Neutralization-reionization mass spectrometry and computational studies of hydrogen atom adducts to the amide bond.

摘要

Since the introduction of electron capture dissociation (ECD) as an efficient method for fragmenting peptide and protein ions in the gas phase, much interest has surrounded the elucidation of its reaction mechanisms. With respect to conventional peptide activation techniques, ECD has been shown to yield unique and abundant fragment ions, particularly the complementary c and z· series, with nearly complete sequence coverage. This novel chemistry is intrinsically odd-electron driven, and presumably involves the formation of a backbone carbon-centered ketyl radical that undergoes facile alpha cleavage of the N---Calpha bond. Unfortunately, direct characterization of these reactive peptide cation radicals (i.e., elucidation of structures, energetics, and kinetics of competing dissociations) is virtually impossible, especially in light of the multitude of gas-phase conformers and sheer size of these ions. Presented in this dissertation are the results of combined experimental and computational studies of ECD prototype compounds based on hydrogen atom adducts to simple amide molecules, namely, formamide, acetamide, N-methylacetamide, and beta-alanine-N-methylamide.; The amide radicals were generated and investigated by a technique known as neutralization-reionization mass spectrometry (NRMS), in which collisional electron transfer to a fast precursor ion beam produced the corresponding radical species. Following a brief (3--5 mus) drift period, surviving neutrals and dissociation products were reionized, decelerated, and mass analyzed. These experiments were carried out on a home-built, tandem quadrupole acceleration-deceleration instrument equipped with interchangeable electron impact (EI) and chemical ionization (CI) sources. Also described is the design and development of an electrospray ionization (ESI) source and interface, that allowed for the generation of high ion currents from larger, less volatile precursor compounds.; Spectral interpretation was aided by collisionally activated dissociation (CAD) and variable time NRMS experiments, that allowed for qualitative and quantitative deconvolution of the ion and neutral contributions to observed NRMS dissociations. Additionally, all ions and neutrals of interest were investigated by density functional and ab initio calculations. Unimolecular rate constants for competing reactions were calculated using RRKM theory, and theoretical branching ratios were compared to those obtained experimentally. Charge-facilitated backbone electron attachment was also studied computationally, using point charges and small cations as models to ECD charge-carrying moieties.