Helical Screw Sense of Peptide Molecules. Crystal Structures of Three Aib-Based Pentapeptides

The molecular and crystal structures of three terminally blocked (Aib)(n)/(L-Xxx)(m) (n = 3, 4; m = 1, 2; n + m = 5) pentapeptides [pBrBz-L-Ala-(Aib)(4)-OtBu (1), pBrBz-L-Val-(Aib)(4)-OtBu (2), pBrBz-L-Ala-(Aib)(3)-L-Ala-OMe (3)] were determined by X-ray diffraction. The three compounds are characterized by the following parameters: (1) orthorhombic, P2(1)2(1)2(1), a = 33.833(3) Angstrom, b = 11.776(2) Angstrom, c = 9.317(1) Angstrom, and Z = 4; (2) triclinic, P1, a = 10.316(6) Angstrom, b = 10.580(5) Angstrom, c = 18.735(6) Angstrom, alpha = 95.46(4)degrees, beta = 90.08(6)degrees, gamma = 111.92(4)degrees, and Z = 2; (3) monoclinic, P2(1), a = 9.002(1) Angstrom, b = 19.242(3) Angstrom, c = 9.668(1) Angstrom, beta = 113.69(1)degrees, and Z = 2. The structures were solved by direct methods. The least-squares refinements led to R values of 0.042, 0.060, and 0.046 for (1), (2), and (3), respectively. The three pentapeptides are folded in a 3(10)-helical structure. However, the structure of (1) is left-handed helical from residue 1, whereas that of (3) begins with a non-helical type-II beta-bend that is followed by a left-handed helix. The two independent molecules, A and B, in the asymmetric unit of (2) are diastereomeric, the former being right-handed helical from residue 1, while the latter is left-handed.