There are many ways to label peptides with tritium. The most prominent examples are shown in the scheme below. The four amino acids commonly used as precursors for the tritium labelling of peptides are 4-iodo-L-phenylalanine, 3,5-diiodo-L-tyrosine, 3,4-dehydro-L-proline and 4,5-dehydro-L-leucine. These unnatural amino acids are replacing the corresponding amino acids in your sequence to get the precursor for the tritium labelling. The iodinated amino acids are labelled by I/T exchange to yield the single or double labelled amino acid. The unsaturated amino acids are hydrogenated with tritium gas in presence of a catalyst leading to a twofold labelled amino acid. In these cases, the allylic positions can additionally undergo an H/T exchange leading to even higher specific activities. In summary, double bond saturation or I/T exchange yields products with suitable high specific activities (ranging from approx. 15 - 100 Ci/mmol; 0.55 – 3.7 TBq/mmol).
A further approach for peptide labelling is the use of . This method does not require a precursor, but specific activities are not predictable unless H/D studies were performed and they might end up to be low. Nevertheless, keep in mind that suitable specific activity is depending on the application of the peptide and as well on the economic feasibility. Just and we will find the most appropriate and efficient way to label your peptide.