Effect of Amino Acid Side-Chain on Fragmentation of Cyclic Peptide Ions: Differences of Electrospray Ionization/Collision-Induced Decomposition Mass Spectra of Toxic Heptapeptide Microcystins Containing ADMAdda Instead of Adda

Characteristics of collision-induced decomposition (CID) spectra recorded following electrospray ionization (ESI) of toxic heptapeptide microcystins containing (2S,3S,8S,9S)-3-amino-9-acetoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (ADMAdda) have been investigated in detail and compared with those from compounds containing (2S,3S,8S,9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (Adda). A significant difference in their CID spectra was observed in the cleavage of cyclic structures in low-energy CID processes due to the effect of the amino acid side-chain on fragmentation. Microcystins containing ADMAdda produced mostly b-type ions. Fragment ions predicted on the basis of the known fragmentation of cyclic peptides and linear-peptide b-type ions, predicted by the Peptide Matching (PEPM) program, matched well with peaks in the CID spectra. A neutral loss of CH₃COOH from the ADMAdda residues was observed in the CID spectra of doubly charged ions, [M + 2H]²⁺. The present study suggested that the CID spectra of [M + H]⁺ and [M + 2H]²⁺ ions can provide their sequence information.