Internal water bridge and antiparallel sheet in the structure of benzyloxycarbonyl-L-alanyl-D-phenylalanyl-L-proline monohydrate

Abstract

Benzyloxycarbonyl-L-alanyl-o-phenylalanyl-L-proline monohydrate, C₂₅H₂₉N₃O₆.H₂O, crystallizes in the orthorhombic space group P2₁2₁2₁ with four molecules in a unit cell of dimensions a = 9.594 (9), b = 9.705 (4) and c = 27.9 17 (12) Å. The structure has been refined to an R value of 0.067 for 2046 observed reflections. All the peptide units in the molecule are trans and the prolyl residue is in the conformation. The lone water molecule in the structure is hydrogen bonded to the carbonyl O atom in the C₂ - C^γ - exo - C^β endo conformation. The lone water molecule in the structure is hydrogen bonded to the carbonyl O atom in the benzyloxycarbonyl group and to one of the O atoms in the terminal carboxyl group. This internal water bridge, observed for the first time in a linear peptide, provides a model for water-mediated chain-reversal. An interesting feature of the crystal structure is the presence of an antiparallel sheet involving the alanyl and the phenyl-alanyl residues.