Fast Atom Bombardment Combined with Tandem Mass Spectrometry for the Determination of Cyclic Peptides

The mass spectra and colllsional activated decomposition (CAD) spectra of a series of underivatlzed cyclic polypeptides Ionized by fast atom bombardment (FAB) are reported. All compounds give easily identifiable protonated molecular Ions. The colllsional activation decomposition spectra of products from the (M + H)⁺ can be understood In terms of protonation of an amide nitrogen, scission of the N-acyl bond, and fragmentation by loss of successive amino acid residues from the resulting ring-opened acyllum ion. Thus, mass spectrometry experiments permit the molecular weight and the amino acid sequence to be assigned.