Crystal and molecular structure of l-histidyl-l-serine trihydrate: occurrence of Cα-H…O=C hydrogen bond motif similar to the motif in collagen triple helix and β-sheets

l -Histidyl-l -serine (HSN) trihydrate, C₉H₁₄N₄O₄H₂O crystallizes in the orthorhombic space group P2₁2₁2₁with a= 4.865(4), b= 15.604(4), c= 18.918(5) and Z= 4. The crystal structure was solved by direct methods and refined to R1 = 0.070 by a full-matrix least-squares method. The peptide exists in a zwitterionic form, with the N-terminus in a protonated form and the C-terminus in an ionized form. The imidazole ring of histidine in its neutral His^e tautomeric state has conformational angles X¹₁ of-53.5(7)° and X²¹₁ of-55.4(8)° and the serine hydroxyl group has X¹₂ of 68.2(7)°. The conformational angles deviate significantly from those of the dipeptide complexed with glycyl-l -glutamic acid in which the histidine is protonated. A noteworthy feature of the crystal packing is the occurrence of a C^α-H O=C hydrogen bond motif similar to that observed in collagen triple helix and β-sheets.