Article pubs.acs.org/accounts
Cryogenic Ion Mobility-Mass Spectrometry: Tracking Ion Structure from Solution to the Gas Phase Kelly A. Servage,† Joshua A. Silveira,‡ Kyle L. Fort,§ and David H. Russell*,† †
Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States Thermo Fisher Scientific, 355 River Oaks Parkway, San Jose, California 95134, United States § Netherlands Proteomics Center, 3584 Utrecht, The Netherlands ‡
CONSPECTUS: Electrospray ionization (ESI) combined with ion mobility-mass spectrometry (IM-MS) is adding new dimensions, that is, structure and dynamics, to the field of biological mass spectrometry. There is increasing evidence that gasphase ions produced by ESI can closely resemble their solution-phase structures, but correlating these structures can be complicated owing to the number of competing effects contributing to structural preferences, including both inter- and intramolecular interactions. Ions encounter unique hydration environments during the transition from solution to the gas phase that will likely affect their structure(s), but many of these structural changes will go undetected because ESI−IM-MS analysis is typically performed on solvent-free ions. Cryogenic ion mobility-mass spectrometry (cryo-IM-MS) takes advantage of the freezedrying capabilities of ESI and a cryogenically cooled IM drift cell (80 K) to preserve extensively solvated ions of the type [M + xH]x+(H2O)n, where n can vary from zero to several hundred. This affords an experimental approach for tracking the structural evolution of hydrated biomolecules en route to forming solvent-free gas-phase ions. The studies highlighted in this Account illustrate the varying extent to which dehydration can alter ion structure and the overall impact of cryo-IM-MS on structural studies of hydrated biomolecules. Studies of small ions, including protonated water clusters and alkyl diammonium cations, reveal structural transitions associated with the development of the H-bond network of water molecules surrounding the charge carrier(s). For peptide ions, results show that water networks are highly dependent on the charge-carrying species within the cluster. Specifically, hydrated peptide ions containing lysine display specific hydration behavior around the ammonium ion, that is, magic number clusters with enhanced stability, whereas peptides containing arginine do not display specific hydration around the guanidinium ion. Studies on the neuropeptide substance P illustrate the ability of cryo-IM-MS to elucidate information about heterogeneous ion populations. Results show that a kinetically trapped conformer is stabilized by a combination of hydration and specific intramolecular interactions, but upon dehydration, this conformer rearranges to form a thermodynamically favored gas-phase ion conformation. Finally, recent studies on hydration of the protein ubiquitin reveal water-mediated dimerization, thereby illustrating the extension of this approach to studies of large biomolecules. Collectively, these studies illustrate a new dimension to studies of biomolecules, resulting from the ability to monitor snapshots of the structural evolution of ions during the transition from solution to gas phase and provide unparalleled insights into the intricate interplay between competing effects that dictate conformational preferences.
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systems.1 Although studies of gas-phase ions remove the complicating effects of solute−solvent interactions, understanding the conformational preferences of ions as they transition from solution to the gas phase is of critical
INTRODUCTION
Electrospray ionization (ESI), a well-established method for transferring intact biomolecular ions from solution to the gas phase, combined with ion mobility-mass spectrometry (IMMS) is a powerful experimental technique that when coupled with molecular dynamics (MD) simulations is capable of generating structural information on important biological © XXXX American Chemical Society
Received: April 11, 2016
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DOI: 10.1021/acs.accounts.6b00177 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research importance for the correlation of gas-phase and solution-phase structure and dynamics. Increasing numbers of studies report that gas-phase peptide and protein ions can retain memory of their solution-phase structures;2−4 however, the preservation of so-called “native” structure(s) is dependent on an ensemble of stabilizing interactions, including intermolecular interactions with water, intramolecular charge solvation, and Coulombic repulsion between charge sites of multiply charged analyte ions. Here, we describe the novel features and capabilities of cryogenic ion mobility-mass spectrometry (cryo-IM-MS) and its utility in performing IM-MS analyses of intact hydrated ions formed by ESI. This is achieved by taking advantage of the freeze-drying capabilities of ESI and additional cooling of ions provided by the cold IM drift cell (80 K), thereby preserving ions of the type [M + xH]x+(H2O)n, where the number of water molecules, n, can vary from zero to several hundred. As a result, the effects of hydration on ion structure and dynamics during the evaporative process of ESI can be elucidated. Cold (80 K) IM drift cells offer a number of advantages over traditional IM experiments carried out at ambient temperature. In order to preserve “native state” conformations, gentle instrument conditions must be employed, which can oftentimes result in decreased resolution and ion transmission.5 On the other hand, decreasing the temperature of the IM drift gas has been shown to reduce diffusional broadening of the ion swarm through the drift cell, resulting in increased sensitivity and resolution.6 In addition, low temperatures are required for the preservation of hydrated analytes on the experimental time scale, because weakly bound clusters would otherwise dissociate rapidly. While hydrated biomolecules have predominantly been studied using spectroscopic methods or condensed-phase techniques,7−9 recent gas-phase studies involving the sequential hydration of biomolecules have provided insights into the effect of hydration on ion structure.10−12 A limitation of these studies is that hydrated ions are typically limited to small cluster sizes, whereas cryo-IM-MS is the first IM-MS apparatus designed specifically for structural studies of extensively hydrated cluster ions formed by ESI. We present here a number of recent examples highlighting the unique capabilities of cryo-IM-MS as a useful tool for investigating the structures of hydrated biomolecules.
Figure 1. Schematic of the home-built cryogenic ion mobility-mass spectrometer; inset contains expanded view of the ESI source region. The source contains a (1) heated capillary ion inlet, (2) skimmer cone, and (3) DC ion guide. Ions are transferred from the DC ion guide into the (4) cold (80 K) IM drift tube. Once ions traverse the drift tube, they are orthogonally pulsed into a (5) time-of-flight mass spectrometer for analysis.
spectrometer that is maintained at ∼1 × 10−7 Torr, for detection. Hydrated ions are preserved throughout the experiment owing to a combination of evaporative cooling that occurs during ESI and additional cooling of ions to ∼80 K upon entering the drift tube.7 The effect of IM field strength on cluster ion preservation was investigated to establish the dissociative “low-field” limit for operation of the drift tube (E/ N < 5.8Td), where E is electric field, N is buffer gas number density, and Td = 10−17 V·cm2.13 Maintaining a weak electric field across the drift tube minimizes loss of water monomers to collision induced dissociation (CID) and ensures that the drift time is directly proportional to the ion-neutral collision cross section (CCS).14 All studies were performed at field strengths below the established limit, with
