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Conformation and structure of acidic dipeptides. Crystal structure of glycyl-L-aspartic acid dihydrate.

The crystal structure of the acidic dipeptide glycyl-L-aspartic acid dihydrate, Gly-L-Asp X 2H2O, C6H10N2O5 X 2H2O, has been determined by means of three-dimensional counter X-ray data. The dipeptide crystallizes in space group P212121 of the orthorhombic system with four formula units in a cell of dimensions of a = 9.611(4), b = 9.775(6), and c = 10.845(4)A. The structure was solved by direct methods and refined by least-squares methods to a final value of the weighted R-factor of 0.035 using all 1191 observed intensity data. The dipeptide occurs in the crystal as a zwitterion, with the glycyl N-terminus protonated and the aspartyl main chain carboxyl group deprotonated. The conformation about the peptide bond is fully trans extended, with an omega angle of -175.9 degrees. While there is extensive intermolecular hydrogen bonding in the crystals, there is no intramolecular hydrogen bonding either here or in the solid state structures of related dipeptides.