Site-specific analysis of gas-phase hydrogen/deuterium exchange of peptides and proteins by electron transfer dissociation.

To interpret the wealth of information contained in the hydrogen/deuterium exchange (HDX) behavior of peptides and proteins in the gas-phase, analytical tools are needed to resolve the HDX of individual exchanging sites. Here we show that ETD can be combined with fast gas-phase HDX in ND(3) gas and used to monitor the exchange of side-chain hydrogens of individual residues in both small peptide ions and larger protein ions a few milliseconds after electrospray. By employing consecutive traveling wave ion guides in a mass spectrometer, peptide and protein ions were labeled on-the-fly (0.1-10 ms) in ND(3) gas and subsequently fragmented by ETD. Fragment ions were separated using ion mobility and mass analysis enabled the determination of the gas-phase deuterium uptake of individual side-chain sites in a range of model peptides of different size and sequence as well as two proteins; cytochrome C and ubiquitin. Gas-phase HDX-ETD experiments on ubiquitin ions ionized from both denaturing and native solution conditions suggest that residue-specific HDX of side-chain hydrogens is sensitive to secondary and tertiary structural features occurring in both near-native and unfolded gas-phase conformers present shortly after electrospray. The described approach for online gas-phase HDX and ETD paves the way for making mass spectrometry techniques based on gas-phase HDX more applicable in bioanalytical research.