Dissociation behavior of tryptic and intramolecular disulfide-linked peptide ions modified in the gas phase via ion/ion reactions

► Tryptic and disulfide-linked peptide cations are covalently modified in the gas phase via Schiff base formation at primary amines via reaction with 4-formyl-1,3-benzenedisulfonic acid (FBDSA) anions. ► Multiply protonated peptides are reduced in charge via reaction with singly deprotonated FBDSA anion whereas singly protonated peptides are inverted in charge via reaction with doubly deprotonated FBDSA. ► Collision-induced dissociation of modified peptide ions leads to fragmentation that is complementary to that noted for unmodified versions of the same peptides, which is due to charge sequestration at the highly acidic sulfonate groups of FBDSA.