The dipeptide pGlu-Pro-NH₂

The crystal structure of the pGlu-Pro-NH₂ dipeptide, cis-1-(5-oxo-L-prolyl)-L-prolinamide hydrate [cis-1-(5-oxo-L-prolyl)pyrrolidine-2-carboxamide hydrate], C₁₀H₁₅-N₃O_a.H₂O, has been determined in order to establish the conformation of the pyrrolidine ring both in natural (L) proline (Pro) and in pyroglutamic acid (pGlu), a cyclic analogue of the the natural L-glutamate amino acid. The structure was solved by direct methods 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. Molecular cohesion is stabilized by a dense network of hydrogen bonds involving the free amine group of pGlu, the three O atoms of the carbonyl groups, the terminal carboxy-protective NH₂ group and a water molecule.