Observation of a mixed antiparallel and parallel β-sheet motif in the crystal structure of Boc-Ala-Ile-Aib-OMe

Abstract

A diastereomeric mixture of the tripeptide Boc-Ala-Ile-Aib-OMe crystallized in the space group Pl from CH₃OH/H₂O. The unit cell parameters are a = 10.593(2) Å, b = 14.377(3) Å, c = 17.872(4) Å, α = 104.41(2)°, β = 90.55(2)°, γ = 106.91(2)°, V = 2512.4 ų, Z = 4. X-Ray crystallographic studies shows the presence of four molecules in the asymmetric unit consisting of two pairs of diastereomeric peptides, Boc-l-Ala-l-Ile-Aib-OMe and Boc-l-Ala-d-Ile-Aib-OMe. The four molecules in the asymmetric unit form a rarely found mixed antiparallel and parallel β-sheet hydrogen bond motif. The Ala and (l,d)-Ile residues in all the four molecules adopt the extended conformations, while the φ, ψ values of the Aib residues are in the right-handed helical region. In one of the molecules the Ile sidechain adopts the unusual gauche conformation about the C^β-C^γ bond.