**3.1 Radioiodination**

Radioiodination of peptides with 125I, 123I, 124I, or 131I can be performed by either direct labeling or indirect labeling via an auxiliary group. During direct radioiodination, radioactive iodine is incorporated covalently into the side chains of tyrosyl or histidyl residues in the presence of an oxidizing agent such as chloramine-T or iodogen. If no tyrosyl or histidyl residue is available, free amino groups in the peptide may be radioiodinated by auxiliary groups, including *N*-succinimidyl-3-(4-hydroxyphenyl)propionate, known as Bolton-Hunter reagent [30]. Other auxiliary groups for radioiodination of peptides are *N*-succinimidyl-3-iodobenzoate (SIB), *N*-succinimidyl-5-iodo-3-pyridinecarboxylate (SIPC), and *N*-succinimidyl 4-guanidinomethyl-3-iodobenzoate (SGMIB) (**Figure 7**) [31–34].

As none of the radioiodination methods is based on complexation with a chelator, we will not focus on this labeling method in this book chapter further.
