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Ion Spectroscopy of Non-Covalent Complexes

spectroscopy1
Noncovalent interactions, including hydrogen bonds, salt bridges, and metal cation binding, play important roles in the structure, function, and reactivity of biomolecules. For example, salt bridge formation is known to be an important feature in high-order protein structure and in the interfaces of many enzyme-substrate complexes, and the presence of metal cations is crucial for the proper folding and function of many proteins. Water molecules can have complex hydrogen-bonding interactions with biomolecules and play an important role in their structure. The interaction of biomolecules with water and cosolutes in vivo thus involves an intricate cooperation and competition of many noncovalent interactions. By isolating model systems in the gas phase, the effects of specific noncovalent interactions on biomolecule structure can be investigated individually or in combination. One of our goals is to relate these effects to intrinsic properties of the components of the biomolecule complex, such as gas-phase basicities and acidities, ion size and charge state, and sequence. Infrared photodissociation (IRPD) spectroscopy is a sensitive probe of structure and has been particularly useful in studying effects of noncovalent interactions in gas-phase ions and complexes. In this technique, the rate of dissociation of an isolated ion or complex due to irradiation with a tunable laser is measured as a function of laser photon frequency, resulting in an infrared “action” spectrum of the ion or complex, which can provide valuable structural information that can be related to the physical properties of the complex substituents.  Contacts: Terry Chang




Ion Spectroscopy of Hydrated Ions and Large Droplet Surfaces

spectroscopy1
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 (see above), 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: Terry Chang



Nanocalorimetry

sanjay spectrum
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: Maria Demireva





Standardless Quantitation

standardless
Electrospray mass spectrometry methods excel at identifying ion mass, composition and bond connectivity. However, a multitude of ionization, transmission, and detection efficiency effects can mask the presence of analytes in mixtures, making the determination of mixture molar fractions directly from electrospray ion abundances extremely unreliable. To counter these effects, we have developed a series of techniques that use nonspecific clusters as probes of solute composition in mixtures. At sufficiently large cluster sizes, nonspecific cluster ions contain an accurate estimate of the solution mole fraction for each analyte that is present, and are largely unaffected by differences in efficiency. These “standard-free” quantitation methods have broad application to mixtures found in pharmaceuticals, proteomics, and other biological fluids. Studies looking at the effects of complex mixture components, dramatic changes in ionization efficiency, and improving the dynamic range and sensitivity of this technique are ongoing. Contacts: Tawnya Flick






Biophysical Mass Spectrometry

sanjay spectrum
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: Catherine Cassou

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