Generation and Action Spectroscopy of Gaseous DNA Cation Radicals

Abstract

Reactions between genetic material and secondary low-energy electrons emitted from high energy radiation can cause various DNA lesions such as base modifications and cleavage, single strand breaks and double strand breaks. The mechanism of DNA lesions has been well studied by many research groups for the past two decades. However, little is known about the electronic structure of DNA radicals which are the immediate precursors in the early-stage of DNA damage. Due to the multistage capacity of the ion trap and great progress of photodissociation action spectroscopy, we are able to generate biomolecule cation radicals in a control manner in the gas phase, purify them and their precursors by mass selection, store the reactive species in an inert environment of low-pressure helium, and investigate the ion structures and electronic excitations via various molecular dissociation processes and quantum chemistry calculations. Here, I focus on novel mass spectroscopic studies and theoretical investigations that range from gaseous DNA nucleoside to oligonucleotide cation radicals. The DNA radical generation relies on a fine-tuned electron transfer from fluoranthene anion to either charge tagged nucleoside cations or hydrogen-rich oligonucleotide multi-cations. As the charge tag, I used the 6-(trimethylammonium)hexane-1-aminocarbonyl group that was attached to O5’ of the ribose moiety. This tag was found to have negligible effect on the electronic transitions of nucleobase radicals. The charge-tagged adenosine, cytidine and guanosine radicals were further characterized by action spectra as N-7-H adenosine, N-7-H guanosine and N-3-H cytidine radical tautomers. All three radicals were found to undergo spontaneous dissociations by loss of the nucleobase and a hydrogen atom. Hydrogen deuterium exchange experiments showed a negligible isotope effect on the unimolecular dissociation of adenosine and cytidine radicals, while the guanosine radical displayed an unusual inverse isotope effect. This might arise from the dynamics of post-transition-state complexes preceding the product separation. The investigation of the three nucleoside radicals provided us a chance to elucidate the structure, energetics and electronic states of the fundamental DNA constituent, and the spectra provided a reference for the electronic properties of larger DNA cation-radical oligomers. The last chapter describes intramolecular interactions of hydrogen-rich DNA tetranucleotide cation radicals GATT+• and AGTT+•. These radicals were generated in the gas phase by one-electron reduction of the respective dications. The spectroscopic study indicated the formation of the 7,8-H-dihydroguanine cation radical isomer via a hydrogen atom migration from adenine N-1-H to the C-8 position in N-7-protonated guanine. This isomerization occurred spontaneously in hot cation radicals produced by electron transfer and was facilitated by low transition state energies as indicated by Rice–Ramsperger–Kassel–Marcus (RRKM) and transition state theory kinetic analysis.