Crystal structure of a dipeptide Boc-Aib-Phe-OMe

In order to understand the effect of the restrictions posed by the Aib residue on peptide conformation we studied the crystal structure of a dipeptide t Boc-Aib-Phe-OMe. Crystals of this compound are triclinic, space group P1 with α = 9.600(1)Å, b = 10.262(1) Å, c = 10.799(1) Å, α = 98.43°(1), β = 99.18(1), γ = 98.87°(1). V = 1021.69(18) ų and Z=2. The structure was solved by direct methods and refined to an R-Factor of 4.98%. The backbone conformational angles for the Aib residue in molecule A are in the left-handed helical region, while in molecule B they are in the right- handed helical region. The Phe residue in molecule A is in the right-handed helical conformation, while in molecule B it is in the β-region. The peptide units are trans and show significant deviation from planarity [ω₁ = 166.67(5) and ω₂ = -177.9(5)].