Facile transition between 3₁₀- and α-helix: structures of 8-, 9-, and 10-residue peptides containing the -(Leu-Aib-Ala)₂-Phe-Aib-fragment

Abstract

A structural transition from a 3₁₀-helix to an α-helix has been characterized at high resolution for an octapeptide segment located in 3 different sequences. Three synthetic peptides, decapeptide (A) Boc-Aib-Trp-(Leu-Aib-Ala)₂-Phe-Aib-OMe, nonapeptide (B) Boc-Trp-(Leu-Aib-Ala)₂-Phe-Aib-OMe, and octapeptide (C) Boc-(Leu-Aib-Ala)₂-Phe-Aib-OMe, are completely helical in their respective crystals. At 0.9 Å resolution, R factors for A, B, and C are 8.3%, 5.4%, and 7.3%, respectively. The octapeptide and nonapeptide form ideal 3₁₀-helices with average torsional angles Φ(N-C^α) and Ψ(C^α-C°) of -57°, -26° for C and -60°, -27° for B. The 10-residue peptide (A) begins as a 3₁₀-helix and abruptly changes to an -helix at carbonyl O(3), which is the acceptor for both a 4 → 1 hydrogen bond with N(6)H and a 5 → 1 hydrogen with N(7)H, even though the last 8 residues have the same sequence in all 3 peptides. The average Φ,Ψ angles in the decapeptide are -58°, -28° for residues 1-3 and -63°, -41° for residues 4-10. The packing of helices in the crystals does not provide any obvious reason for the transition in helix type. Fourier transform infrared studies in the solid state also provide evidence for a 3₁₀- to α-helix transition with the amide I band appearing at 1,656-1,657 cm^-1 in the 9- and 10-residue peptides, whereas in shorter sequences the band is observed at 1,667 cm^-1.