MECHANISTIC STUDIES OF ELECTRON CAPTURE DISSOCIATION BY DEUTERIUM LABELING

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our recent data on ECD of cyclic peptides implied extensive free-radical mediated secondary fragmentation was occurring. In retrospect, the mere observation of the [M+2H]1+ radical ion resulting from the electron capture implies the existence of long lived ( 1 second) radical cations. Since radicals are known to be highly unstable, and since the previous data on ECD of cyclic peptides implied extensive rearrangment, a simple experiment was devised to test this hypothesis. A pair of synthetic peptides were purchased which contained 4 glycine residues. In one peptide, the glycines were deuterium labeled on the alpha carbon thus creating D2-glycine. The other peptide used normal glycine. If a long lived radical undergoes extensive rearrangment, it was expected that the radical will abstract hydrogens and deuteriums indiscriminately from the alpha carbon sites as the radical migrates up and down the peptide. There is likely to be a kinetic isotope effect that will slow down the migration of the deuterium, but this is unlikely to be observable on a 1 second timescale. ECD of the normal versus D2-Glycine modified peptide showed deuterium scrambling which implies that the radical does rearrange and migrate via hydrogen (or deuterium) abstraction from the alpha carbons (of glycine at least). After much effort and many revisions, this paper was finally published in spring 2006.