Endothermic Ion-Molecule Reactions: Strategies for Tandem Mass Spectrometric Structural Analyses of Large Biomolecules

Very low energy collisions between the protonated peptide leucine-enkephalin and ammonia on a tandem mass spectrometer lead to the formation of a proton-bound collision complex, which dissociates to form fragment ions of the peptide or by transferring a proton to ammonia (neutralization). The endothermlclty of the proton transfer reaction suggests that the proton was initially located on the amide bonds of the peptide. From these studies we conclude that endothermic Ion-molecule reactions may be effective for the fragmentation of large peptides and/or as the first step in a neutralization/chemical reionlzation scheme in which the reverse (exothermic) reaction is used for reprotonation.