Tritium
Tritium has a specific activity of 28.6Ci per matom (1.07TBq per matom) and therefore the incorporation of one tritium atom per peptide molecule will yield material with a specific activity of 28.6Ci per mmole.
The specific activity of tritium labelled peptide can be increased by increasing the number of tritium atoms present per molecule. The specific activities that can be achieved with various amino acids are summarised below.
Higher specific activities can be achieved by labelling multiple amino acids within a peptide.
Conversely, the specific activity of the labelled peptide can be reduced to any desired value by diluting the material with unlabelled material.
| Amino acid | Maximum specific activity achievable |
|---|---|
| Proline | 57Ci/mmole |
| Leucine | 200Ci/mmole |
| Phenylalanine | 28Ci/mmole |
| Tyrosine | 57Ci/mmole |
| Alanine | 57Ci/mmole |
Carbon-14
Carbon-14 has a specific activity of 62.5mCi per mmole (2.31GBq per mmole) and peptides containing one carbon-14 will be expected to have a specific activity in the range 50-60mCi per mmole. The simplest amino acid, glycine, can be prepared with one or both carbon atoms labelled with carbon-14 leading to a maximum specific activity of 100-120mCi/mmole; similarly the N-terminus of the peptide can be labelled with acetic acid at specific activity of up to 120mCi/mmole.
Other amino acids can be labelled up to a maximum specific activity determined by the number of carbon atoms present. As with tritium, the specific activity can be increased by incorporating several amino acids labelled with carbon-14 and can be reduced by diluting with unlabelled material.