Chain length dependent transition of 3 10 ‐ to α‐helix of Boc‐(Ala‐Aib) n ‐OMe

An apolar synthetic octapeptide, Boc‐(Ala‐Aib) 4 ‐OMe, was crystallized in the triclinic space group P1 with cell dimensions a = 11.558 Å, b = 11.643 Å, c = 9.650 Å, α = 120.220°, β = 107.000°, γ = 90.430°, V = 1055.889 Å 3 , Z = 1, C 34 H 60 O 11 N 8 ·H 2 O. The calculated crystal density was 1.217 g/cm 3 and the absorption coefficient ϕ was 6.1. All the intrahelical hydrogen bonds are of the 3 10 type, but the torsion angles, ϕ and ψ, of Ala(5) and Ala(7) deviate from the standard values. The distortion of the 3 10 ‐helix at the C‐terminal half is due to accommodation of the bulky Boc group of an adjacent peptide in the nacking. A water molecule is held between the N‐terminal of one peptide and the C‐terminal of the other. The oxygen atom of water forms hydrogen bonds with N (1) ‐H and N (2) ‐H, which are not involved in the intrahelical hydrogen bonds. The hydrogen atoms of water also formed hydrogen bonds with carbonyl oxygens of the adjacent peptide molecule. On the other hand, 1 H‐nmr analysis revealed that the octapeptide took an α‐helical structure in a CD 3 CN solution. The longer peptides, Boc‐(Ala‐Aib) 6 ‐OMe and Boc‐(Ala‐Aib) 8 ‐OMe, were also shown to take an α‐helical structure in a CD 3 CN solution. An α‐helical conformation of the hexadecapeptide in the solid state was suggested by x‐ray analysis of the crystalline structure. Thus, the critical length for transition from the 3 10 ‐ to α‐helix of Boc‐(Ala‐Aib) n ‐OMe is 8. © 1993 John Wiley & Sons, Inc.