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The solubility of quercetin, isoquercitrin, rutin, chrysin, naringenin, and hesperetin was quantified in acetonitrile, acetone, and tert-amyl alcohol. The solubility was strongly affected by both the nature of the solvent and the flavonoid structure. The highest solubility was obtained in acetonitrile for hesperetin (85 mmol·L-1) and naringenin (77 mmol·L-1) and in acetone (80 mmol·L-1) for quercetin. The lowest solubility value was obtained with rutin in acetonirile (0.50 mmol·L-1). The thermodynamic properties of flavonoids were also measured (melting point, enthalpy of fusion, and solid heat capacity) and predicted (liquid heat capacity, solid phase activity, and activity coefficient). Glycosylated flavonoids are characterized by a low melting point and a high enthalpy of fusion compared to the aglycon ones. Contrary to the data reported for other compounds, there is no clear correlation between the solubility of flavonoids and their thermodynamic properties. However, the conformational study showed that the flavonoids having a torsion angle OC2C1‘C6‘ of 40° are characterized by a high solubility.

The conversion yield at equilibrium, the initial rate, and the regioselectivity of the enzymatic acetylation of aglycone flavonoids (quercetin, naringenin, hesperetin, and chrysin) were investigated and compared to those obtained with a glycosylated one (isoquercitrin). The effects of a wide range of operating conditions were quantified. Fourier transform infrared spectrometry (FT-IR), NMR, and high performance liquid chromatography electrospray ionization mass spectrometry (HPLC-ESI-MS) analyses showed that for glycosylated flavonoids, in the presence of Candida antarctica (CAL-B), the acetylation occurred on the 2''-OH, 3''-OH, and 6''-OH of the glucose part, while with Pseudomonas cepacea lipase (PSL-C) acetylation takes place on 6''-OH of the sugar and 4'-OH of the B-ring. For aglycone flavonoids, the acetylation occurred only with PSL-C on 4'-OH, 3'-OH, and 7-OH hydroxyls. The conversion yield and the number and the relative proportions of the synthesized products were found dependent on the nature of the enzyme, the molar ratio, and the flavonoid structure. The initial rate was affected only by the origin of the enzyme.

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