Chiroptical Properties and Conformation of Dehydrophenylalanine Peptides

Some N‐acylated α, β‐unsaturated peptides (dehydropeptides) containing one, two or three dehydrophenylalanine (dehydro‐Phe) residues and a C‐terminal L‐amino acidic residue have been prepared and characterized by NMR, IR and UV absorption. For N‐actyl‐(dehydro‐Phe) 2 ‐L‐Ala the crystal and molecular structure have been determined by X‐ray diffraction analysis. CD spectra of N‐acetyl‐(dehydro‐Phe)‐ϵ‐Cbzo‐L‐Lys and N‐acetyl‐(dehydro‐Phe) 2 ‐ϵ‐Cbzo‐L‐Lys have been measured in different solvents and, of N‐acetyl‐(dehydro‐Phe) 2 ‐L‐Val, in the presence of different amounts of urea. The results have been compared with those for N‐acetyl‐(dehydro‐Phe)‐(dehydro‐Tyr)‐L‐Glu, for the tetrapeptide N‐acetyl‐(dehydro‐Phe) 3 ‐L‐Val, and for analogous compounds already reported [ J. Am. Chem. Soc ., 97 , 5820 (1975)]. Crystalline molecular structure and chiroptical properties are consistent with the hypothesis that the dehydropeptides assume in solution a chiral conformation based on a 4 → 1 hydrogen‐bonded ten‐membered ring, also referred to as β‐turn. The importance of these systems for the understanding of the conformational properties of polypeptides containing D‐L sequences is discussed.