants from damaging UV radiation. The fluorescence excitation spectra of both kaempferol and quercetin-7-glucosides at high concentrations in aqueous solutions indicate the presence of significant amounts of the enolic tautomeric form in the ground-state. At lower concentrations only the kaempferol glucoside spectrum shows this. When kaempferol- and quercetin-7-glucosides were deposited on a cellulosic support, their reflectance and fluorescence excitation spectra could be resolved into contributions from keto and enol ground-state chromophores. The absorption of the enolic tautomer is at longer wavelengths (ca. 450 nm) than that of the keto tautomer (ca. 370 nm) and as it extends into the blue spectral region, would account for the yellow appearance of these flavonols in aggregation on cellulose and in concentrated solution in petal vacuoles.