Kinetic and Thermodynamic Control of Flavylium Hydration in the Pelargonidin-Cinnamic Acid Complexation. Origin of the Extraordinary Flower Color Diversity of Pharbitis nil

During the past decade, structural elucidation of heavily substituted anthocyanins present, for instance, in the bright ornamental flowers, has brought to light the role played by sugar and phenolic acid residues in the fascinating pigmentation properties of such natural molecules. It now appears that higher plants have developed in their flowers and fruits extremely sensitive and powerful color stabilization and variation mechanisms related to the presence of glycosidic acylated anthocyanidins. In these molecules, a sugar unit, bearing the anthocyanidin chromophore and a cinnamic acid residue, brings sufficient flexibility for the latter two moieties to interact through complexation, according to a mechanism called intramolecular copigmentation. Here, on the basis of UV-visible spectroscopic measurements, we give kinetic and thermodynamic evidence supporting the existence of folded conformations which involve the stacking of either one cinnamic acid residue on the anthocyanidin chromophore or two cinnamic acid residues on both sides of the chromophore (sandwich-type association). Pigments investigated in this work were obtained from red-purple cultivars of Pharbitis nil (morning glory). They correspond to four pelargonidin derivatives (1–4): one which is not acylated (reference compound 1), two of the monoacylated type (2 and 3), and one diacylated (4). The cornerstone of our study rests on the hydration reaction of the anthocyanidin chromophore when in its flavylium form. Indeed, the kinetic and thermodynamic parameters of this reaction and the way they are affected by the presence of one or two cinnamic acid residue(s) is of considerable value in the understanding of anthocyanin intramolecular complex formation. Copigmentation more frequently occurs as an intermolecular process, pigment and copigment being in that case two distinct molecules. By running competitive intra- and intermolecular copigmentation experiments, we also demonstrate that the phenomenon in which pigment and copigment are linked together is much more efficient.