Photodissociation Spectroscopy of Gaseous Bio-ions in a Commercial Quadrupole Ion Trap Mass Spectrometer

This thesis presents the results of photodissociation spectroscopy experiments performed on biologically interesting molecular and cluster ions within a commercial quadrupole ion trap mass spectrometer. The experimental apparatus uses an excitation laser which is tuneable between the visible and mid-UV. Both cationic and anionic species have been studied using this instrument.

Electronic photodissociation spectroscopy has been used to distinguish between the gaseous protonation isomers of nicotinamide and para-aminobenzoic acid. Similar “protomers” have previously been identified using other gaseous techniques, but these studies are the first to use electronic photodissociation within a commercial mass spectrometer. It is shown that the electronic absorption spectra of individual “protomers” can be resolved by monitoring the production of photofragments as the laser is scanned.

The gaseous electronic absorption spectra of deprotonated alloxazine and lumichrome are recorded using photodissociation; these molecules are the simplest flavin chromophores which form the basis for much cellular chemistry. It is found that both molecules undergo resonant transitions near their calculated electron affinities, which are assigned as dipole-bound excited states. By monitoring the production of a photofragment, it is shown that this excited state is sufficiently long lived to undergo relaxation for alloxazine but not for lumichrome. This subtle difference is explained through the structural differences between these molecules.

Finally, photodissociation spectroscopy is used to study the clusters formed between hexachloroplatinate and the nucleobases. These clusters represent model systems for identifying photochemical reactions that occur between a photoactivatable pharmaceutical and DNA. The electronic absorption spectra of these clusters are broadly similar and are dominated by ligand-to-metal charge transfer transitions within the hexachloroplatinate moiety. It is shown through an analysis of the photofragments across the spectral range that the excitation wavelength controls the distribution of photofragments and that the nucleobase influences the available fragmentation pathways.