Studies on mechanism of thermal crystal transformation of sitafloxacin hydrates through melting and recrystallization, yielding different anhydrates depending on initial crystalline forms

The polymorphic and pseudopolymorphic forms of sitafloxacin, a novel fluoroquinolone antibiotic, were characterized by infrared spectroscopy, X-ray diffractometry, and thermal analysis. Hydrates of sitafloxacin underwent thermal transformation during the course of heating to 300 °C. Monohydrate melted at 130 °C and crystallized at 147 °C to yield α-form (anhydrate) while sesquihydrate melted at 127 °C and crystallized at 146 °C to yield β-form (anhydrate). The crystal structural analysis revealed that monohydrate and sesquihydrate had opposite torsion at quinolone ring and the conformation of quinolone ring tended to be retained during hydrates to anhydrates crystal conversion. The infrared spectroscopy showed that hydrates and anhydrate α-from exists in zwitterion while β-from is consist of neutral molecule. Detail investigation of thermal behavior of hydrates suggested that water vapor also affected anhydrous crystal forms obtained by heating hydrates, though promoting ionization at carboxyl group and amine group.

Graphical abstractTorsion of quinolone ring (left) and conformation of 5-membered ring of sitafloxacin observed in monohydrate and β-form. Upper: observed in monohydrate, lower: observed in β-form. Quinolone ring in monohydrate takes opposite torsion to that in β-form. Amino group takes equatorial position in monohydrate while it takes axial position in β-form.Figure optionsDownload full-size imageDownload as PowerPoint slide