Tandem Mass Spectrometry Methodology for the Sequence Determination of Cyclic Peptides

The unequivocal sequence determination of cyclic penta- and hexapeptides, including biologically active enkephaline and somatostatine analogues, is achieved by using the following procedure. Production of gas-phase [M + H]⁺ by using fast atom bombardment is followed by investigating either unimolecular or collision-induced dissociations of mass selected [M + H]⁺ and [MH - amino acid residues]⁺ fragments. The unimolecular and collisionally activated decompositions of these ions and of their fragments resulting from decarbonylation enable one to elucidate unequivocally the sequence of cyclic peptides. As an example, the sequence determination of an unknown synthetic enkephalin analogue is presented. The amount of sample required for the determination is in the range of 5–100 nmol. Unambiguous sequencing cannot normally be achieved by only analyzing the decompositions of [M + H]⁺ ions.