Navigating the Complex Solid Form Landscape of the Quercetin Flavonoid Molecule

Quercetin, a naturally occurring bioflavonoid substancewidelyused in the nutraceutical and food industries, exists in various solidforms that can have different physicochemical properties, thus impactingthis compound's performance in various applications. In thiswork, we will clarify the complex solid-form landscape of this molecule.Two elusive isostructural solvates of quercetin were obtained fromethanol and methanol. The obtained crystals were characterized experimentally,but the crystallographic structure could not be solved due to theirhigh instability. Nevertheless, the desolvated structure resultingfrom a high-temperature treatment (or prolonged storage at ambientconditions) of both these two labile crystals was characterized andsolved via powder X-ray diffraction and solid-state nuclear magneticresonance (SSNMR). This anhydrous crystal structure was compared withanother anhydrous quercetin form obtained in our previous work, indicatingthat, at least, two different anhydrous polymorphs of quercetin exist.Navigating the solid-form landscape of quercetin is essential to ensureaccurate control of the functional properties of food, nutraceutical,or pharmaceutical products containing crystal forms of this substance.