A method of distinguishing between aspartic acid and asparagine and between glutamic acid and glutamine during sequence analysis by the dansyl‐Edman procedure

The dansyl-Edman procedure is widely used for sequence analysis of peptides due to its high sensitivity and rapidity [ 1,2] . The method suffers from the major disadvantage that, under the conditions required to liberate N-terminal Dns-amino acids from peptides, complete hydrolysis of the amide side chains of asparagine and glutamine occurs, thus making it impossible to distinguish between aspartic acid and asparagine or between glutamic acid and glutamine. One solution to the problem is to measure the mobility of the intact peptide on electrophoresis at pH 6.5 relative to the mobility of aspartic acid; for peptides not containing histidine, a good correlation exists between mobility, molecular weight and net ionic charge [3]. This method, however, only provides an unambiguous assignment of acid and amide side chains if a single such residue occurs in the peptide or alternatively if all such residues are either acids or amides. This restriction can be removed by measuring the mobility of the peptide after each step of Edman degradation, but this is very wasteful of time and material. Clearly, the most satisfactory approach would be to modify side chain carboxyl groups before sequence analysis in such a way that the modification is