| dc.description.abstract | The ability to retain and observe solution-phase structures using the gas-phase ion mobility (IM) technique is paradoxical. Although many studies have shown kinetic trapping during electrospray ionization (ESI) can allow IM to observe solution-phase structures, i.e. “native” ion mobility-mass spectrometry (IM-MS), there remain many uncertainties as to for how long and to what extent solution-phase structures are retained. Cryogenic IM-MS (cryo-IM-MS) is used to investigate structural characteristics of electrosprayed ions here. The advantage offered by the cryogenic drift cell is the rapid thermalization of “freeze-dried” ions (~130 K) to the ~80 K drift cell temperatures, preserving kinetically trapped solution-phase structures and extensively hydrated ions i.e. [M + xH]x+(H2O)n. With n approaching up to several hundred, cryo-IM-MS offers a unique experimental approach to survey the interactions of water and the late-stages of ESI.
The preservation of an unusual like-charged ion pair interaction between guanidinium ions, which has been observed in protein-protein interactions, is investigated. Charge solvation is imperative for stabilizing these like-charged ion pairs, and without sufficient water molecule adducts bridging between the ions, the like-charged ions repel and break apart. A second like-charged interaction was also isolated, where a proton was stabilized within a hydrated guanidinium cluster. These studies are extended to the peptide bradykinin, which has two arginine residues, and showcases similar charging behavior; extensive hydration of BK permits an additional charge to be stabilized within the solvated clusters.
A late-stage ESI proton transfer event in dehydrating 4-aminobenzoic acid ions is investigated. Molecular dynamics simulations (MDS) are used to calibrate a theoretical collisional cross section (CCS) with the experimental arrival time distributions (ATD) of sequential water clusters. The structures generated indicate that a proton transfer occurs via a water wire at n = 6, i.e. via a Grotthuss mechanism. The structural characterizations provided by the addition of CCS to cryo-IM-MS allow for identification of larger structural families up to ~500 Daltons. | en |
