Acta Crystallographica Section B

Structural Science

Volume 55, Part 6 (December 1999)

research papers

Acta Cryst. (1999). B55, 975-984 [ doi:10.1107/S0108768199004656 ]

Structure and molecular modelling of protected dipeptide fragment (Boc-Phe-Leu-OBzl) of enkephalin

S. Antolic, M. Teichert, G. Sheldrick, B. Kojic-Prodic, M. Cudic and S. Horvat

Abstract: 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, [beta] = 97.15 (3)°, V = 9511 (3) Å³, Z = 14, D_c = 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 [Phi] ₁ and [Psi] ₁ angles of the Phe residue and [Phi] ₂ 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-HO hydrogen bonds of peptide groups forming an infinite sheet similar to a parallel [beta] -sheet. Molecular dynamics simulations performed in vacuo reproduce the conformers and rotamers detected in the solid state.

Keywords: Enkephalin; dipeptide.