Hydrogen-bonding patterns in cis-4-ammoniocyclohexanecarboxylate hemihydrate.

In the title compound, C 7 H 13 NO 2 .0.5H 2 O, cis-4-aminocyclohexanecarboxylic acid exists as a zwitterion and co-crystallizes with water molecules in a 2:1 amino acid-water ratio. The cyclohexane ring adopts a chair conformation, with the carboxylate and ammonium groups in axial and equatorial positions, respectively. The basic motif in the crystal structure is a sandwich structure, formed by two amino acid units linked by head-to-tail hydrogen bonds. Hydrogen bonds of the type N + -H...O-C-O - link these motifs, forming helicoidally extended chains running along the c axis. The water molecule lies on a twofold axis and interacts with the chains by means of O-H...O hydrogen bonds.