SEQUENCING OF CYCLOSPORINS BY FAST-ATOM-BOMBARDMENT AND LINKED-SCAN MASS-SPECTROMETRY

The mass spectrometric sequence determinations of amino acid residues in cyclosporins using fast atom bombardment, collisionally activated dissociation in the first field-free region and linked B/E scan is described. The general fragmentation scheme was derived from the spectra of cyclosporins A, B, C, D, F, G, L and [DH-MeBmt1]CS. The main fragmentation pathways start by primary splitting between amino acids 2-3, 1-11 and 5-6. corresponding-terminal b-type ions are common fragment types in the mass spectra. The 1-11 splitting can be enhanced by the introduction of a lactone group into the peptide ring by conversion of cyclosporins into isocyclosporins. The fragmentation scheme was used for amino acid sequence determination in four new natural cyclosporins, [MeLeu1] CS, [Leu4] CS, [Ile4] CS and [Leu5] CS.