TAUTOMERISM OF FLAVONOL GLUCOSIDES - RELEVANCE TO PLANT UV PROTECTIONAND FLOWER COLOR

The fluorescence emission spectra of the naturally occurring flavonoid s, 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 e xtent of keto-enol phototautomerism in both flavonoids is greatest at high concentrations: a situation which favours molecules aggregation/d imerization. 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 d issipates absorbed energy harmlessly and as such provides a possible m echanism by which these molecules may function in the protection of pl ants from damaging UV radiation. The fluorescence excitation spectra o f 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-stale. At lower concentrations o nly 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 absor ption of the enolic tautomer is at longer wavelengths (ca. 450 nm) tha n that of the keto tautomer (ca. 370 nm) and as it extends into the bl ue spectral region, would account for the yellow appearance of these f lavonols in aggregation on cellulose and in concentrated solution in p etal vacuoles. (C) 1998 Elsevier Science S.A. All rights reserved.