Hippuryl-1-histidyl-1-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 (C21H27N5O5.5H(2)O). 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(T)(1) = 14.5(8) and psi(T)(2) = -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 histidyl N epsilon atom.