Structural and Physicochemical Studies of Olopatadine Hydrochloride Conformational Polymorphs

Crystal and molecular structures of 2 conformational polymorphs (forms I and II) of olopatadine hydrochloride, an antiallergic agent, are presented. Both crystal modifications crystallize in the monoclinic crystal system with 1 olopatadine hydrochloride molecule in the Z configuration in the asymmetric unit. Molecules are arranged into the centrosymmetric association through the interactions of the intermolecular strong and weak hydrogen bonds of N-H…Cl, O-H…Cl and C-H…Cl, C-H…O types. Conformational change between polymorphs is proved by calculations of a maximum torsion angle deviation (max[Δθ]) and a root-mean-square deviation between the atomic positions (rmsd[r]). The physicochemical characterization of polymorphs is performed by X-ray powder diffraction, infrared and Raman spectroscopy, differential scanning calorimetry. The comparison of the melting points and heats of fusions shows that the forms are monotropically related.