Design rules for peptides with α, β-dehydro-residues: synthesis of a model peptide Boc-Ile-ΔAla-OCH3 and its crystal structures obtained from two different solvents

The peptide Boc-Ile-ΔAla-OCH3 was synthesized and its three dimensional structure was determined using crystals grown under two different conditions using solvents (a) ethyl acetate and (b) an 80:20 mixture of dichloromethane and heptane. In the first case, the peptide crystallized in space group P21 with six crystallographically independent molecules while in the second condition the peptide crystallized in space group P32 with four molecules in the asymmetric unit. In both structures the conformations of all the crystallographically independent molecules were almost identical. ΔAla adopted a planar conformation with the ϕ torsion angle having values centred at ±180° in all the molecules. The crystal structures in two different crystalline states showed that ΔAla induced a distorted inverse γ-turn in the preceding residue with ϕ and ψ torsions centred at 100(1) and −125(1)°, respectively. The molecular packing in both crystal forms generated the sheet-like structures that were stabilized by intermolecular hydrogen bonds between the layers of molecules.