Hydrated Clusters of Nucleic Acid Bases in Supersonic Beams Probed by Multiphoton Ionization (MPI) Mass Spectrometry

Barć, Bartlomiej (2012). Hydrated Clusters of Nucleic Acid Bases in Supersonic Beams Probed by Multiphoton Ionization (MPI) Mass Spectrometry. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f115


In the present thesis clusters of nucleic acid bases and water are used as model systems of cellular DNA to investigate UV induced radiation damage in the gas phase. A new experimental system has been designed and commissioned to perform multiphoton ionization experiments on hydrogen bonded clusters of the nucleic acid bases: adenine, thymine and uracil, as well as on the related chromophores: 5-fluorouracil and hypoxanthine. As the result of pulsed nanosecond laser irradiation in the wavelength range of 220-230 nm, cluster, monomer and fragment ions were detected using a time-of-flight mass spectrometer. Possible multiphoton ionization pathways of clustered and isolated molecules including the role played by short (singlet) and long-lived (triplet) excited states as well as excited state tautomeric transitions are discussed.

Signal intensities as a function of laser pulse fluence were measured. The slope of a logarithmic plot of the signal intensity versus fluence yields the so-called ‘photon order’ and provides information on the number of photons leading to the production of cluster, monomer and fragment ions. The results indicate two-photon ionization for uracil and thymine in dry molecular beams (i.e. photon orders equal to 1.9±0.2 and 1.4±0.1, respectively). An interesting result in the form of photon order equal to 3.0±0.5 has been measured for adenine in a dry molecular beam irradiated with an unfocused laser beam. This suggests a three photon ionization process, however further experimental work is required in order to exclude factors related to stability of the laser beam spatial profile.

Hydrated cluster ions with up to 7 water molecules attached to a single uracil and 4 water molecules attached to adenine were unambiguously identified after adding water vapour to the molecular beam source. A maximum of 7 water molecules attached to adenine-uracil base pairs were detected. Possible ionization pathways of such hydrated clusters mediated by excited states tautomeric transitions are discussed. On the basis of photon orders measured under hydrated conditions, it is proposed that sequential MPI processes with 2, 3, and 4 photon absorption lead to production of cluster ions, cluster fragment ions (including protonated monomers), and molecular fragment ions, respectively.

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