Amino acid destruction, significance

Detection and quantification of protein by measuring absorbency at 280 nm is perhaps the simplest such method. This approach is based on the fact that the side chains of the amino acids tyrosine and tryptophan absorb at this wavelength. The method is popular, as it is fast, easy to perform and is non-destructive to the sample. However, it is a relatively insensitive technique, and identical concentrations of different proteins will yield different absorbance values if their content of tyrosine and tryptophan vary to any significant extent. Hence, this method is rarely used to determine the protein concentration of the final product, but it is routinely used during downstream processing to detect protein elution off chromatographic columns, and hence track the purification process. 

Protein and free amino acids found in tobacco leaf contribute significantly through pyrodegradation and pyrosynthesis to the formation of many nitrogenous compounds found in tobacco smoke. The nonvolatility of these compounds either as free acids, proteins, or members of tobacco pigment, for example, porphyrins, make them particularly liable to pyrolytic destruction because they, unlike nicotine and the other plant alkaloids, are not readily volatilized and swept away as the more intense heat of the cigarette coal approaches (3724). 

These methods are, however, completely destructive of the rest of the peptide chain—a single piece of information is gained at the cost of significant material use. When the Af-terminus of a peptide is reacted with phenyl isothiocyanate, PhN=C=S, the Edman reagent, a phenylthiohy-dantoin, 22.39, is obtained and the rest of the peptide chain is left intact (Figure 22.36). So the process can be repeated, and this methodology is the basis for automatic sequenators. The phen-ylthiohydantoin contains the R group from the Af-terminal amino acid and can thus be identified. 

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