Science

Single Particle Analyzer of Mass and Mobility (SPAMM)

While mass spectrometry is a very important tool for studying a wide variety of proteins, complexes, and clusters, it is often of limited use for larger species with masses beyond 1 MDa. With common mass analyzers (TOF, FTICR, etc.), which sort ions by mass to charge ratio (m/z), these large particles produce complicated spectra with unresolved charge states, making it impossible to find the exact mass of what you are measuring. Charge detection mass spectrometry (CDMS) provides a way around this problem, by directly finding the mass of single ions with both the m/z and the charge, z, of an ion measured at the same time. In CDMS, single ions are passed through an electrode, inducing a charge pulse on the electrode with amplitude proportional to charge and duration proportional to velocity and m/z. By measuring single ions, there is no complicating factor of multiple ions with similar m/z being detected at the same time and obscuring the individual charge states. With CDMS, mass spectrometry can be extended to interesting new systems that could not be readily studied before, like viruses, very large protein complexes and nanoparticles.

Contacts: Andrew Elliott and Conner Harper

Infrared Spectroscopy of Hydrated Ions

Many ions that occur as cosolutes with biomolecules in vivo play a crucial role in biomolecule structure, function, and reactivity. The Hofmeister series, which ranks various ions according to their effects on proteins in aqueous solution, has been studied for over a hundred years, although the precise origins of these effects of ions on the hydrogen-bond network of water and on biomolecules structure remain poorly understood. In addition to studying direct effects of Hofmeister series ions on biomolecules, our group is interested in how water molecules interact with different ions and how the identity of the ion affects the development of hydrogen bonding structure in successively larger solvation shells. In small gas-phase hydrates, many ions tend to organize the water molecules nearest them in a manner that differs substantially from the hydrogen-bonding network of bulk water. As the number of water molecules in the hydrated ion complex increases, it is interesting to ask at what point a transition from gas-phase to solution-phase properties occurs. IRPD spectroscopy in the hydrogen-stretch region (~2500–4000 cm-1) is sensitive to the hydrogen-bonding environment of both the ion and water molecules and can provide insight into the structure of the hydrated ion complex. In this way, hydrated ion complexes serve as a bridge between the gas phase and solution. Our efforts focus on characterizing the interior and surface structure of hydrated ion complexes as well as relating these results to known solution-phase properties, including ion size, solution-phase coordination number, hydrolysis constants, Jahn-Teller distortion, surface activity, and Hofmeister series effects.

Contacts: Christiane Stachl

Relevant Publications:
1) Role of Water in Stabilizing Ferricyanide Trianion and Ion-Induced Effects to the Hydrogen-Bonding Water Network at Long Distance
2) Hydration of Guanidinium: Second Shell Formation at Small Cluster Size
3) Hydration of Gaseous m-Aminobenzoic Acid: Ionic vs. Neutral Hydrogen Bonding and Water Bridges

Nanocalorimetry

Determining the energy deposition during a gas-phase dissociative reaction is difficult due to various factors, such as multiple reaction pathways and broad ranges of incident energies. To accurately measure the energy distribution, not just the average energy deposition, hydrated cluster ions can be used as chemical thermometers. Even at relatively small sizes, these ‘nanocalorimeters’ provide a ladder of discrete energy values based on the binding energy of each water molecule to the cluster. When energy is deposited as the result of a gas-phase reaction, water molecules evaporate to cool the heated cluster. By using mass spectrometry, the number of water molecules lost from a cluster is readily determined, and the incident energy of the reaction may be obtained. This technique has been applied to various hydrated cluster systems, from metal ions to peptides, and has provided detailed information for ion-neutral, ion-electron, and ion-photon reactions. These measurements have broad applicability to proteomics, electrochemistry, and the characterization of gas-phase ion structure.

Contacts: Christiane Stachl

Relevant Publications:
1) Ions in Size-Selected Aqueous Nanodrops: Sequential Water Molecule Binding Energies and Effects of Water on Ion Fluorescence
2) Gas-phase Electrochemistry: Measuring Absolute Potentials and Investigating Ion and Electron Hydration
3) Average Sequential Water Molecule Binding Enthalpies of M(H₂O)²⁺_19-124 (M= Co, Fe, Mn, and Cu) Measured with Ultraviolet Photodissociation at 193 and 248 nm

Supercharging of Protein Ions in Electrospray Ionization

Increasing the charge on protein ions produced by electrospray ionization can lead to improvements in tandem MS efficiency, detection sensitivity, and resolution. High charge-state protein ions produced from native, aqueous solutions can yield important information on the native or native-like structures of proteins and protein complexes via tandem MS coupled with covalent labeling techniques. Our lab has developed three “supercharging” methods for adding charge to proteins from either denaturing or native solutions: 1) reagent supercharging, 2) electrothermal supercharging, and 3) trivalent metal ion supercharging. We are continuously investigating new applications for these techniques and developing new methods for supercharging.

Contact: Zijie Xia

Relevant Publications:
1) Desalting Protein Ions in Native Mass Spectrometry using Supercharging Reagents
2) Anions in Electrothermal Supercharging of Proteins with Electrospray Ionization Follow a Reverse Hofmeister Series
3) Electrothermal Supercharging in Mass Spectrometry and Tandem Mass Spectrometry of Native Proteins

Investigating Factors Influencing Charging in ESI

Electrospray ionization (ESI) is a widely used technique to produce intact, multiply charged, gas-phase macromolecular ions directly from solution for analysis my mass spectrometry. The extent of charging of protein ions depends on solution-phase conformation, solvent surface tension, basicities of the analyte and solvent, and many other factors. Low-charge protein ions are formed from buffered aqueous solutions, such as ammonium acetate, where proteins are in native conformations. When other acetate salts, such as alkali metal, tetramethylammonium (TMA+) or tetraethylammonium (TEA+) acetate, are added to the ESI solution, significant cation adduction is observed. These cation-adducted protein ions can be mass selected and collisionally activated to either fragment the protein ion or form very low- charge protein ions from loss of the cation. This technique is useful to produce singly charged (1+) protein ions from native buffered aqueous ESI solutions as well as probing the factors that influence protein charge from native ESI.

Contact: Zijie Xia

Relevant Publications:
1) Effects of Cations on Protein and Peptide Charging in Electrospray Ionization from Aqueous Solutions

Biophysical Mass Spectrometry

In solution, the proximity of two residues predicts whether the effect of removing them both equals the sum of the effects from removing them singly, or whether they are coupled. We will apply the same logic to gas phase protein-protein complexes, using the coupling between residue relative free energies to produce a distance map of contacts in the interface: a first approximation of the local gas-phase structure and an experimental probe of water effects. We have alanine-scanned the binding interface of two proteins, barnase and barstar, and have simple enzymological means to see how mutations affect binding energy in solution. The mass spectrum of a solution of competing monomers gives an instant and high-throughput readout of their relative solution affinities for heterodimer formation.

Contact: Zijie Xia