Helical screw sense of peptide molecules. Diastereomeric -Pro-Ala-(Aib)4-sequences
Abstract
The molecular and crystal structures of two terminally blocked diastereomeric -Pro-Ala-(Aib)4- hexapeptide sequences, pBrBz-L-Pro-D-Ala-(Aib)4-OtBu (1) and pBrBz-D-Pro-D-Ala-(Aib)4-OtBu dihydrate (2), have been determined by X-ray diffraction. The two compounds are characterized by the following parameters: (1) orthorhombic, P212121, a = 15.827(2) Å, b = 22.625(2) Å, c = 11.336(2) Å, and Z = 4; (2) triclinic, P1, a = 10.225(2) Å, b = 12.474(2) Å, c = 8.827(1) Å, α = 104.2(1)°, β = 92.8(1)°, γ = 85.0(1)°, and Z = 1. The structures were solved by direct methods. The least-squares refinements led to R values of 0.057 and 0.079 for (1) and (2), respectively. The heterochiral L-Pro-D-Ala- sequence of hexapeptide (1) is folded in a type-II β-bend conformation, which is followed by a left-handed 310-helix spanning the sequence -D-Ala-(Aib)3-. Conversely, the homochiral -D-Pro-D-Ala- sequence of hexapeptide (2) adopts a distorted type-II′ β-bend conformation, which induces a right-handed 310-helix in the following -D-Ala-(Aib)3- sequence. Interestingly, in compound (2) the D-Ala residue is forced to take a φ torsion angle typical of an L-amino acid.
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