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Biophysical Mass Spectrometry Techniques for Probing the Higher-Order Structure of Proteins and Complexes


Electrospray ionization mass spectrometry (ESI-MS) is a powerful analytical platform for answering a wide variety of questions about the identity, quantity, structure, function, dynamics and energetics of biological molecules. Key advantages of ESI-MS include unrivaled specificity, attomole sensitivity, and the capacity for simultaneous analysis of complex mixtures with analyte masses that differ by less than 1 ppm. The low flow rates and sub-micron sized droplets formed with "nano" ESI allows biomolecular ions to be readily formed from purely aqueous or buffered aqueous solutions, and these ions have been shown to retain a "memory" of their solution-phase structures so that higher-order structural information can be obtained directly from a gas-phase measurement. All of the work described in this dissertation was undertaken in an effort to develop new nanoESI-based techniques that augment the existing array of biophysical mass spectrometry techniques for probing the structure/function relationships of biological molecules in their native environments. In part one, a hypothesis for the origin of nanoESI "supercharging" is developed and exhaustively tested utilizing a variety of solution- and gas-phase techniques with a range of different proteins and protein complexes. The results of all of these studies support the hypothesis that the origin of aqueous solution supercharging is the rapid chemical and/or thermal denaturation of a protein or protein complex analyte in an evaporating ESI droplet due to enrichment of the reagent caused by its high boiling point relative to that of water. Aqueous solution supercharging has recently been used in a variety of new applications and an understanding of its underlying mechanism is therefore essential. In part two, two new biophysical mass spectrometry applications are described. The first is a tandem-MS application of aqueous solution supercharging for obtaining hydrogen/deuterium exchange (HDX) rate constants in real-time with nearly single amino acid spatial resolution, and the second describes an MS method to obtain the quaternary structure of protein complexes that require high concentrations of essential salts. Finally, two ideas for new HDX-MS methods that capitalize on the mechanism of aqueous solution supercharging are outlined.

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