Alpha,beta-dehydro residues in the design of peptide and protein structures

The results of X-ray studies on 19 structures containing dehydro residues have been analysed. The observed average C alpha = C beta distance in the dehydro residues is 1.331 (2) A. The average values of the C alpha = C beta - C gamma bond angle in dehydro-Phe and dehydro-Leu are 131.2 (2) and 127.3 (1) degrees, respectively. The dehydro residue is essentially planar. A beta-turn of type II is formed if the dehydro residue is placed either at the (i + 1) or at the (i + 2) corner position of the beta-turn. If the dehydro residues occur consecutively in an amino-acid sequence, the backbone folds into an alternating right- and left-handed alpha-helix. The peptide bond is planar in all these structures. The beta-turn is stabilized by an intramolecular hydrogen bond between CO of the ith and NH of the (i + 3)th residue.