Histidine reaction with iodine

The reaction of IODO-GEN with iodide ion in solution results in oxidation with subsequent formation of a reactive, mixed halogen species, IC1 (Fig. 266). Either 125I or 13 1 can be used in this reaction. The IC1 then rapidly reacts with any sites within target molecules that can undergo electrophilic substitution reactions. Within proteins, any tyrosine and histidine side-chain groups can be modified with iodine within... 

Directing the iodination reaction toward histidine residues in proteins, as opposed to principally tyrosine modification, is possible simply by increasing the pH of the lodobeads reaction from the manufacturer s recommended pH 7.0-8.2 (Tsomides et ai, 1991). No reducing agent is required to stop the iodination reaction as is the case with chloramine-T and other methods. 

Tryptophan can also be iodinated directly. Both mono and diiodinated derivatives are known. Reaction is slower than with tyrosine and histidine, and hence occurs rarely. 

If the experimenter wishes for monoiodized proteins, however, just a single iodine can keep the protein from binding. In this case, the experimenter changes the reaction conditions to try to iodize another tyrosine residue or a histidine residue, in the hope that iodine in the new position does not interfere with the binding. If this hope turns out to be unfulfilled, or you do not want to get involved in endless screening of reaction conditions, Bolton-Hunter reagent or marking remain an option. 

The experimenter separates heterooligomers before cleaving them into subunits. Then she denatures the purified subunit and reduces possible disulfide bridges. A carboxymethylation with iodoacetic acid protects against the reoxidation of the cysteines (Lind and Baker 1982). This reaction requires a sure instinct, in that other amino acids such as methionine, lysine, and histidine also react at high concentrations of iodine acetate or at the wrong pH. If you just need a few partial sequences, you can skip the carboxymethylation. 

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