Proteins

As mentioned in the first paragraph of this chapter, the analysis of proteins is one of the oldest applications of Py-MS (ref. 1). Rather than break-up of the polymer backbone into large fragments characteristic of the original building blocks, the dominant mechanism appears to be the splitting off of appendages (ref. 95). As a rule, highly characteristic signals are found for the aromatic- and sulphur-containing amino acid moieties, e.g. hydrogen sulphide for cyst(e)ine and in combination with methanethiol also for methionine; pyrrole, pyrrolidine and methyl -pyrrole for (hydroxy)proline; phenol and cresol for tyrosine; toluene, styrene and phenylacetonitrile for phenylalanine; and indole and methyl indole for tryptophan (see Figure 6). As is evident from this far from extensive list, some fragments, e.g. pyrroles and phenylacetonitrile, must involve scission of the polymer chain. In fact, some of the aliphatic amino acid moieties also produce corresponding nitriles. Nevertheless, significant nitrile formation is not observed for tyrosine or tryptophan.

There seem to be two possible explanations for the limited amount of structural information obtained thus far from the Py-MS analysis of proteins. First, proteins are composed of a greater variety of building blocks than any other class of biopolymers. This in turn provides for a great variety of pyrolysis products, many of which possess the same nominal mass. Nominal resolution mass spectra obviously

Met-Glu-His-Phe-Arg-Trp-Gly

Met-Glu-His-Phe-Arg-Trp-Gly

80 100 120 m/z WO 160

Figure 6. Curie-point pyrolysis mass spectra of synthetic oligopeptides representing sequences from adrenocorticotropic and p-lipotropin. Note characteristic ion signals of methionine (m/z 34, 48), phenylalanine (m/z 92, 104, 117), tyrosine (m/z 94, 108, 120, 122) and tryptophan (m/z 117, 131). Conditions: samples 10 yg; T 510°C; E , 14 eV. c el

80 100 120 m/z WO 160

Figure 6. Curie-point pyrolysis mass spectra of synthetic oligopeptides representing sequences from adrenocorticotropic and p-lipotropin. Note characteristic ion signals of methionine (m/z 34, 48), phenylalanine (m/z 92, 104, 117), tyrosine (m/z 94, 108, 120, 122) and tryptophan (m/z 117, 131). Conditions: samples 10 yg; T 510°C; E , 14 eV. c el provide very limited information on such an extremely complex pyrolysate. Secondly, there may well be a fundamental problem in the pyrolysis reaction mechanisms of proteins, namely a pronounced tendency for charring (ref. 95) involving the polymer backbone. This prevents the release of more or less complete amino acid moieties and instead results in the splitting off of appendages such as the phenol and indole fragments. As charring during pyrolysis appears to be inversely proportional to the rate of heating of the sample (ref. 23) - providing an important argument for

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