HISTIDYL CONFORMATIONS AND SHORT N-H-N HYDROGEN-BONDS - STRUCTURE OF D,L-HISTIDYL-L,D-HISTIDINE PENTAHYDRATE

D,L-Histidyl-L,D-histidine pentahydrate, C12H16N6O3.5H2O, M(r) = 382.38, F(000) = 408, crystallizes in the monoclinic space group Pc with the cell dimensions a = 9.971 (2), b = 4.745 (2), c = 19.572 (3) angstrom and beta = 96.08 (1)-degrees, V = 920.6 angstrom3, Z = 2, D(x) = 1.379 g cm-3, mu = 1.083 cm-1, T = 295 K, Mo K-alpha, lambda = 0.71073 angstrom. Final R (on F) = 0.040 for 1658 observed reflections with I greater-than-or-equal-to 3-sigma-(I). This dipeptide crystallizes in a zwitterionic form with protonation of the C-terminal imidazole ring. Both histidine units exist in the g+ or 'closed' conformation with C-alpha-C-beta-torsion angles of 67.2 (3) and 63.6 (3)-degrees. Principal torsion angles, omega = 176.8 (2), psi-1 = 161.8 (3) and phi-2 = -152.1 (3)-degrees, are indicative of a highly extended trans conformation. Intramolecular hydrogen bonding occurs between the imidazole rings [N2D-H2D1...N1D = 2.724 (4) angstrom]. Intermolecular hydrogen bonding occurs between symmetry-related histidine molecules forming chains along the y axis and includes another short [2.764 (4) angstrom] N-H...N interaction. The five water molecules occupy channels between adjacent histidine layers.