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(.5)H2O). 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 N delta atom of the histidine ring stabilizes the helical conformation. The principal torsion angles are phi 1 = -67.7 (8), psi 1 = 140.8 (5), omega 1 = 171.0 (6), phi 2 = -156.5 (5), psi 2 = 162.7 (5), omega 2 = 175.0 (5), phi 3 = -96.4 (6), psi T1 = 14.5 (8) and psi T2 = -164.6 (6) degrees [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 histidy1 N epsilone atom.