Solid state and solution conformation of Boc-L-Met-Aib-L-Phe-OMe

The conformation of the peptide Boc-L-Met-Aib-L-Phe-OMe has been studied in the solid state and solution by X-ray diffraction and ¹H n.m.r., respectively. The peptide differs only in the N-terminal protecting group from the biologically active chemotactic peptide analog formyl-L-Met-Aib-L-Phe-OMe. The molecules adopt a type-II ß-turn in the solid state with Met and Aib as the corner residues (øMet =- 51.8^o, øMet = 139.5^o, øAib = 58.1^o, øAib = 37.0^o). A single, weak 4 -> 1 intramolecular hydrogen bond is observed between the Boc CO and Phe NH groups (N—O 3.25 Å, N-H—O 128.4^o). ¹Hn.m.r. studies, using solvent and temperature dependencies of NH chemical shifts and paramagneti radical induced line broadening of NH resonances, suggest that the Phe NH is solvent shielded in CDCI₃ and (CD₃)₂SO. Nuclear Overhauser effects observed between Met Cα H and Aib NH protons provide evidence of the occurrence of Met-Aib type-II ß-turns in these solvents.