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

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L-Histidyl-L-serine (HSN) trihydrate, C9H14N4O4·H2O, crystallizes in the orthorhombi space group P212121 with 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 switterionic 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 Hisε tautomeric state has conformational angles χ11 of-53.5(7)° and χ121 of -55.4(8)° and the serine hydroxyl group has χ21 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 Cα-H O=C hydrogen bond motif similar to that observed in collagen triple helix

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