Water-soluble phosphines for direct labeling of peptides with technetium and rhenium: insights from electrospray mass spectrometry

Direct labeling of salmon calcitonin (sCT) is possible in one step using water-soluble phosphines (sulfonated triphenylphosphines) as the reducing agent both for disulfide and for pertechnetate. Phosphines were the most efficient reducing agent for disulfide bonds among those examined. The phosphines both reduced the pertechnetate to Tc(III), and contributed to the technetium coordination sphere in the labeled product. In contrast, the phosphines did not reduce rhenium below oxidation state V, nor did they participate in the rhenium coordination sphere in the labeled peptide. Instead, the expected oxorhenium(V) moiety was incorporated. Both Tc and Re labeling processes gave rise to dimers with two peptides linked by the metal center, as well as simple monomeric species. Positive mode electrospray mass spectrometry not only revealed the presence of phosphine bound to technetium and oxygen bound to rhenium in the metallopeptides but also revealed the oxidation states of the metals. Electrospray mass spectrometry is proving to be an exceptionally valuable technique for characterizing radiopharmaceuticals. Although the one-step direct labeling method described gives mixed products and poor receptor affinity when applied to the small peptide sCT, it might be readily adapted to monoclonal antibodies.