Y-type ions

The formation of y type ions was observed65 from several of the precursor ions studied and the threshold energies E0 were compared with proposed mechanisms. The interested reader can find these results and many additional findings in the original work 65 The purpose of the present account was to provide an illustration of the contribution that the determinations of threshold curves and threshold energies can provide to the elucidation of the mechanisms involved. 

Fragmentation of peptides can also be observed with FTICR instruments. Infrared multiple photon dissociation (IRMPD) and electron capture dissociation (ECD) have been introduced as two alternative dissociation methods to the low-energy CID method. The IRMPD method produces many fragments that make the spectrum very complex and difficult to interpret. Some of the fragment types observed with IRMPD are b and y type ions or these ions that have lost ammonia or water. However, most of them are not these types of fragment ions. 

Example The direct bond cleavages upon ECD can be used to locate postrans-lational peptide modifications that would be disguised in CID or IRMPD spectra The difference between CID and IRMPD on one side and ECD on the other arises from the loss of most posttranslational modifications prior to backbone cleavage to from b- and y-type ions, while in ECD the chain is immediately cleaved to yield c- and 2-type ions, independent of the peptide sequence [160], Thus, the neutral loss reflects the mass of the modification, e.g., additional 80 u (HPO3) in case of the phosphorylation that can be identified to occur at the tyrosine of the peptide RLpYIFSCFR (Fig. 9.38) [150]. The opportunity to obtain conplementary peptide sequence information is the reason why CID or IRMPD spectra are sometimes combined with ECD spectra for structure elucidation of peptides [136,149,161]. 

Figure 18.3. ESI-QTOF-MS/MS spectra of tryptic peptide generated from normal water (top) and 0-labeled water (bottom). Product ions shifted by 2 Da are y-type ions, whereas b ions show no increase in mass. Product y ions are indicated by the dot symbols.

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