Conformation and crystal structure of L-prolyl-L-glutamic acid dihydrate

The crystal structure of the dipeptide L-prolyl-L-glutamic acid dihydrate, L-Pro-L-Glu . 2H2O, C10H20O7N2, has been determined from three-dimensional X-ray diffractometer data. The dipeptide crystallizes in the space group P21 of the monoclinic system with two formula units in a cell of dimensions a = 5.629(2), b = 11.832(5), c = 10.485(4)A, and beta = 103.06(3) degrees. The structure was solved by direct methods and refined by least squares techniques to a final value of the conventional R-factor (on F) of 0.039 based on 1798 independent intensities with I greater than or equal to 3 sigma(I). The dipeptide occurs as a zwitterion in the crystal with the pyrrolidine nitrogen atom protonated and the main chain carboxyl group deprotonated. The conformation of the peptide linkage is trans, the omega torsional angle being 173.7 degrees. The pyrrolidine ring adopts the Cs-C beta endo conformation and the conformation of the glutamyl side chain is fully extended. There is considerable intermolecular hydrogen bonding in the crystals.