Beam-type collisional activation of polypeptide cations that survive ion/ion electron transfer.

Doubly protonated peptides that undergo an electron transfer reaction without dissociation in a linear ion trap can be subjected to beam-type collisional activation upon transfer from the linear ion trap into an adjacent mass analyzer, as demonstrated here with a hybrid triple quadrupole/linear ion trap system. The activation can be promoted by use of a DC offset difference between the ion trap used for reaction and the ion trap into which the products are injected of 12-16 V, which gives rise to energetic collisions between the transferred ions and the collision/bath gas employed in the linear ion trap used for ion/ion reactions. Such a process can be executed routinely on hybrid linear ion trap/triple quadrupole tandem mass spectrometers and is demonstrated here with several model peptides as well as a few dozen tryptic peptides. Collisional activation of the peptide precursor ions that survive electron transfer frequently provides structural information that is absent from the precursor ions that fragment spontaneously upon electron transfer. The degree to which additional structural information is obtained by collisional activation of the surviving singly charged peptide ions depends upon peptide size. Little or no additional structural information is obtained from small peptides (<8 residues) due to the high electron transfer dissociation (ETD) efficiencies noted for these peptides as well as the extensive sequence information that tends to be forthcoming from ETD of such species. Collisional activation of the surviving electron transfer products provided greatest benefit for peptides of 8-15 residues.