LINEAR OLIGOPEPTIDES .252. ALPHA,BETA-DEHYDRO-AMINO ACID RESIDUES IN THE DESIGN OF PEPTIDE STRUCTURES - MOLECULAR AND CRYSTAL-STRUCTURES OF 2 FOLDED DEHYDRO PEPTIDES

The molecular and crystal structures of two N-alpha-protected tripeptide amides, containing in the central position the alpha-beta-dehydro-amino acid residue DELTA-Phe (Z-configurational isomer), were determined by X-ray diffraction. While Z-Gly-DELTA-Phe(z)-L-Pro-NH2 is characterized in the crystal state by the presence of a type 1 beta-bend conformation (at the DELTA-Phe(z)-L-Pro sequence), Z-D-Ala-DELTA-Phe(z)-Gly-NH2 is folded into two consecutive beta-bends (type II' followed by type I), at the D-Ala-DELTA-Phe(z) and DELTA-Phe(z)-Gly sequences, respectively. In both cases the achiral DELTA-Phe(z) residue adopts a set of phi, psi angles typical of the right-handed helical conformation. The DELTA-Phe residue may be exploited to design aromatic peptides with preferred secondary structures.