Side chain chemistry mediates backbone fragmentation in hydrogen deficient peptide radicals
A crown ether based, photolabile radical precursor which forms noncovalent complexes with peptides has been prepared. The peptide/precursor complexes can be electrosprayed, isolated in an ion trap, and then subjected to laser photolysis and collision induced dissociation to generate hydrogen deficient peptide radicals. It is demonstrated that these peptide radicals behave very differently from the hydrogen rich peptide radicals generated by electron capture methods. In fact, it is shown that side chain chemistry dictates both the occurrence and relative abundance of backbone fragments that are observed. Fragmentation at aromatic residues occurs preferentially over most other amino acids. The origin of this selectivity relates to the mechanism by which backbone dissociation is initiated. The first step is abstraction of a β-hydrogen from the side chain, followed by beta-elimination to yield primarily a-type fragment ions. Calculations reveal that those side chains which can easily lose a β-hydrogen correlate well with experimentally favored sites for backbone fragmentation. In addition, radical mediated side chain losses from the parent peptide are frequently observed. Eleven amino acids exhibit unique mass losses from side chains which positively identify that particular amino acid as part of the parent peptide. Therefore, side chain losses allow one to unambiguously narrow the possible sequences for a parent peptide, which when combined with predictable backbone fragmentation should lead to greatly increased confidence in peptide identification.
Additional Information© 2008 American Chemical Society. Received August 4, 2008. Publication Date (Web): December 29, 2008. The authors thank the National Science Foundation for funding (CHE-0747481 for R.R.J.). H.N. thanks the Ford Foundation for a predoctoral scholarship. B.M.S. and H.N. thank Caltech for funding. Supporting Information: Additional mass spectra and the mechanism for c and z formation. This material is available free of charge via the Internet at http://pubs.acs.org.
Supplemental Material - Sun2009p19010.1021pr800592t_supp.pdf