Search

We have measured mass spectra for positive ions produced from neutral 5-fluorouracil by electron impact at energies from 0 to 100 eV. Fragment ion appearance energies of this (radio-)chemotherapy agent have been determined for the first time and we have identified several new fragment ions of low abundance. The main fragmentations are similar to uracil, involving HNCO loss and subsequent HCN loss, CO loss, or FCCO loss. The features adjacent to these prominent peaks in the mass spectra are attributed to tautomerization preceding the fragmentation and/or the loss of one or two additional hydrogen atoms. A few fragmentions are distinct for 5-fluorouracil compared to uracil, most notably the production of the reactive moiety CF+. Finally, multiphoton ionization mass spectra are compared for 5-fluorouracil from a laser thermal desorption source and from a supersonic expansion source. The detection of a new fragment ion at 114 u in the supersonic expansion experiments provides the first evidence for a clustering effect on the radiation response of 5-fluorouracil. By analogy with previous experiments and calculations on protonated uracil, this is assigned to NH3 loss from protonated 5-fluorouracil.

The fundamental nanoscale processes that initiate radiation damage in biological material have not yet been fully elucidated. This represents a significant barrier to developing multidimensional simulations of radiation effects that can lead to advances in radiotherapy and radioprotection. This thesis explores UV- and electron-induced processes in DNA and RNA bases. Pure and hydrated clusters are studied in order to better understand the effects of the chemical environment on the radiation response of these important biomolecules. Although extensive research has been carried out on the relaxation pathways of UV-excited nucleobases, no previous experiments have investigated bond breaking in neutral electronic excited states. This thesis reveals a new fragment ion from uracil (C3H4N2O+) that can be accessed by multi-photon ionization (MPI) but not by electron impact ionization (ElI). This provides the first experimental demonstration that neutral excited state dynamics in a nucleobase can lead to bond breaking in the aromatic ring, as predicted in recent theoretical studies. The specific excited state dynamics have not yet been identified definitively and are the subject of on-going ultrafast pump-probe experiments in collaboration with Townsend and co-workers (Heriot-Watt University). The time-resolved measurements provide new evidence supporting a theoretically predicted relaxation pathway into long-lived triplet states. Dissociative ionization of hydrated nucleobases and uracil-adenine clusters has been studied experimentally for the first time. Evidence for deamination reactions is observed in hydrated adenine complexes. The production of C3H4N2O+ fragments from uracil is strongly suppressed by clustering with water whereas the channel remains open in uracil-adenine complexes. To unravel the specific cluster-mediated dynamics and reactions re

Multi-photon ionization experiments have been carried out on thymine-water clusters in the gas phase. Metastable H2O loss from T+(H2O)n was observed at n ≥ 3 only. Ab initio quantum-chemical calculations of a large range of optimized T+(H2O)n conformers have been performed up to n = 4, enabling binding energies of water to be derived. These decrease smoothly with n, consistent with the general trend of increasing metastable H2O loss in the experimental data. The lowest-energy conformers of T+(H2O)3 and T+(H2O)4 feature intermolecular bonding via charge-dipole interactions, in contrast with the purely hydrogen-bonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters.

The fundamental nanoscale processes that initiate radiation damage in biological material have not yet been fully elucidated. This represents a significant barrier to developing multidimensional simulations of radiation effects that can lead to advances in radiotherapy and radioprotection. This thesis explores UV- and electron-induced processes in DNA and RNA bases. Pure and hydrated clusters are studied in order to better understand the effects of the chemical environment on the radiation response of these important biomolecules. Although extensive research has been carried out on the relaxation pathways of UV-excited nucleobases, no previous experiments have investigated bond breaking in neutral electronic excited states. This thesis reveals a new fragment ion from uracil (C3H4N2O+) that can be accessed by multi-photon ionization (MPI) but not by electron impact ionization (ElI). This provides the first experimental demonstration that neutral excited state dynamics in a nucleobase can lead to bond breaking in the aromatic ring, as predicted in recent theoretical studies. The specific excited state dynamics have not yet been identified definitively and are the subject of on-going ultrafast pump-probe experiments in collaboration with Townsend and co-workers (Heriot-Watt University). The time-resolved measurements provide new evidence supporting a theoretically predicted relaxation pathway into long-lived triplet states. Dissociative ionization of hydrated nucleobases and uracil-adenine clusters has been studied experimentally for the first time. Evidence for deamination reactions is observed in hydrated adenine complexes. The production of C3H4N2O+ fragments from uracil is strongly suppressed by clustering with water whereas the channel remains open in uracil-adenine complexes. To unravel the specific cluster-mediated dynamics and reactions re

Multi-photon ionization experiments have been carried out on thymine-water clusters in the gas phase. Metastable H2O loss from T+(H2O)n was observed at n ≥ 3 only. Ab initio quantum-chemical calculations of a large range of optimized T+(H2O)n conformers have been performed up to n = 4, enabling binding energies of water to be derived. These decrease smoothly with n, consistent with the general trend of increasing metastable H2O loss in the experimental data. The lowest-energy conformers of T+(H2O)3 and T+(H2O)4 feature intermolecular bonding via charge-dipole interactions, in contrast with the purely hydrogen-bonded neutrals. We found no evidence for a closed hydration shell at n = 4, also contrasting with studies of neutral clusters.

We have measured mass spectra for positive ions produced from neutral 5-fluorouracil by electron impact at energies from 0 to 100 eV. Fragment ion appearance energies of this (radio-)chemotherapy agent have been determined for the first time and we have identified several new fragment ions of low abundance. The main fragmentations are similar to uracil, involving HNCO loss and subsequent HCN loss, CO loss, or FCCO loss. The features adjacent to these prominent peaks in the mass spectra are attributed to tautomerization preceding the fragmentation and/or the loss of one or two additional hydrogen atoms. A few fragmentions are distinct for 5-fluorouracil compared to uracil, most notably the production of the reactive moiety CF+. Finally, multiphoton ionization mass spectra are compared for 5-fluorouracil from a laser thermal desorption source and from a supersonic expansion source. The detection of a new fragment ion at 114 u in the supersonic expansion experiments provides the first evidence for a clustering effect on the radiation response of 5-fluorouracil. By analogy with previous experiments and calculations on protonated uracil, this is assigned to NH3 loss from protonated 5-fluorouracil.