Helical screw sense of peptide molecules. Crystal structures of three Aib-based pentapeptides1
Abstract
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, P212121, a = 33.833(3) Å, b = 11.776(2) Å, c = 9.317(1) Å, and Z = 4; (2) triclinic, P1, a = 10.316(6) Å, b = 10.580(5) Å, c = 18.735(6) Å, α = 95.46(4)°, β = 90.08(6)°, γ = 111.92(4)°, and Z = 2; (3) monoclinic, P21, a = 9.002(1) Å, b = 19.242(3) Å, c = 9.668(1) Å, β = 113.69(1)°, 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 310-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 β-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.
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Articles in the same Issue
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