The Bush Lab is a research group in the Department of Chemistry and the Biological Physics, Structure & Design Program at the University of Washington. Our research focuses on the development and application of mass spectrometry and ion mobility spectrometry techniques to elucidate the structures and assembly of protein complexes and subcellular machines.
Experimental data from ion mobility measurements and electron transfer dissociation were combined with extensive computational analysis of ion structures and dissociation energetics for Gly-Leu-Gly-Gly-Lys cations and cation radicals. Experimental and computational collision cross sections of (GLGGK + 2H)2+ ions pointed to a dominant folding motif that is represented in all low free-energy structures. The local folding motifs were preserved in several fragment ions produced by electron transfer dissociation. Gradient optimizations of (GLGGK + 2H)+• cation-radicals revealed local energy minima corresponding to distonic zwitterionic structures as well as aminoketyl radicals. Both of these structural types can isomerize to low-energy tautomers that are protonated at the radical-containing amide group forming a new type of intermediates, −C•O–NH2+– and −C•(OH)NH2+–, respectively. Extensive mapping with B3LYP, M06-2X, and MP2(frozen core) calculations of the potential energy surface of the ground doublet electronic state of (GLGGK + 2H)+•provided transition-state and dissociation energies for backbone cleavages of the N–Cα and amide C–N bonds leading to ion–molecule complexes. The complexes can undergo facile prototropic migrations that are catalyzed by the Lys ammonium group and isomerize enolimine c-type fragments to the more stable amide tautomers. In contrast, interfragment hydrogen atom migrations in the complexes were found to have relatively high transition energies and did not compete with fragment separation. The extensive analysis of the intermediate and transition-state energies led to the conclusion that the observed dissociations cannot proceed competitively on the same potential energy surface. The reactive intermediates for the dissociations originate from distinct electronic states that are accessed by electron transfer.
Comprehensive Analysis of Gly-Leu-Gly-Gly-Lys Peptide Dication Structures and Cation-Radical Dissociations Following Electron Transfer: From Electron Attachment to Backbone Cleavage, Ion-Molecule Complexes, and Fragment Separation Robert Pepin, Kenneth J. Laszlo, Bo Peng, Aleš Marek, Matthew F. Bush, František Tureček. J. Phys. Chem. A 2014, DOI: 10.1021/jp411100c. (Link|PUBMED)
Congratulations to Sam Allen, who has been selected as one of the recipients of the Department of Chemistry’s Graduate Student Merit Fellowships.
Native mass spectrometry and ion mobility spectrometry were used to investigate the gas-phase structures of selected cations and anions of proteins and protein complexes with masses ranging from 6–468 kDa. Under the same solution conditions, the average charge states observed for all native-like anions were less than those for the corresponding cations. Using an RF-confining drift cell, similar collision cross sections were measured in positive and negative ion mode suggesting that anions and cations have very similar structures. This result suggests that for protein and protein complex ions within this mass range, there is no inherent benefit to selecting a specific polarity for capturing a more native-like structure. For peptides and low-mass proteins, polarity and charge-state dependent structural changes may be more significant. The charged-residue model is most often used to explain the ionization of large macromolecules based on the Rayleigh limit, which defines the upper limit of charge that a droplet can hold. Because ions of both polarities have similar structures and the Rayleigh limit does not depend on polarity, these results cannot be explained by the charged-residue model alone. Rather, the observed charge-state distributions are most consistent with charge-carrier emissions during the final stages of analyte desolvation, with lower charge-carrier emission energies for anions than the corresponding cations. These results suggest that the observed charge-state distributions in most native mass spectrometry experiments are determined by charge-carrier emission processes; although the Rayleigh limit may determine the gas-phase charge states of larger species, e.g., virus capsids.
Effects of Polarity on the Structures and Charge States of Native-like Proteins and Protein Complexes in the Gas Phase Samuel J. Allen, Alicia M. Schwartz, Matthew F. Bush. Anal. Chem. 2013, DOI: 10.1021/ac403139d. (Link)
Congratulations to Chrissy Stachl, who has been awarded the Pfizer – La Jolla Academic and Industrial Relations (AIR) 2013-2014 Diversity Research Fellowship in Chemistry! This highly selective Fellowship will support her research this academic year.
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Prof. Bush just returned from the Department of Chemistry at Mercer University, where he presented a seminar and met with undergraduate students. He thanks the American Society for Mass Spectrometry for support and Prof. Kathryn Kloepper for hosting his visit.
Prof. Bush will present the following seminars this November:
- Department of Chemistry, Pacific Lutheran University, Tacoma, WA, 11/25/13. MFB thanks Prof. Tina Saxowsky for hosting this visit.
- Joint Chemistry Seminar, Claremont McKenna, Harvey Mudd, Pomona, Pitzer, and Scripps Colleges, Claremont, CA, 11/19/13. MFB thanks the American Society for Mass Spectrometry for travel support and Prof. Aaron Leconte for hosting this visit.
- Department of Chemistry & Biochemistry, Northern Illinois University, DeKalb, IL, 11/11/13. MFB thanks Prof. Victor Ryzhov and Prof. Marc Adler for hosting this visit.
- Department of Chemistry, Mercer University, Macon, GA, 11/1/13. MFB thanks the American Society for Mass Spectrometry for travel support and Prof. Kathryn Kloepper for hosting this visit.