Acta Cryst. (2003). C59, m18–m20
Martin Lutz and Ruud Bakker
Hydrogen-bond formation in the crystal structure. The proline residues in the chain on the left-hand side have the R configuration, while those on the right-hand side have the S configuration [symmetry code: (i) x, 1 + y, z].Neutral, zwitterionic amino acids form adducts (neutral salts) with metal salts like LiCl, CaCl2 or ZnCl2. Thereby the negatively charged carboxylate group binds to the cation of the salt, and in most cases the positively charged ammonium group donates hydrogen bonds to the anions. This paper reports on the neutral salt of DL-proline with ZnCl2. The Zn cation is tetrahedrally four-coordinated with two chlorine anions and two proline residues as ligands. Because the Zn is located on a twofold axis, these two proline residues have the same absolute configuration. By hydrogen bonding infinite linear chains of these molecules are formed, consisting of molecules with the same chirality (partial separation of enantiomers). Due to the centrosymmetry of the space group, the same amount of R and S chains is present in the crystal. Under different crystallization conditions a complete separation of the enantiomers was observed, resulting in a crystalline powder of the enantiopure compounds.