Quantification of azo-coupled lysine in azo proteins by amino acid analysis

Hydrochloric acid hydrolysis of azo proteins in which lysine residues are azo coupled, results in conversion of modified lysines to alpha-amino-epsilon-hydroxy caproic acid plus alpha-amino-epsilon-chloro caproic acid. The latter can interfere with the determination of tyrosine by amino acid analysis. This potential problem can be avoided either by making basic and then neutralizing HCl hydrolysates or by hydrolyzing the protein in methane sulfonic acid.