Effect of stereochemistry (Z and E) and position of alpha,beta-dehydrophenylalanine (Delta Phe) on beta-turn stability

Several model peptides containing alpha, beta -dehydrophenylalanine (Delta Phe) in both Z and E configurations were studied for beta -turn stability a t the AM1 level of theory. Both configurations of Delta Phe are well able t o stabilize beta -turns in the backbone. However, the beta -turns for pepti des bearing Z-Delta Phe are energetically more stable than the E-counterpar ts. The difference in energies between the global minima of these peptides having the Z and E configuration of Delta Phe, is dictated by the size and stereochemistry of residues flanking Delta Phe. One distinct feature of E-D elta Phe is that it pushes peptides to adopt a Type II beta -turn with the Delta Phe residue in the (i + 1) position of the turn. This unique feature may be exploited in peptide design.