83554-49-8Relevant articles and documents
ELUCIDATION OF THE MULTIPLE EQUILIBRIA OF MALVIN IN AQUEOUS SOLUTION BY ONE- AND TWO-DIMENSIONAL NMR
Santos, Helena,Turner, David L.,Lima, Joao C.,Figueiredo, Paulo,Pina, Fernando S.,Macanita, Antonio L.
, p. 1227 - 1232 (1993)
One- and two-dimensional NMR were used to characterize the several forms of malvin present in aqueous solution in the pH range 0.3-4.5 and to determine their molar fractions as a function of pH.In addition to the flavylium cation, two hemiacetal forms and both the cis and trans form of chalcone were firmly identified.The pathways for the interconversion of different forms were derived and and the molar absorbances of the species calculated by coupling NMR and UV/Vis data.Equilibrium constants were determined at different temperatures, and enthalpy and entropy changes were calculated for the interconversion processes.The conclusions are supported by molecular orbital calculations. Key words: Anthocyanins; malvin; multi-equilibria; two-dimensional NMR.
The hemiacetal-cis-chalcone equilibrium of malvin, a natural anthocyanin
Brouillard, Raymond,Lang, Jacques
, p. 755 - 761 (2007/10/02)
In slightly acidic aqueous solutions, malvin essentially exists, in a stable form, as a fast equilibrating reaction between a ring structure, the hemiacetal B, and a chain structure, the cis-chalcone CE.The temperature-jump technique has been used for the determination of the relaxation time τ characterizing this equilibrium.At 20 deg C τ-values were found to be in the range 1 s (for the more acidic solutions) to 2 x 10-4 s (for the neutral solutions).Both buffered and unbuffered solutions were investigated.It was found that τ is largly dependent upon pH andupon the concentration of the buffer.For instance, at constant pH, a plot of the observed first-order rate constant τ-1 versus the acetic acid - sodium acetate buffer concentration shows a nonlinear variation that can only be interpreted as a change in the proton transfer rate-limiting step with the buffer concentration.At 20 deg C, the constant K = E>/ is 0.25 (+/- 0.05) and the enthalpy change ΔH associated with this equilibrium is 20.7 (+/-2) kJ mol-1.Therefore, increasing the temperature greatly enhances the stability of the open cis-chalcone isomer.Possible implications arising from the existence of a fast chemical ring-closure reaction, in the case of natural anthocyanins, for the biosynthesis of flavonoids are discussed.In particular, it is suggested that, along with the main biosynthetic pathway to flavonoids, a minor pathway involving the anthocyanin cis-chalcone CE could also occur under favorable circumstances.Physico-chemical conditions in the plant organelles containing flavonoids (vacuoles) make this alternative plausible.
