Alternatively, the deuteration pattern of a labeled protein
can be determined by a “top-down” approach, which fragments
intact proteins in the gas-phase inside the mass spectrometer.
Since this approach bypasses the in-solution protein digestion
step and the chromatographic peptide separation, it has attracted
a lot of attention in the HDX-MS community in recent years.
Collision-induced dissociation (CID) was initially evaluated for
this purpose by a number of research groups [18–21]. However,
the potential problems of H/D migration across the polypeptide
chain (scrambling) prior to bond dissociation, as well as limited
sequence coverage, makes this technique not generally viable
for top-down HDX-MS measurements [22–24]. In contrast, electron
capture dissociation (ECD) [25], which gives ultrafast peptide
bond cleavage with minimal vibrational excitation of the precursor
ions [26,27], holds great promise for top-down HDX. It
has recently been demonstrated that no significant H/D scrambling
occurred during ECD fragmentation of isotopically labeled
proteins [28–30], as long as the instrument settings for ion transfer
are set at relatively gentle conditions to minimize collisional
activation. A closely related technique, electron transfer dissociation
(ETD) has also been shown to be a viable gas-phase
sequencing method for top-down HDX without the problem of
scrambling [31–34].