Crystal Engineering of Pharmaceutical Co-crystals: Application of Methyl Paraben as Molecular Hook

Applicability of the O−H···N heterosynthon for synthesis of a pharmaceutical co-crystal comprised of a commonly used tablet excipient methyl paraben and quinidine, an anti-malarial constituent of Cinchona tree bark, has been successfully demonstrated. Insights into local conformation and hydrogen-bonding were derived from advanced multinuclear solid-state NMR techniques, where interpretation of the obtained NMR data was supported by DFT quantum-chemical computations. Furthermore, an approach for selective separation of quinidine from its stereoisomer quinine based on the molecular specificity of methyl paraben is presented. It was found that methyl paraben picked its target via hydrogen-bond-mediated molecular recognition, thereby acting as “molecular hook”.