Zwitterionic versus neutral molecules of fluoroquinolones: crystal structure of danofloxacin dihydrate

Abstract

The crystal structure of danofloxacin (DFX) as a zwitterionic dihydrate [systematic name: (1S,4S)-5-(3-carboxylato-1-cyclopropyl-6-fluoro-4-oxo-1,4-dihydroquinolin-7-yl)-2-methyl-2,5-diazabicyclo[2.2.1]heptan-2-ium dihydrate], C19H20FN3O3·2H2O, has been determined for the first time. The stability of the crystal structure is ensured by N—H...O and O—H...O hydrogen bonds, some of which are assisted by charge. The zwitterionic molecules of DFX are packed in stacks and this structural motif can be defined as a basic one from a supramolecular and energetic point of view. Hirshfeld surface analysis was used for an additional interpretation of the close contacts. The nature of the intermolecular interactions was evaluated in a pairwise manner. Destabilizing interactions were also discussed. In the context of our study on danofloxacin, we addressed the question: how common is a zwitterionic form for fluoroquinolones (FQ) in the crystalline state? A survey of the Cambridge Structural Database limited to anhydrous/hydrated/solvated forms of the most typical FQs revealed that neutral molecules are preferred, while zwitterions are less common.