Crystal and molecular structure of l‐valyl‐l‐lysine hydrochloride

l‐Valyl‐l‐lysine hydrochloride, C₁₁N₃O₃H₂₃ HCl, crystallizes in the monoclinic space group P2, with a = 5.438(5), b = 14.188(5), c = 9.521(5) Å, β= 95.38(2)° and Z = 2. The crystal structure, solved by direct methods, refined to R = 0.036, using full matrix least‐squares method. The peptide exists in a zwitterionic form, with the N atom of the lysine side‐chain protonated. The two γ‐carbons of the valine side‐chain have positional disorder, giving rise to two conformations, χ¹¹₁= ‐67.3 and 65.9°, one of which (65.9°) is sterically less favourable and has been found to be less popular amongst residues branching at β‐C. The lysine side‐chain has the geometry of g⁻ tgt, not seen in crystal structures of the dipeptides reported so far. Interestingly, χ³₂ (63.6°) of lysine side‐chain has a gauche⁺ conformation unlike in most of the other structures, where it is trans. The neighbouring peptide molecules are hydrogen bonded in a head‐to‐tail fashion, a rather uncommon interaction in lysine peptide structures. The structure shows considerable similarity with that of l‐Lys‐l‐Val HO in conformational angles and H‐bond interactions [4].