Hippuryl-L-histidyl-L-leucine, a Substrate for Angiotensin Converting Enzyme

The tripeptide crystallizes as a zwitterion with a protonated histidyl ring and the C-terminus ionized and with five water molecules of hydration (C₂₁H₂₇N₅O₅.5H₂O). The tripeptide adopts an all trans extended conformation with the histidine and phenyl rings parallel to one another. The C-terminus coils into a helical conformation. An intramolecular hydrogen bond between the C-terminus and the Nδ atom of the histidine ring stabilizes the helical conformation. The principal torsion angles are φ₁ = −67.7 (8), ψ₁ = 140.8 (5), ω₁ = 171.0 (6), φ₂ = −156.5 (5), ψ₂ = 162.7 (5), ω₂ = 175.0 (5), φ₃ = −96.4 (6), ψ_T¹ = 14.5 (8) and ψ_T² = −164.6 (6)° [IUPAC-IUB Commission on Biochemical Nomenclature (1970). J. Mol. Biol. 52, 1–17]. The tripeptides are linked in infinite chains through a short intermolecular hydrogen bond between the C-terminal carboxylate group and the protonated histidyl N[epsilon] atom.

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