| Popis: |
Two major lines of investigation, both involving collisions of ions with organic thin films on metal, are described in this dissertation. The first topic involves studies of low-energy (e.g., 10-250 eV) ion-surface interactions with Langmuir-Blodgett (L-B) films labeled at the outermost surface carbon atom, either by isotopes (deuterium or 13C) or by fluorine. The L-B films are prepared from the labeled fatty acids. The ion-surface collision results suggest that the outermost surface atoms/groups are the main determinant of energy transfer, electron transfer, and ion-surface reactions for polyatomic projectile ions (e.g., benzene, pyrazine). The results presented in this highly interdisciplinary area could be of interest to ion chemists, surface scientists, molecular physicists, physical chemists and others. In addition, the results promise a novel surface characterization technique using ion-surface interactions in the future. The second research topic involves studies that utilize surface-induced dissociation (SID) for mechanistic investigations of peptide fragmentation. Easily prepared self-assembled monolayers (SAMs) of alkanethiols or fluoroalkanethiols on gold are used in SID of peptides. A long-term goal associated with the work on peptide fragmentation is to provide improved and additional predictive rules of peptide dissociation for the computer-aided interpretation of MS/MS spectra in MS-based high throughput peptide/protein sequencing. For example, MS/MS results of fixed-charge derivatized peptides unequivocally demonstrate that in the absence of an available mobile proton, selective cleavages at the peptide bond immediately C-terminal to an Asp residue (Asp-Xxx), initiated by the Asp side chain acidic hydrogen, dominate the MS/MS spectra. SID on a series of dendrimers was also performed to investigate the effect of different charge states on the ion fragmentation. The dendrimers serves as model compounds that have a number of protonation sites with similar gas-phase basicities compared to peptides that have a more heterogeneous population of basic sites. In contrast to previously reported results for multiply protonated peptides of comparable size and charge states, no dependence of SID characteristic collision energy on the charge state of the dendrimers is observed. This supports the idea that it is a mobile proton available to the amide group moiety, instead of simply an additional positive charge, that promotes the lower energy cleavage of peptide bonds. |
