The crystal structure of the protected C-terminal dipeptide fragment (Boc-Phe-Pro-NH2) of the μ-opioid receptor highly selective agonist, morphiceptin (Tyr-Pro-Phe-Pro-NH2), was determined; the crystals are monoclinic with space group P21 and unit cell dimensions: a = 11.5731(5), b = 6.4490(3), c = 15.4082(5) Å, β = 100.359(5)° and Z = 2. To examine the influence of proline on the conformation of peptide bond, the molecular conformation was studied in solid state and solution (using 1H and 13C NMR data). The X-ray analysis revealed the following conformations of peptide backbone: φ1 = -63.2(5)°, ψ1 = 156.1(4)°, ω1 =-174.3(4)°, φ2 =-66.0(5)° and ψ2 = 152.0(4)°. The conformation of the Boc group is trans-trans. Experimental data revealed the trans conformation about the Phe-Pro amide bond, both in solid state and solution (DMSO). The possibility of cis/trans isomerization about the peptide bond (ω1) was examined by theoretical calculations using BIOSYM software. Molecular modelling, including molecular dynamics simulations of the title dipeptide, is in favour of trans peptide bond
