Discrimination of isobaric leucine and isoleucine residues and analysis of post-translational modifications in peptides by MALDI in-source decay mass spectrometry combined with collisional cooling.

Collisional cooling employed in an orthogonal time-of-flight mass spectrometer (o-TOF MS) stabilizes fragment ions that are generated by matrix-assisted laser desorption ionization in-source decay (MALDI ISD). By variation of the buffer gas pressure, ISD and "post-source decay" (PSD) dissociation channels can be switched on and off to some extent. Under high-pressure conditions, ISD type fragments of post-translationally modified (PTM) peptides are generated that contain even labile bound side groups; the examples of a phosphorylated and an O-glycosylated peptide are shown. At elevated laser fluences, d- and w-type fragment ions of peptides are detected as a result of high-energy side chain cleavage. This allows for differentiation of the isobaric amino acid residues leucine and isoleucine. Reduction of the cooling efficiency by lowering the buffer gas pressure results in the loss of the d-/w-type species, presumably in secondary metastable dissociation processes. This also enhances cleavage of the side groups from the PTM peptides and can be used to corroborate identification of the modification site. Furthermore, these measuring conditions generate small amounts of fragments containing sequence information about the cyclic part of a disulfide peptide by inducing symmetric and asymmetric cleavage of the intramolecular S-S bond.