A technique for radioactively labeling proteins is electrophilic radioiodination in which an aromatic substitution of ¹³¹I onto a tyrosine residue is performed as follows:

Using the activity of ¹³¹I, one can measure protein lifetimes in a variety of biological processes. ¹³¹I undergoes beta decay with a half-life of 8.02 days. Initially a protein labeled with ¹³¹I has a specific activity of 1.0 μCi, which corresponds to 37,000 decay events every second. The protein is suspended in aqueous solution and exposed to oxygen for 5 days. After isolating the protein from solution, the protein sample is found to have a specific activity of 0.32 μCi. Is oxygen reacting with the tyrosine residues of the protein, resulting in the loss of ¹³¹I?