Tyrosine diazonium group

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

Fig. 2.10. Illustration of the mechanism of affinity labeling. When the hapten is in its most favored position in the active site the reactive diazonium group is in close proximity to a side chain (tyrosine in the figure) with which it is capable of forming a covalent bond.

Tyrosine may be targeted specifically for modification through its phenolate anion by acylation, through electrophilic reactions such as the addition of iodine or diazonium ions, and by Mannich condensation reactions. The electrophilic substitution reactions on tyrosine s ring all occur at the ortho position to the —OH group (Figure 1.11). Most of these reactions proceed effectively only when tyrosine s ring is ionized to the phenolate anion form. 

The phenolic group of tyrosine undergoes iodina-tion (Eq. 3-44), acylation, coupling with diazonium compounds, and other reactions. 

The catalytically essential nature of tyrosine 85 and its proximity to the substrate binding site and to tyrosine 115 were demonstrated from studies of modification with tetranitromethane (71) and from studies of intramolecular cross-linking of aminotyrosyl residues (72). The bro-moacetamidophenyl (69) and diazonium (70) reagents obtained from aminophenyl-pdT both react selectively and exclusively with tyrosine 85. This residue is situated, stereochemically, such that its hydroxyl group can interact with the 3 -phosphate of pdTp. 

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. - ... 

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