| Summary: | Anti-solvent crystallization of carbapenem CS-023 was performed at 25 °C. The following results were obtained: (1) A solvate crystal, Form A (5/2 Ethanol·1/2 H<sub>2</sub>O), was recovered from 80 <i>v</i>/<i>v</i>% ethanol solution; (2) Form A transformed to Form H (4H<sub>2</sub>O) through solid-phase transition through the solvate-free polymorph, Form A-2, and Form A also transformed into Form C (1Ethanol·3H<sub>2</sub>O) through solvent-mediated transformation. In the present study, we found that Form C also transforms to Form H through the solid-phase transition through the solvate-free polymorph Form C-2. The three polymorphs, Forms A, H, and C, were composed of helical chain structures. However, there was an incomprehensible affair in the solid-phase transition among the three polymorphs. Namely, Form A comprised a left-handed helix. On the other hand, Form C’s and Form H’s helix chains were in a left- and right-handed helix complex, respectively. The solid-phase transition of Form A into Form H suggested a switch in helicity in the solid. We attempted to explain the helicity change in the solid-phase transition. As a result, we suggest that the over-absorption of water by Form A-2 at high humidity plays a vital role in the helicity change.
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