Crystal and molecular structure of the dehydropeptide Ac-delta Phe-Val-delta Phe-NH-Me

The dehydropeptide Ac-delta Phe-L-Val-delta Phe-NH-Me, containing two dehydrophenylalanine (delta Phe) residues, crystallizes from methanol/water in space group P212121, with a = 12.622 (1), b = 12.979 (1), and c = 15.733 (1) A. In the solid state, the molecular structure is characterized by the presence of two intramolecular hydrogen bonds which form two consecutive beta-bends. The (phi, psi) torsion angles of the three residues are very similar and close to the standard values of type III beta-bends, so the molecular conformation corresponds to an incipient right-handed 3(10)-helix, only slightly distorted. In the crystal, the molecules are linked by head-to-tail hydrogen bonds, thus forming continuous helical columns packed in antiparallel mode. There are no lateral hydrogen bonds; the only interactions are hydrophobic contacts between the apolar side chains of neighboring helical columns.