CRYSTAL AND MOLECULAR-STRUCTURE OF L-HISTIDYL-L-SERINE TRIHYDRATE - OCCURRENCE OF C-ALPHA-H-O=C HYDROGEN-BOND MOTIF SIMILAR TO THE MOTIF INCOLLAGEN TRIPLE-HELIX AND BETA-SHEETS

L-Histidyl-L-serine (HSN) trihydrate, C9H14N4O4 . H2O, crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 4.865(4), b = 15. 604(4), c = 18.918(5) and Z = 4. The crystal structure was solved by d irect methods and refined to R1 = 0.070 by a full-matrix least-squares method. The peptide exists in a zwitterionic form, with the N-terminu s in a protonated form and the C-terminus in an ionized form. The imid azole ring of histidine in its neutral His(epsilon) tautomeric state h as conformational angles chi(1)(1) of -53.5(7)degrees and chi(1)(21) o f -55.4(8)degrees and the serine hydroxyl group has chi(2)(1) of 68.2( 7)degrees. The conformational angles deviate significantly from those or rue dipeptide complexed with glycyl-L-glutamic acid in which the hi stidine is protonated. A noteworthy feature of the crystal packing is the occurrence of a C-alpha-H O=C hydrogen bond motif similar to that observed in collagen triple helix and beta-sheets. (C) Munksgaard 1998 .