Protein monoiodized

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. 

An iodination reaction often creates a mixture of different molecules. For example, the iodination of proteins that contain several tyrosine residues yields monoiodized, di-iodized. 

If the monoiodized molecule species cannot be isolated, you have a mixture of iodized and uniodized molecules (i.e., you have to determine the specific radioactivity). This is calculated as the amoimt of incorporated radioactivity divided by the amoimt of molecules (iodized + uniodized) in mol. The specific activity is given in Ci/mM. On the basis of the specific activity, you can calculate how many atoms of the radioactive isotope are contained in a molecule (Figure 2.2). Thus, if you have a protein derivatized with iodine and with a specific radioactivity of 2,200 Ci/mM, each protein molecule contains one iodine atom on average. 

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