Secondary Losses via γ-Lactam Formation in Hot Electron Capture Dissociation: A Missing Link to Complete de Novo Sequencing of Proteins?

Differentiating between the isomeric isoleucine and leucine amino acid residues can be accomplished by a gas-phase fragmentation reaction in which an N-terminal radical site of a C-terminal fragment initiates formation of a five-membered ring with the beta-carbon in the adjacent residue.