The role of hydroxyl group of ethanol in the self-assembly of pharmaceutical cocrystal of myricetin with 4,4′-bipyridine

In order to further explore the role that hydroxyl plays in synthesizing and improving physicochemical properties of pharmaceutical cocrystals of flavonoids, an unreported pharmaceutical cocrystal (MYR·BPY·EtOH) of myricetin (MYR) with 4,4′-bipyridine (BPY) has been synthesized and characterized. However, something unexpected happened. MYR·BPY·EtOH is a cocrystal with supramolecular helical structure which is different from the supramolecular layered structure of MYR·BPY·H2O that we have reported on. This may be due to the O9-H9···O6 hydrogen bond, which was formed by the hydroxyl of the ethanol molecule and the 6-oxygen atoms of the MYR, maintains the cocrystal helical structure. Additionally, MYR·BPY·EtOH shows better solubility in comparison to pure MYR and MYR·BPY·H2O. but the hygroscopic stability of MYR·BPY·EtOH and MYR·BPY·H2O are lower than that of pure MYR. It may be compared with planar MYR and layered-structured MYR·BPY·H2O, helically-structured MYR·BPY·EtOH is more likely to allow water molecules to enter. This study implies that we should not ignore the influential that hydroxyl groups of solvent molecules have on the structures and properties of flavonoid pharmaceutical cocrystals when considering the influence of hydroxyl groups of the A, B, and C rings of flavonoid on the cocrystal structure of flavonoids.