Reaction of diazonium salt with tyrosine residues in polypeptides

Some copolymers of L ‐glutamic acid and L ‐tyrosine are treated with p ‐carboxybenzenediazonium chloride. Monoazotyrosine chromophores can be obtained along the polypeptide chains in the presence of a stoichiometric quantity of the reagent, whereas an excess of the reagent gives rise to the formation of a mixture of mono‐ and bis‐azoderivatives. In all the cases only a part of the total number of the tyrosine residues can be modified. The results are discussed on the basis of the p K a values for the ionization of tyrosine and azo‐tyrosine OH‐groups.