Synthetic and conformational studies on dehydroalanine-containing model peptides

Abstract

Three model dipeptides containing a dehydroalanine residue (ΔAla) at the C-terminal, Boc-X-ΔAla-NHCH₃ [X = Ala, Val, and Phe,] have been synthesized and their solution conformations investigated by ¹H-NMR, IR, and CD spectroscopy. NMR studies on these peptides in CDCl₃ clearly indicate that the NH group of dehydroalanine is involved in an intramolecular hydrogen bond. This conclusion is supported by IR studies also. Nuclear Overhauser effect (NOE) studies are also accommodative of an inverse γ-turn-type of conformation that is characterised by conformational angles of Φ ~ −70° and ψ ~ +70° around the X residue, and a C^α_i+1H-N_i+2H interproton distance of 2.5 Å. It appears that unlike dehydrophenylalanine or dehydroleucine, which tend to stabilize β-turn type of structures occupying the i + 2 position of the turn, dehydroalanine favors the formation of an inverse -turn, centered at the proceeding L-residue in such solvents as CDCl₃ and (CD₃)₂SO. A comparison of solution conformation of Boc Val-ΔAla-NHCH₃ with the corresponding saturated analogue, Boc-Val-Ala-NHCH₃, is also presented and shows that dehydroalanine is responsible for inducing the turn structure. It may be possible to design peptides with different preferred conformations using the suitable dehydroamino acid.