Chain length dependent transition of 310‐ 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 Å3, Z = 1, C34H60O11N8·H2O. The calculated crystal density was 1.217 g/cm3 and the absorption coefficient ϕ was 6.1. All the intrahelical hydrogen bonds are of the 310 type, but the torsion angles, ϕ and ψ, of Ala(5) and Ala(7) deviate from the standard values. The distortion of the 310‐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, 1H‐nmr analysis revealed that the octapeptide took an α‐helical structure in a CD3CN 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 CD3CN 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 310‐ to α‐helix of Boc‐(Ala‐Aib)n‐OMe is 8.