Characterization of dipeptide isomers by tandem mass spectrometry of their mono- versus dilithiated complexes

The Li+ complexes of the isomeric dipeptide pairs PheGly/GlyPhe, PheAla/AlaPhe, and TrpAla/AlaTrp, namely, [Pep + Li]+, and of the corresponding lithium carboxylates, namely, [Pep - H + 2Li]+, are produced in the gas phase by desorption ionization, and their unimolecular chemistry is probed by tandem mass spectrometry experiments at various activation conditions. At low internal energies, monolithiated isomers dissociate to the same products, formed through a mixed anhydride intermediate in which the sequence information is lost. Isomerization to the mixed anhydride is less competitive at higher internal energies, which start promoting sequence-specific fragmentations. On the other hand, dilithiated isomers (they contain a permanent COO-Li+ salt bridge) do not rearrange to an anhydride and give rise to substantially different fragmentation patterns; structurally diagnostic c1- and y1-type fragments are observed at all internal energies, allowing for unequivocal sequence assignment. The mono- and dilithiated peptides undergo loss of their aromatic side chain to form distonic radical ions carrying Li+ charge(s) and one unpaired electron at an alpha-C atom of the peptide backbone. The yield of such metal-bound peptide radicals is particularly high from the dilithiated complexes, [Pep - H + 2Li]+. Upon activation, the Li+ ions become mobile and can be shuttled to the various basic sites of the dipeptides, where they may initiate backbone fragmentation or the elimination of small neutral molecules.