Standard radioiodination methods lack site-selectivity and either mask charges (Bolton-Hunter) or involve oxidative reaction conditions (chloramine-T). Opioid peptides are very sensitive to certain structural modifications, making these labeling methods untenable. In our model opioid peptide, α-neoendorphin, we replaced a tyrosyl hydroxyl with an iodine, and in cell lines stably expressing mu, delta, or kappa opioid receptors, we saw no negative effects on binding. We then optimized a repurposed Sandmeyer reaction using copper(I) catalysts with non-redoxing/non-nucleophilic ligands, bringing the radiochemical yield up to around 30%, and site-selectively incorporated radioactive iodine into this position under non-oxidizing reaction conditions, which should be broadly compatible with most peptides. The (125)I- and (131)I-labeled versions of the compound bound with high affinity to opioid receptors in mouse brain homogenates, thus demonstrating the general utility of the labeling strategy and of the peptide for exploring opioid binding sites.
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