Design of specific structures using α, β-dehydro-phenylalanine residues: synthesis, crystal structure, and molecular conformation of Boc-L-Val-ΔPhe-ΔPhe-L-Val-ΔPhe-ΔPhe-L-Val-OCH₃, a 3₁₀-helical heptapeptide

Abstract

The peptide design using α,β-dehydro-residues has wide applications. To design an extensive 3₁₀ helical conformation, a heptapeptide Boc-L-Val-ΔPhe-ΔPhe-L-Val-ΔPhe-ΔPhe-L-Val-OCH₃ with a repeat of two consecutive ΔPhe residues has been synthesized using an azlactone method in solution phase. This is the first design using a repeat of two consecutive ΔPhe residues. It is observed that the ΔPhe in a sequence of two consecutive ΔPhe residues, adopts only one set of Φ,Ψ values, i.e., ±60°, ±30°, thus making it a specific design tool. The peptide crystallized from its solution in a methanol-water mixture in the space group P2₁ with a = 10.159(5)A, b = 20.057(2)A, c = 14.448(3)Å, β = 99.41(2)°, V = 2904(2)Å.³ The structure has been determined by direct methods and refined to an R value of 0.048 for 5404 observed [I ≥3 δ(I)] reflections. The structure consists of a heptapeptide Boc-L-Val-ΔPhe-ΔPhe-L-Val-ΔPhe-ΔPhe-L-Val-OCH₃ and a solvent methanol molecule in the asymmetric unit. All peptide units in the structure are trans. As a result of six overlapping type III β-turns formed involving seven residues and five intramolecular 4 → 1 hydrogen bonds, the peptide adopts a right-handed 3₁₀ helical conformation with more than two complete helical turns. It is noteworthy that starting from the Boc group to the C-terminal residue of Val, the 3₁₀helical structure is maintained well. The carbonyl oxygen atom of the Boc group is the first acceptor whereas the carbonyl oxygen atom of Val⁴ is the last acceptor in the helical structure of the peptide. The side chains of four ΔPhe residues in this helical arrangement exists in a slightly staggered arrangement. The solvent methanol molecule interacts through its hydroxyl group and forms two intermolecular hydrogen bonds, one as a donor with a C-terminal CO group and ΔPhe⁶ and second as an acceptor with the NH group of ΔPhe² from the N-terminal region of the peptide. Thus the solvent molecule plays a significant role in promoting a head-to-tail packing of 3₁₀helices of the peptide. There are no lateral hydrogen bonds between the helices, but there exist several van der Waals interactions involving the hydrophobic side chains of peptide molecules.