DOUBLE RESONANCE ECD

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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. The Double Resonance (DR) technique was adapted to electron capture dissociation (ECD) and applied to the analysis of the fragmentation timeframe of peptides. DR-ECD involves continuously ejecting an ion as it is being formed durine ECD. This ion is usually, but not necessarily, the charge reduced molecular radical cation. Since the ejection timeframe for on-resonant ejection in a 7T FTMS of our geometry is rougly 100 microseconds - 1 millisecond, any ions that are formed on a timeframe greater than 1 ms will be ejected. Thus, one can classify ions as "fast" or "slow" in terms of their ECD formation rate. The DR-ECD experiment on linear peptides showed that approximately half of all fragment ions are generated on a >1 millisecond timeframe. These are presumably fragments that are held together by hydrogen bonds - the dissociation of which determines the formation rate. This model does correlate with the DR-ECD data as reduction in the number of residues that can H-bond reduces the number of long-duration fragments ions. However, the cyclic peptide data is more interesting. Cyclosporin, which has no residues with sidechains that can hydrogen bond, generates many secondary fragments, but ALL of them are formed faster than 100 microseconds. On the other hand, gramicidin S, which has several ornithine residues, shows some slow-forming fragments. Furthermore, some of the fragments ions show "survival ratios" > 1, but this is correlated with double electron capture. The manuscript was published in the Journal of the American Society for Mass Spectrometry. Implementation of the infrared laser heating with the DR-ECD experiment allowed us to determine several important kinetic parameters for a set of peptides. We could measure the peptide refolding rate (by varying the delay between IR and ECD), we could measure the H-bond dissociation rate in a similar fashion, and we could measure the rate of H-atom abstraction in the radical cation complex, and it was measured to be ~5-10x higher than the H-bond dissociation rate. Finally, by calibrating the laser heating using a BIRD experiment, we were able to build Van 't Hoff plots from which we can extract thermodynamic parameters. We published one paper (Lin et al. 2008) in the Journal of the American Society for Mass Spectrometry (2008, 19, 870-879) regarding the kinetic data and have a second one in revision regarding the thermodynamic data.