Histidine diazonium group

Figure 11.17 The diazonium group of p-diazobenzoylbiocytin can react with tyrosine or histidine residues in proteins to form diazo bonds.

Another problem was the number of chelating groups attached to each protein. Diazonium reagents react with many different amino acid side chains including those of lysine, tyrosine, and histidine (24). There-... 

Much of Landsteiner s pioneer work was carried out with haptens that were aromatic amines. The compounds were converted to diazonium salts with nitrous acid and aUowed to react with proteins at alkaline pH (approximately 9). Reaction occurred primarily with histidine, tyrosine, and tryptophan residues of the protein carrier. For a representative procedure, see Kabat (p. 799 seq.). An interesting application of this procedure was the preparation of a chloramphenicol-protein conjugate which was used to elicit antibodies specific for chloramphenicol. In this case, a prior reduction of the nitro group of chloramphenicol to an amino group was required. As early as 1937, carcinogenic compounds were conjugated to protein carriers by means of their isocyanate derivatives which were prepared from amines. Immune sera were raised, and their properties were studied. - ... 

The experiments of Wofsy and Parker indicated that one cannot generalize concerning the presence of tyrosine, capable of being affinity labeled, in the active sites of antibodies (119). Horse antibodies specific for haptens containing carbohydrate (a /3-lactoside or a j8-galactoside derivative) were affinity labeled with an appropriate diazonium reagent. The affinity label was found principally in association with histidine in both the H and the L chains. Some tyrosine groups were labeled, however, in rabbit H chains in antibody of the same specificity L chains were also affinity labeled, but the amino acid side chain involved was not established. 

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