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Structure and molecular modelling of protected dipeptide fragmet (Boc-Phe-Leu-Obzl) of enkephalin

The conformational characteristics of a flexible totally protected C-terminal dipeptide fragment (Boc–Phe–Leu–OBzl) of enkephalin are studied using X-ray data, molecular modelling and data retrieved from the Cambridge Structural Database. The dipeptide crystallizes with seven conformers in the asymmetric unit. C27H36N2O5, T = 133 K, monoclinic, P21, a = 13.706 (3), b = 22.800 (3), c = 30.674 (5) Å, β = 97.15 (3)°, V = 9511 (3) Å3, Z = 14, Dc = 1.145 Mg m−3. Six of the seven molecules exhibit folded conformations with hydrophobic groups disposed at the opposite side of the peptide backbone. The characteristic Φ1 and Ψ1 angles of the Phe residue and Φ2 of the Leu fragment are in the allowed region defined in the Ramachandran diagram. However, they do not belong to the family of the lowest energy conformations. In the crystal, molecules are interconnected via N—H⋯O hydrogen bonds of peptide groups forming an infinite sheet similar to a parallel β-sheet. Molecular dynamics simulations performed in vacuo reproduce the conformers and rotamers detected in the solid state.

protected dipeptide enkephalin fragment ; X-ray structure ; molecular modelling

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