α,β-Dehydro-amino acid residues in the design of peptide structures: Synthesis,crystal structure,and molecular conformation of two homologous peptides—N-Ac-Dehydro-Phe-L-Leu-OCH3 and N-Ac-Dehydro-Phe-NorVal-OCH3

The dehydro-residue containing peptides N-Ac-dehydro-Phe-L -Leu-OCH₃ ( I ) and N-Ac-dehydro-Phe-NorVal-OCH₃ ( II ) were synthesized by the usual workup procedures. The peptides crystallize from their solutions in methanol in space group P6₅: ( I ) a = b = 12.528(2) Å, c = 21.653(5) Å; ( II ) a = b = 12.532(2) Å, c = 21.695(4) Å. The structures were determined by direct methods. Both peptides adopt similar conformations with ?,ψ of dehydro-Phe as follows: ( I ) ?57.0(5)° and ?37.0(5)°; ( II ) ?56.0(5),° and ?37.5(5)°. The observed data on dehydro-Phe when placed at the (i + 1) position show that the ?,ψ values of dehydro-Phe are either ?60°, 140° or ?60°, ?30°. The conformation of ?60°, 140° can be accommodated only with a flexible residue at the (i + 2) position while the ?,ψ values of ?60°, ?30° are obtained with a bulky residue at the (i + 2) position as in the present structures. The molecules are packed in a helical way along the c axis. These are held by two strong intermolecular hydrogen bonds involving both NH as donors and acetyl group and dehydro-Phe oxygen atoms as acceptors. © 1994 John Wiley & Sons, Inc.