FACILE TRANSITION BETWEEN 3(10)-HELIX AND ALPHA-HELIX - STRUCTURES OF8-RESIDUE, 9-RESIDUE, AND 10-RESIDUE PEPTIDES CONTAINING THE -(LEU-AIB-ALA)(2)-PHE-AIB- FRAGMENT

A structural transition from a 3(10)-helix to an alpha-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 Angs trom resolution, R factors for A, B, and C are 8.3%, 5.4%, and 7.3%, r espectively. The octapeptide and nonapeptide form ideal 3(10)-helices with average torsional angles Q(N-C-alpha) and psi(C-alpha-C') of -57 degrees, -26 degrees for C and -60 degrees, -27 degrees for B. The 10- residue peptide (A) begins as a 3(10)-helix and abruptly changes to an alpha-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 tho ugh the last 8 residues have the same sequence in all 3 peptides. The average phi,psi angles in the decapeptide are -58 degrees, -28 degrees for residues 1-3 and -63 degrees, -41 degrees 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 3(10)- to alpha-helix t ransition 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).