Peptides separation mode alternatives

In addition to suitable protection of other coreactive functionalities, activation of the carboxy group prior to reaction with the amino group is required for a controlled formation of the amide bond between two a-amino acid or peptide components (Scheme 1). For this purpose activated species can be generated separately, isolated, and stored for subsequent use. Alternatively, the activation can be carried out in situ with specific coupling reagents. Independently of the mode of activation, an electron-withdrawing group X must be incorporated at the acyl carbon in a transient mode in the in situ activation or in the formation of reactive intermediates. 

An interesting alternative to the more frequentiy applied postisotope dilution [69]. In this case, the species-unspecific spike was directly added to the eluents used for separation, which is only possible, of course, if the spike compound does not react with the different analytes and if the column does not fractionate the spike from the analyte, which always needs to be carefully assessed. Successful application of this type of species-unspecific isotope dilution has been demonstrated for the quantification of sulfur[Pg.214]

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