Peptide Structure and Radical Chemistry in the Gas Phase Revealed by Product Ion Abundance Analysis in Electron Capture and Transfer Dissociation Mass Spectrometry
Peptide and protein ion abundances in mass spectrometry (MS) and tandem mass spectrometry (MS/MS) may provide orthogonal, or complementary, information to their mass measurements. Collision induced dissociation (CID), the most commonly used MS/MS technique, demonstrates high variability of fragmentation pathways and their relative product abundances as a function of experimental parameters. Moreover, a reliable prediction of relative abundances of CID products is possible only for relatively small systems, but not for larger biomolecules. Recently developed radical-induced fragmentation techniques, e.g., electron capture and transfer dissociation (ECD/ETD), appear to demonstrate more repeatable and reproducible fragmentation patterns. However, it has been reported that ECD/ETD product ion abundance distribution for peptides is rather flat and extracting additional peptide sequence information is yet to be done. In the present work, an attempt is made to understand the peptide's ECD/ETD product ion abundance formation fundamentals and to reveal the possible implications of this source of information for improving peptide and protein structural characterization. Specifically, we have shown that ECD/ETD of peptides is sensitive to the chemical environment in terms of peptide primary structure (amino acid nature) and possibly higher order structure (hydrogen bond connectivity). After demonstrating the analytical validity of the ECD/ETD product ion abundance methodology, its parameter dependence and its applicability to different experimental setups, results will be presented and rationalized along two main axes of comprehension: gas phase peptide radical chemistry and structure. Whereas fundamental aspects of ECD/ETD (mechanism, thermodynamics) can be addressed with the former, additional ECD/ETD yield modulations were observed and presumably relate to structural properties of ions in the gas phase. ECD specificity will be demonstrated on the class of amphipathic peptides, where periodicity is revealed in product ion abundance and correlated to peptide primary and secondary structure. To decipher between both contributions, sequence variations of amphipathic peptides were probed by ECD and correlated to gas phase structures suggested by ion mobility mass spectrometry and supported by molecular dynamics computations. After probing amino acid side chain influence, unraveling the chemical environment specificity of ECD/ETD led us to investigate peptide backbone modifications. Peptides containing beta amino acids revealed two main effects in ECD and ETD. We demonstrate that individual substitutions yield c and z type fragments only for amino acids that provide sufficient z• radical stabilization, whereas multiple substitutions shift the preferential fragmentation pathway to the otherwise minor a-type ion formation pathway indicating possible change in peptide protonation. Finally, understanding the partitioning of radical ions between N- and C-terminal fragments led us to investigate ion internal energy dependence. The extent of hydrogen atom transfer, the [c+z] complex lifetime and secondary fragmentation pathways (w-type ion formation) are directly modulated by ion internal energy as demonstrated by decoupling ETD from charge reduced CID. To summarize, ECD/ETD product ion abundance methodology developed in the thesis advances biomolecule radical chemistry understanding and provides a potentially useful methodology for peptide and protein structural characterization.
Keywords: peptide ; protein ; alpha-helix ; radical chemistry ; beta amino acid ; mass spectrometry MS ; tandem mass spectrometry MS/MS ; Fourier transform mass spectrometry FTMS ; ion cyclotron resonance ICR ; electron capture dissociation ECD ; electron transfer dissociation ETD ; collision induced dissociation CID ; gas phase conformation ; hydrogen bonds ; hydrophobicity ; amphipathic peptides ; ion mobility mass spectrometry IM-MS ; replica exchange molecular dynamics REMD ; peptide ; protéines ; hélice alpha ; chimie radicalaire ; acide aminé beta ; spectrométrie de masse MS ; spectrométrie de mass en tandem MS/MS ; spectrométrie de masse à transformée de Fourrier FT-MS ; résonance cyclotronique d'ions ICR ; dissociation par capture électronique ECD ; dissociation par transfert électronique ETD ; dissociation induite par collisions CID ; conformation en phase gazeuse ; ponts hydrogène ; hydrophobicité ; peptides amphipathiques ; spectrométrie de masse à mobilité ionique IM-MS ; dynamiques moléculaire à échange de répliques REMDThèse École polytechnique fédérale de Lausanne EPFL, n° 5003 (2011)
Programme doctoral Chimie et Génie chimique
Faculté des sciences de base
Institut des sciences et ingénierie chimiques
Laboratoire de spectrométrie de masse de biomolécules
Record created on 2011-02-17, modified on 2016-08-09