Ion/Ion Reactions of MALDI-Derived
Peptide Ions: Increased
Sequence Coverage via Covalent and Electrostatic Modification upon
Charge Inversion
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Abstract
Atmospheric pressure matrix-assisted laser desorption/ionization
(AP-MALDI)-derived tryptic peptide ions have been subjected to ion/ion
reactions with doubly deprotonated 4-formyl-1,3-benzenedisulfonic
acid (FBDSA) in the gas-phase. The ion/ion reaction produces a negatively
charged electrostatic complex composed of the peptide cation and reagent
dianion, whereupon dehydration of the complex via collision-induced
dissociation (CID) produces a Schiff base product anion. Collisional
activation of modified lysine-terminated tryptic peptide anions is
consistent with a covalent modification of unprotonated primary amines
(i.e., N-terminus and ε-NH<sub>2</sub> of lysine). Modified
arginine-terminated tryptic peptides have shown evidence of a covalent
modification at the N-terminus and a noncovalent interaction with
the arginine residue. The modified anions yield at least as much sequence
information upon CID as the unmodified cations for the small tryptic
peptides examined here and more sequence information for the large
tryptic peptides. This study represents the first demonstration of
gas-phase ion/ion reactions involving MALDI-derived ions. In this
case, covalent and electrostatic modification charge inversion is
shown to enhance MALDI tandem mass spectrometry of tryptic peptides