Crystal structures of Z-Gly-Aib-O-·0.5Ca2+·H2O and Z-Gly-Aib-OH.

Both deprotonated and neutral achiral title dipeptides assume similar structures of two conformations, which are related by a unit-cell inversion centre. Two mol­ecules of both conformations of the metal-free neutral dipeptide are linked by two hydrogen bonds, while two mol­ecules of both conformations of the ionized form coordinate a calcium ion in calcium(II) bis­[2-(2-{[(benz­yl­oxy)carbon­yl]amino}­acetamido)-2-methyl­propano­ate] monohydrate, 0.5Ca2+·C14H17N2O5−·0.5H2O, which lies on an inversion centre and forms a distorted octa­hedral complex with the metal ion. These CaII complexes are connected in the crystal via hydrogen bonds in the b- and c-axis directions, whereas in the a-axis direction, they stack via apolar contacts. In the metal-free crystal, namely 2-(2-{[(benz­yloxy)carbon­yl]amino}­acetamido)-2-methyl­propanoic acid, C14H18N2O5, mol­ecules are hydrogen bonded in the a- and c-axis directions, and stack in the b–axis direction via apolar contacts.