Conformational changes of apigenin 7-O-(6-O-malonylglucoside), a vacuolar pigment from parsley, with solvent composition and proton concentration.

Malonylated apigenin 7-O-glucoside was prepared from malonyl-CoA and apigenin 7-O-glucoside using a malonyltransferase from parsley cell cultures. The enzyme product, as well as the flavonoid substrate, was analyzed by high-resolution nuclear magnetic resonance spectroscopy, confirming that malonic acid had been attached to the primary hydroxyl of the glucose moiety of apigenin 7-O-glucoside. During these studies in several solvents, it became apparent that, in particular, the chemical shift of H-6" A and the coupling constants J5",6"A and J5",6"B were dependent on solvent composition and proton concentration. Similar changes of sugar proton resonance frequencies were also observed with methyl 6-O-malonyl-beta-D-glucopyranoside, but not with apigenin 7-O-[6-O-(4-coumaroyl)glucoside]. The change of coupling constants is ascribed to a conformational modification of the sugar portion in the malonylglucosides. Malonylated flavonoid glycosides are exclusively located within the vacuoles of parsley cells. We propose that acylation of flavonoid glycosides which are synthesized in the cytoplasm facilitates transport of the substrates into the vacuole. The conformational modification which is induced by changes in proton concentration may provide a mechanism to trap flavonoids in situ.