In ESI-source H/D exchange under atmospheric pressure for peptides and proteins of different molecular weights from 1 to 66 kDa: the role of the temperature of the desolvating capillary on H/D exchange

Transition of proteins from the solution to the gas phase during electrospray ionization remains a challenging problem despite the large amount of attention it has received during the past few decades. One of the major questions relates to the extent to which proteins in the gas phase retain their condensed phase structures. We have used in-electrospray source hydrogen/deuterium exchange to determine the number of deuterium incorporations as a function of protein mass, charge state and temperature of the desolvating capillary where the reaction occurs. All experiments were performed on a Thermo LTQ FT Ultra equipped with a 7-T superconducting magnet. Ions were generated by an IonMax Electrospray ion source operated in the positive ESI mode. Deuterium exchange was performed by introducing a droplet of D2 O beneath the ESI capillary. We systematically investigated gas phase hydrogen/deuterium (H/D) exchange under atmospheric pressure for peptides and proteins of different molecular weights from 1 to 66 kDa. We observed that almost all proteins demonstrate similar exchange rates for all charge states and that these rates increase exponentially with the temperature of the desolvating capillary. We did not observe any clear correlation of the number of H/D exchanges with the value of the cross section for a corresponding charge state. We have demonstrated the possibility of performing in-ESI source H/D exchange of large proteins under atmospheric pressure. The simplicity of the experimental setup makes it a useful experimental technique that can be applied for the investigation of gas phase conformations of proteins.