CHAIN-LENGTH EFFECTS ON HELIX-FORMING TENDENCY IN SEQUENTIAL PEPTIDESCONTAINING 2-DEHYDROPHENYLALANINE RESIDUES

THe present paper describes chain-length effects on helix-forming tend ency in sequential peptides containing Z-dehydrophenyLalanine (Delta(Z )Phe) residues, i.e., (X-Delta(Z)Phe). For sequential peptides Boc-(L- Leu-Delta(Z)Phe)(n)-L-Leu-OMe (n = 2--6) that take 3(10)-helical confo rmation in CHCl3, solvent dependence on their helical structures was i nvestigated by CD spectroscopy. CD patterns corresponding to a right-h anded helix were observed for peptides n = 3--6 in CHCl3, tetrahydrofu ran, acetonitrile, methanol, and trimethyl phosphate. Such helical str uctures became less stable in dimethyl sulfoxide and trifluoroethanol. Conformational energy calculation was carried out for acetyl-(L-Ala-D elta(Z)Phe)(n)-NHCH3 (n = 1-12). Peptides above n = 2 were predicted t o form helical structures preferentially. In addition, rigidity of the helical structure containing Delta(Z)Phe residues was found to be som ewhat lower than that containing alpha-aminoisobutyric acid residue wi th strong helix-forming tendency. This can be ascribed to the conforma tional properties inherent in Delta(Z)Phe residue that induces not onl y helical backbones, but beta-turn backbones. Thus, Delta(Z)Phe residu e will be useful to design functional peptides having such backbones.