Charge Site Mass Spectra: Conformation-Sensitive Components of the Electron Capture Dissociation Spectrum of a Protein
A conventional electron capture dissociation (ECD) spectrum of a protein is uniquely characteristic of the first dimension of its linear structure. This sequence information is indicated by summing the primary cm+ and zm+• products of cleavage at each of its molecular ion’s inter-residue bonds. For example, the ECD spectra of ubiquitin (M + nH)n+ ions, n = 7–13, provide sequence characterization of 72 of its 75 cleavage sites from 1843 ions in seven c(1–7)+ and eight z(1–8)+• spectra and their respective complements. Now we find that each of these c/z spectra is itself composed of “charge site (CS)” spectra, the cm+ or zm+• products of electron capture at a specific protonated basic residue. This charge site has been H-bonded to multiple other residues, producing multiple precursor ion forms; ECD at these residues yields the multiple products of that CS spectrum. Closely similar CS spectra are often formed from a range of charge states of ubiquitin and KIX ions; this indicates a common secondary conformation, but not the conventional α-helicity postulated previously. CS spectra should provide new capabilities for comparing regional conformations of gaseous protein ions and delineating ECD fragmentation pathways.
Key wordsElectron capture dissociation (ECD) Charge site (CS) mass spectra Protein ion conformation
- 5.Oh, H., Breuker, K., Sze, S.K., Ge, Y., Carpenter, B.K., McLafferty, F.W.: Secondary and tertiary structures of gaseous protein ions characterized by electron capture dissociation mass spectrometry and photofragment spectroscopy. Proc. Natl. Acad. Sci. U. S. A. 99, 15863–15868 (2002)CrossRefGoogle Scholar
- 15.Kong, X., Breuker, K., McLafferty, F.W.: IR Photodissociation Spectra of Gaseous Protein Ions: Hydrogen Bonding of Side-Chain protonated amino groups is unusually strong. Proceedings of the 57th ASMS Conference on Mass Spectrometry and Allied Topics, Philadelphia, June (2009).Google Scholar