Identification of isocyclosporins by collision-induced dissociation of doubly protonated species

Abstract Nonribosomal cyclopeptide cyclosporin A (CsA), produced by fungus Tolypocladium inflatum, is an extremely important immunosuppressive drug used in organ transplantations and for therapy of autoimmune diseases. Here we report the first time production CsA, along with related cyclosporins B C, inflatum strains marine origin (White Sea). Cyclosporins A–C contain unusual amino acid, (4R)-4-((E)-2-butenyl)-4,N-dimethyl- l -threonine (MeBmt), are prone to isomerization non-active isocyclosporin N→O acyl shift valine connected MeBmt acidic conditions. CsA isoCsA not distinguishable MS analysis [M+H]+ ions due rapid [CsA + H]+→[isoCsA + H]+ conversion. We found that completely suppressed cyclosporine [M+2H]2+ ions, their collision-induced dissociation (CID) can be unambiguous isocylosporins. Fragmentation patterns [CsA+2H]2+ [isoCsA+2H]2+ were analyzed explained. The developed approach could useful other peptides containing β-hydroxy-α-amino acids.