The hemiacetal–cis-chalcone equilibrium of malvin, a natural anthocyanin

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Abstract
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 °C τ-values were found to be in the range 1s (for the more acidic solutions) to 2 × 10−4 s (for the neutral solutions). Both buffered and unbuffered solutions were investigated. It was found that τ is largely dependent upon pH and upon 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 °C, the constant K = [CE]/[B] 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. Keywords: flavonoids, malvin, chalcone, catalysis, temperature jump.

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