C-type ions

The sequence of an underivatized hetero-CHO can be deduced from the occurrence of °4A and 4A cross-ring fragments in MS of the pseudomolecular ion as well as MS of sodiated C-type ions (unpublished results from this laboratory). In this case, the sodium ion may be understood as a tag, which allows identification of the reducing end sugar. 

More recently, c-type fragments have been studied in the negative mode [37]. In this case, several peptide systems were studied to determine the relative influence of amino acid chain length and side-chain makeup. Because c-type ions are known to form structures corresponding to linear peptide fragments with a C-terminus amide moiety, the important question for IRMPD to answer in this study is where the site of deprotonation resides for various systems. The possible stmctures are given in Fig. 14. 

As with protonated peptides, b-type ions have proved to be, perhaps, the most compelling in terms of potential stmctures. Peptide motifs with and without nucleophilic side chains have been investigated with quite interesting results which are not completely unlike their protonated counterparts. The first two systems studied, AlaAlaAla and AlaTyrAla, both gave rise to b2 ions taking the deprotonated oxazolone structure [38]. It is notable that tyrosine was seemingly uncompetitive as a deprotonation site, in clear contrast to c-type ions. 

Scheme 17.37. Proposed mechanisms for decompositions of long-chain hydrocarbons and formation of C-type ions (A) 1,4-Elimination involving even-electron hydrogen transfers. (Adapted with permission from Ref. 5.) (B) Homolytic cleavage and ion-molecule complex formation leading to two possibilities for transfer of hydrogen radical. (Adapted with permission from Ref. 167.) (C) Initial diradical formation upon high-energy CID of alkaU-cationized fatty acid ester precursor leading to hydrogen radical departures. (Adapted with permission from Ref. 168.)...

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