THE DIPEPTIDE PGLU-PRO-NH2

The crystal structure of the pGlu-Pro-NH2 dipeptide, cis-1-(5-oxo-L-pr olyl)-L-prolinamide hydrate [cis-1-(5-oxo-L-prolyl)pyrrolidine-2-carbo xamide hydrate], C10H15N3O3.H2O, has been determined in order to estab lish the conformation of the pyrrolidine ring both in natural (L) prol ine (Pro) and in pyroglutamic acid (pGlu), a cyclic analogue of the th e natural L-glutamate amino acid. The structure was solved by direct m ethods and refined by least-squares calculations to a final R value of 0.026. While the pyrrolidine ring of the Pro residue adopts a twisted conformation, this ring is planar in the pGlu amino acid. The proline residue is in a cis orientation with respect to the peptide bond. Mol ecular cohesion is stabilized by a dense network of hydrogen bonds inv olving the free amine group of pGlu, the three O atoms of the carbonyl groups, the terminal carboxy-protective NH2 group and a water molecul e.