First Observation of Left-Handed Helical Conformation in a Dehydro Peptide Containing Two l-Val Residues. Crystal and Solution Structure of Boc-l-Val-ΔPhe-ΔPhe-ΔPhe-l-Val-OMe,

The solution and solid structure of Boc-l-Val-ΔPhe-ΔPhe-ΔPhe-l-Val-OMe, containing three consecutive ΔPhe residues, have been determined by X-ray diffraction, nuclear magnetic resonance, and circular dichroism methods. The crystals grown from aqueous methanol are orthorhombic, space group P212121, a = 11.624(2), b = 17.248(2), c = 21.532 Å, V = 4216 (1) Å3, Z = 4. In the solid state, the peptide exhibits a left-handed 310-helical conformation, in spite of the presence of two l-Val residues. NMR and CD studies in different solvents also support the crystal structure data, suggesting that the solid state structure is maintained in solution as well. This is the first report of a dehydropeptide containing three consecutive ΔPhe residues and exhibiting left-handed 310-helical conformation, which demonstrates the remarkable conformational consequences produced by consecutive occurrence of ΔPhe residues in a peptide.