Reactive properties of aryl diazonium salts

Reactive properties of aryl diazonium salts Aryl diazonium cations are electrophilic reagents and appear to react predominantly with four types of amino acids - lysine, tyrosine, histidine and cysteine. Although the precise structure of the cysteine derivatives is unclear, aryl diazonium salts can react with lysine, histidine and tyrosine as indicated in eq. (5.3) below to give the various substituted derivatives. 

Each of the monosubstituted amino acids can react with a second mole of diazonium salt to give the derivatives XVI, XVII and XVIII. However, if the diazonium salt is incorporated into an affinity label. 

The monazo and bisazo derivatives of lysine, tyrosine, and histidine are unstable to acid hydrolysis and the native amino acids are not regenerated. Therefore, the best method to quantitate the extent of derivatization is with radioactive reagent. The characteristic absorp- 

The relative lability of diazonium salts dictates that the modification reaction be carried out between 0-5°C and that whenever possible the counterion of the diazonium salt be an inert anion such as tetra-fluoroborate. Diazonium salts readily undergo solvolytic reactions when exposed to ambient temperatures in hydroxylic solvents. They easily lose molecular nitrogen and will react with any available nucleophilic anion. For example, in an aqueous solution of potassium iodide the following reaction will take place (eq. 5.4). 

Frequently if the diazonium salt is isolated in dry crystalline form, with a nucleophilic anion such as chloride, an explosive reaction will ensue (eq. 5.5). However, the diazonium salts with a non-nucleophilic anion such as tetrafluoroborate are relatively stable. These salts are therefore of particular value if the synthesis of a stable radioactive reagent is desired. 

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