Enhanced protein characterization through selective derivatization and electrospray ionization tandem mass spectrometry
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There continue to be great strides in the field of proteomics but as samples become more complex, the ability to increase sequence coverage and confidence in the identification becomes more important. Several methods of derivatization have been developed that can be used in combination with tandem mass spectrometry to identify and characterize proteins. Three types of activation, including infrared multiphoton dissociation, ultraviolet photodissociation, and electron transfer dissociation, are enhanced in this dissertation and compared to the conventional method of collisional induced dissociation (CID) to demonstrate the improved characterization of proteins. A free amine reactive phosphate group was synthesized and used to modify the N-terminus of digested peptides. This phosphate group absorbs at the IR wavelength of 10.6 µm as well as the Vacuum-ultraviolet (VUV) due to an aromatic group allowing modified peptides to be dissociated by infrared multi-photon dissociation (IRMPD) or ultraviolet photodissociation (UVPD) whereas peptides without this chromophore are less responsive to IR or UV irradiation. The PD spectra for these modified peptides yield simplified MS/MS spectra due to the neutralization of all N-terminal product ions from the incorporation the negatively charged phosphate moiety. This is especially advantageous for UVPD due to the great number of product ions produced due to the higher energy deposition of the UV photons. The MS/MS spectra also produce higher sequence coverage in comparison to CID of the modified or unmodified peptides due to more informative fragmentation pathways generated upon PD from secondary dissociation and an increased ion trapping mass range. IRMPD is also implemented for the first time on an orbitrap mass spectrometer to achieve high resolution analysis of IR chromophore-derivatized samples as well as top-down analysis of unmodified proteins. High resolution/high mass accuracy analysis is extremely beneficial for characterization of complex samples due to the likelihood of false positives at lower resolutions/accuracies. For electron transfer dissociation, precursor ions in higher charge states undergo more exothermic electron transfer and thus minimize non-dissociative charge reduction. In this dissertation, cysteine side chains are alkylated with a fixed charge to deliberately increase the charge states of peptides and improve electron transfer dissociation. ETD can also be used to study protein structure by derivatizing the intact structure with a hydrazone reagent. A hydrazone bond will be preferentially cleaved during ETD facilitating the recognition of any modified residues through a distinguishing ETD fragmentation spectrum.