Polymorphs of the Antiviral Drug 6-[2-(1<i>H</i>-Imidazol-4-yl)ethylamino]-5-oxopentanoic Acid (C₁₀H₁₅N₃O₃, IPA): Crystal Structures, DFT Studies, NMR, and Vibrational Spectra

6-[2-(1H-Imidazol-4-yl)ethylamino]-5-oxopentanoic acid (IPA) is a nontoxic broad-spectrum antiviral drug approved in Russia for treatment and prophylaxis of seasonal influenza and other acute respiratory viral infections. Although it has been used since 2007, data about its crystal structure(s) are still missing. Herein, we report on crystal structures; thermal analysis; 15N and 13C NMR, FTIR, and Raman spectra; and DFT studies of two forms of IPA. Vibrational spectra also revealed the presence of additional forms in solids. Structural analysis demonstrated similarity of molecular structures and H-bonded dimers in forms I and II and allowed us to suggest possible relations between an H-bonded framework and layers observed in these forms. DFT studies indicate that form II is 0.84 kJ/mol more stable than form I. Pressure-dependent FTIR study revealed the irreversible transition of polymorph II into I at 28 GPa.