FAB-MS and Peptide Sequencing

Example The FAB-CID-MS/MS spectrum of thymosin-Tl [M+H] quasimolecular ions, m/z 1427.7, as obtained from a magnetic four-sector instrument [144] shows numerous fragment ions due to A-terminal, C-terminal, and internal fragmentations (Fig. 9.18). [145] 

Note One of the advantages of MS/MS techniques is that they do not require the full isolation of all compounds of interest, because the precursor ion selection of MSI excludes accompanying ions from contributing to the CID spectrum of the actually selected precursor ion as acquired by MS2 (Chaps. 4.2.7, 4.3.6, 4.4.5 and 12.3). 

It would be complicating if not impossible having to obtain sequence information from such a spectrum without rules to follow. [138-141,146,147] The most abundant ions obtained from the fragmenting peptide ion usually belong to six series named a, b, and c if the proton (charge) is kept in the A-terminus or x, y, and z, respectively, where the proton is located in the C-terminal part. Within each se- 

The ability of FAB mass spectra to deliver peptide sequence information was soon recognized [15,130]. Initially, the sequence was derived from fragment ions observed in the full scan spectra [15,96]. Another approach to sequence information is to subject the peptide to enzymatic hydrolysis by a mixture of several carboxy-peptidases to produce a series of truncated molecules. The FAB spectrum of the mixture then reveals the C-terminal sequence [131,132]. In the MALDI community, this approach became known as peptide ladder sequencing [133]. 

As protein ions are too big to effect fragmentation by collision-induced dissociation (CID, Chap. 9), they are enzymatically degraded to peptides prior to their mass spectrometric examination, e.g., by tryptic digestion [134]. The digest may be used directly to obtain MS/MS spectra of peptide [M+H] or [M+Na] ions. Alternatively, the peptides may be separated by liquid chromatography (LC), capillary electrophoresis (CE) [135], or 2D gel electrophoresis prior to MS. 

Nowadays, sequencing of peptides and other biopolymers by tandem mass spectrometry represents a major field of work for many mass spectrometrists [136-139] FAB and LSIMS no longer play a role here (for an example of peptide sequencing by FAB-MS cf. Chap. 9.6.6). 

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