Fragmentation reactions of singly and doubly charged alkali metal ion–peptide complexes: A reaction specific to C‐terminal arginine residues

Ionspray ionization of solutions containing peptides (M) plus alkali metal ions X + produces [M + X] + , [M + H + X] 2+ and [M + 2X] 2+ ions. Low‐energy collision‐induced dissociation of these species demonstrated a marked trend towards decreased strength of the M–X + interaction with increasing size of X + . For example, the [M +2X] 2+ species yield predominantly [M + X] + plus X + for X + = K + , Rb + , Cs + , but rich spectra of fragment ions incorporating X + when X + = Li + or Na + . In general, these fragmentation reactions correspond to those established previously for the corresponding [M + X] + precursor, including formation of [ b i + X + OH] + and [ b i + X – H] + fragments. As a special case, complexes of Li + or Na + with peptides having Arg (but not Lys or other basic residues) at the C ‐terminus undergo a facile loss of 60 u. The resulting [M + X – 60] + ions also fragment further to yield [ y i ′ + X – 60] + fragments. Characterization of the 60 u neutral loss was undertaken by using tandem mass spectrometry plus hydrogen–deuterium exchange, and a mechanism is proposed to account for the characteristics of the process.