Internal Water Bridge and Antiparallel Sheet in the Structure of Benzyloxycarbonyl-L-alanyl-D-phenylalanyl-L-proline Monohydrate

Benzyloxycarbonyl-L-alanyl-o-phenylalanyl-L-proline monohydrate, $C_{25}H_{29}N_3O_6. H_2O$, crystallizes in the orthorhombic space group ${P2}_12_12_1$ with four molecules in a unit cell of dimensions a = 9.594 (9), b = 9.705 (4) and c = 27.9 17 (12) A. 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 $C_2-C^{\gamma}-exo-C^{\beta}-endo$ 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