Fundamental Properties of Biologically Relevant Radicals Studied by Neutralization-Reionization Mass Spectrometry and Computations.

摘要

Electron capture dissociation and electron transfer dissociation (electron-based methods, ExD, collectively) have gained much prominence in recent years due to their application in protein sequencing. An interesting feature of ExD is the formation of cation-radicals in the open-shell, doublet state. The fundamental properties of biologically-relevant open-shell, doublet radicals have gained prominence in recent years due to their implications in ExD, and neutralization-reionization mass spectrometry (NRMS) provides a general synthetic methodology to prepare such radicals and study their unimolecular properties on the microsecond time-scale. Radicals were generated by collisional electron transfer to fast ions, causing near neutralization and conversion to the corresponding radical species. The neutrals were then allowed to drift through a 60 cm conduit before the surviving neutrals and dissociation products were reionized, decelerated, energy filtered, and mass analyzed. Spectral interpretation was aided by collisionally-induced dissociation (CID) experiments that allowed for recognition of products from CID in the NRMS experiments. Additional information on the energetics and kinetics for isomerizations and dissociations of ions and radicals was provided by ab initio calculations and unimolecular rate constants calculated by RRKM theory.;Presented in this dissertation are the results of combined experimental and computational studies of the aminodihydroxymethyl radical, Calpha -amide radicals, N-methylthioacetamide, and protonated tryptophan. The aminodihydroxymethyl radical is an electron super-rich radical with similar properties to the amide aminoketyl radical produced in ExD, and, unlike previously studied electron super-rich radicals, is stable on the experimental time scale. Calpha-amide radicals are models for simple z-ions produced in ExD, and have very different ion and radical geometries which results in extremely high Franck-Condon factors and extensive fragmentation. N-methylthioacetamide is a thio-analogue of a model system for studying electron-based dissociations of the peptide backbone, and shows very different dissociations from the native oxo-backbone. Finally, the protonated tryptophan radical is examined and represents the utility of NRMS as a method for studying a natural biomolecule as well as model systems.