Molecular salts of quinine: a crystal engineering route to enhance the aqueous solubility

The antimalarial drug quinine (QUN) has poor aqueous solubility and belongs to Biopharmaceutical Classification System (BCS) Class-II. We report 12 novel molecular salts of QUN with α,ω-aliphatic dicarboxylic acids and aromatic coformers. The high basicity of QUN and ΔpKa of ∼5 make the complexes ionic, and most of them are hydrates. The solid forms showed enhanced aqueous solubility compared to the pristine QUN. The single-crystal and powder X-ray diffraction, thermal, and microscopy data provide structural, compositional, and stability profiles of the salts. The calculated Full Interaction Maps (FIMs) provide statistical insights into the salt formation and high probability of hydration in QUN. Though with prospective torsional freedom, QUN in most complexes adopts a unique conformation; this indicates that the structure class has a higher statistical probability and belongs to a relatively deep potential energy trough in the vast crystal landscape.