Cyclic pentapeptide analogs based on endomorphin-2 structure: Cyclization studies using liquid chromatography combined with on-line mass spectrometry and tandem mass spectrometry

The cyclization of linear analogs based on endomorphin-2 structure, Tyr/Dmt-d-Lys-Phe-Phe-Asp-NH2 and Tyr/Dmt-d-Cys-Phe-Phe-Cys-NH2 (where Dmt = 2′,6′-dimethyltyrosine), resulting in obtaining lactam or disulfide derivatives, was studied using liquid chromatography combined with on-line mass spectrometry (LC-MS) and tandem mass spectrometry (LC-MS/MS). In case of cyclization via an amide bond, the formation of the cyclic monomers, cyclic but not linear dimers and even traces of cyclic trimers was observed. Disulfide bridge containing peptides was obtained by the solid-phase synthesis of the linear sequences, followed by either in-solution or on-resin cyclization. In case of the in-solution cyclization, the expected cyclic monomers were the only products. When oxidation of the cysteine residues was performed when the peptides were still on the resin, cyclic monomer and two cyclodimers, parallel and antiparallel, were found. Digestion of the isolated cyclodimers with α-chymotrypsin allowed for their unambiguous identification. The comparison of the cyclic monomer/dimer ratios for analogs with Tyr versus Dmt in position 1 revealed that the presence of the exocyclic Dmt favored formation of the cyclic monomer, most likely due to the increased steric bulk of this amino acid side-chain as compared with Tyr.