Label scrambling during CID of covalently labeled peptide ions

Covalent labeling along with mass spectrometry is finding more use as a means of studying the higher order structure of proteins and protein complexes. Diethylpyrocarbonate (DEPC) is an increasingly used reagent for these labeling experiments because it is capable of modifying multiple residues at the same time. Pinpointing DEPC-labeled sites on proteins is typically needed to obtain more resolved structural information, and tandem mass spectrometry after protein proteolysis is often used for this purpose. In this work, we demonstrate that in certain instances, scrambling of the DEPC label from one residue to another can occur during collision-induced dissociation (CID) of labeled peptide ions, resulting in ambiguity in label site identity. From a preliminary study of over 30 labeled peptides, we find that scrambling occurs in about 25% of the peptides and most commonly occurs when histidine residues are labeled. Moreover, this scrambling appears to occur more readily under non-mobile proton conditions, meaning that low charge-state peptide ions are more prone to this reaction. For all peptides, we find that scrambling does not occur during electron transfer dissociation, which suggests that this dissociation technique is a safe alternative to CID for correct label site identification.