Direct bond cleavage

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

The study of the radiation products induced in methyl oleate, methyl stearate, trioleate, and tristearin also supports the argument for direct bond cleavage. Data are given in Table II, and a representation of the process is indicated below. 

In meats, of course, there are components which arise from the protein which cannot be present in the products from pure fat. Table III shows some of the sulfur compounds and aromatic compounds which are also found in irradiated meats. Many of these can be postulated as arising from direct bond cleavage of amino acid moieties. Benzene and toluene may come from phenylalanine and phenol and p-cresol from tyrosine. Recent studies have been directed to considering the origin of some of the compounds from proteinaceous substances. Some of the sulfides, disulfides, and mercaptans can derive directly from cysteine or methionine, but those containing more than two carbon atoms in a chain require more than a superficial explanation. In order to evaluate the contribution of the volatiles from the protein as well as the lipid constituents of meat, volatile components produced in various protein substances have also been analyzed. 

While direct bond cleavage can occur, the spectral energies involved suggest that this is not a primary mechanism of photolysis of the colorants. 

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]. 

By definition alkanes possess no heteroatoms (no free electron pairs) or double bonds (no Jt electrons). They consist of simple C-C and C-H bonds and therefore count only o electrons that are more difficult to remove than n and n electrons. The molecular ions are generally scarce because they are very unstable. The El removal of one of the two electrons composing a o bond leads to directly bond cleavage. Figure 9.29 shows the main ions thereby formed in the case of n-heptane. The abundance of ions decreases with the increase of the m/z ratio beyond m/z 57. 

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