Neutralization and reionization mass spectrometry and computational studies of small biomolecule radicals in the gas phase.

摘要

Radical additions to biomolecules such as DNA, RNA, and proteins, represent an important component of the complex process of ageing, oxidative stress and radiation damage. Among the recognized mechanisms of DNA radiation damage, capture of a low-energy electron followed by protonation of the transient anion radical results in the formation of hydrogen-atom adducts that can undergo further degradation. Electron capture dissociation (ECD) of the multiply charged peptide and protein radical cations, yields unique and abundant fragments, particularly the complementary c and z• series, and has been shown to be an efficient method for peptide and protein sequencing in the gas phase. Assisted by tandem mass spectrometric techniques and the advanced stage of quantum chemistry calculation, it has now become practical to elucidate the transient radical dissociation processes in great detail, and thus disclose some unexpected features of fragmentation mechanisms. However, tautomerization of both neutral nucleobase molecules and their relevant ions in the gas phase has been a drawback factor in nucleobase radicals study. Meanwhile, the complexity of the multiply charged peptide ions has prevented detailed studies on the structure and energetics of their radical intermediates. Direct characterization of reactive peptide and protein radical dissociation is virtually impossible.; This dissertation presents the results of combined experimental and computational studies of: (1) specially generated 1-methylcytosine radicals; (2) hydrogen atom adducted simple peptide model molecules, namely, methylammonium, ethylammonium, and beta-alanine-N-methyl amide (BANMA). Small biomolecule radicals of interest were generated and analyzed by neutralization-reionization mass spectrometry (NRMS). Precursor ions were produced in an interchangeable electron impact (EI), chemical ionization (CI), or electrospray ionization (ESI) source. The fast precursor ions were neutralized to form transient radicals through a vertical electron-transfer collision in a few femtoseconds. The neutral intermediates drifted for a short period of time (3--6 mus), then the surviving neutrals and dissociation products were reionized, decelerated and mass analyzed. Interpretation of NR spectra was assisted by collisionally activated dissociation (CAD) and variable-time NRMS experiments. These methods allow qualitative and quantitative deconvolution of ion and neutral dissociations observed in mass-resolved NRMS spectra. Additionally, standard ab initio and density functional theory calculations were performed for ion and radical structures and energetics. Unimolecular rate constants for the competing reaction were obtained by Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. Meanwhile, experimental and theoretical branching ratios were evaluated for competing radical dissociations, according to different experimental conditions.; Hydrogen atom adducted 1-methylcytosine radicals at either the N-3 or C-5 positions in the pyrimidine ring were found to be very stable species on the microsecond time scale. These radicals were generated and distinguished in NRMS experiments. Investigations of peptide model systems were assisted by ab initio/RRKM calculation. As a result, hypervalent ammonium and BANMA radicals were unstable and dissociated completely on the microsecond time scale. In unimolecular dissociations of methylammonium radical, loss of the ammonium hydrogen atom is more favorable than the N-C bond cleavage. This was reversed in the dissociations of the ethylammonium radical, which was probably attributed to the formation and dissociation of excited electronic states. Studies on BANMA radicals predicted that the formation of a reactive aminoketyl radical through hydrogen atom transfer might result from the isomerization at the B excited electronic state.