Dependence of Flavonol Functions on Their Chemical Structure in Chloroplast Energy Reactions

Abstract

The effect of a number of flavonols (quercetin, morin, quercetin-glucoside, quercetin-glucosylp-coumarate, kaempferol-glucoside, and kaempferol-glucosyl-p-coumarate) on electron and proton transfer reactions as well as on ATP synthesis and hydrolysis in pea chloroplasts has been studied. It was shown that NADP+ photoreduction and coupled photophosphorylation by chloroplasts were suppressed by flavonols within the concentration range of 10-5-10-4 M. The inhibitory effect was weakened as the structure of the substances became more complicated. The inhibition of ATPase activity of CF1 was decreased after glucosylation and acylation of the aglycones. Quercetin increased the values for proton uptake. On the contrary, QuGC and morin decreased H+ uptake and increased the proton gradient dissipation constant. Dissociation constants of flavonol OH-groups were calculated and experimentally determined. The dissociation constants of ionogenic groups were pK2, = 5.1; pK3, = 8.6; pK4, = 9.0-9.4; pK5 > 10; pK7 = 7.4-7.6. Flavonols form unstable complexes with metal ions in the presence of Mg2+ and Ca2+. At pK > 10 OH-groups take part in the complex formation. Regulatory aspects of flavonol function mechanism in chloroplast energy conserving reactions are discussed.