Crystal structures of Z–Gly–Aib–O(−)·0.5Ca(2+)·H(2)O 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, 0.5Ca(2+)·C(14)H(17)N(2)O(5) (−)·0.5H(2)O, which lies on an inversion centre and forms a distorted octa­hedral complex with the metal ion. These Ca(II) 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, C(14)H(18)N(2)O(5), mol­ecules are hydrogen bonded in the a- and c-axis directions, and stack in the b–axis direction via apolar contacts.