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

The conformational characteristics of a flexible totally protected C-termin al dipeptide fragment (Boc-Phe-Leu-OBzl) of enkephalin are studied using X- ray data, molecular modelling and data retrieved from the Cambridge Structu ral Database. The dipeptide crystallizes with seven conformers in the asymm etric unit. C27H36N2O5, T = 133 K, monoclinic, P2(1), a = 13.706 (3), b = 2 2.800(3), c = 30.674(5) Angstrom, beta = 97.15(3)degrees, V = 9511 (3) Angs trom(3), Z = 14, D-c = 1.145 Mg m(-3). Six of the seven molecules exhibit f olded conformations with hydrophobic groups disposed at the opposite side o f 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 define d in the Ramachandran diagram. However, they do not belong to the family of the lowest energy conformations. In the crystal, molecules are interconnec ted via N-H ... O hydrogen bonds of peptide groups forming an infinite shee t similar to a parallel beta-sheet. Molecular dynamics simulations performe d in vacuo reproduce the conformers and rotamers detected in the solid stat e.