Removing the C-terminal protecting group enlarges the crystal size: Z-(Gly-Aib)2-OH•H2O

The achiral tetrapeptide monohydrate N-(benzyloxycarbonyl)glycyl-alpha-aminoisobutyrylglycyl-alpha-aminoisobutyric acid monohydrate, Z-Gly-Aib-Gly-Aib-OH center dot H2O (Z is benzyloxycarbonyl, Aib is alpha-aminoisobutyric acid and Gly is glycine) or C20H28N4O7 center dot H2O, exhibits two conformations related by the symmetry operation of an inversion centre. It adopts only one of two possible intramolecular hydrogen bonds in a type I (and I') beta-turn and forms a maximum of intermolecular hydrogen bonds partly mediated by water. The space group, the molecular structure and the crystal packing differ from two already described (Gly-Aib)(2) peptides which vary only in the protecting groups. This structure confirms the high structural flexibility of Gly-Aib peptides and points to a strong relationship between intermolecular hydrogen bonding and crystal quality and size.