Novel Mass Spectroscopy Techniques Implementing UV Action Spectroscopy for the Characterization of DNA Cation Radicals and Transition Metal Complexes
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Mass spectrometry has become a powerful analytical tool for the detection and characterization of chemical compounds for biological and environmental studies. Advancements in mass spectrometry including new ionization sources, ion traps, and ion activation and separation methods have allowed for enhanced structure elucidation of gas-phase ions. With electrospray ionization, multiply-charged ions in solution can now be readily transferred into the gas phase. 3D quadrupole and linear ion traps can now facilitate multi-staged tandem (MSn) mass spectrometry experiments. That is, ions (based on their mass-to-charge ratios) can be isolated and fragmented, desired fragment ions can be subsequently isolated and fragmented, and so on. Ion activation methods including collision-induced dissociation (CID), electron transfer dissociation (ETD), and UV-Vis photodissociation (UVPD) can now offer different fragmentations of precursor ions and can be combined for more robust structural analyses. Here, novel tandem mass spectrometry techniques implementing UVPD action spectroscopy for the characterization of DNA cation radicals and transition metal complexes is presented. To begin, the employment of new automated action spectroscopy methods on two different commercial mass spectrometers is discussed. UVPD action spectroscopy involves exposing isolated ions to high-energy photons and measuring the resulting photo-fragmentation as a function of photon wavelength (i.e. 200-700 nm). The generated action spectrum is representative of the gas-phase ion population’s electronic excitations, and can be complemented with theoretical calculations to probe the predominant gas-phase structures. The modifications imposed on a 3D quadrupole and a linear ion trap mass spectrometer, including the optical setups needed to direct an external laser beam into the ion traps, are described in detail. Utilization of the instruments’ auxiliary interface features, as well as the specific software (LabView) codes created to orchestrate automated MSn-UVPD at specified photon wavelengths, is also reported. To compare the performance of the automated action spectroscopy methods between the two mass spectrometers, the experimental action spectrum of a well-studied compound was generated using both systems. Next, the focus is shifted towards the characterization of DNA molecules using tandem mass spectrometry with UVPD action spectroscopy. Radiative damage of DNA can lead to the formation of nucleobase cation radicals along DNA chains, resulting in nucleobase loss and strand breaks. Here, DNA-based cation radicals of guanine, 9-methylguanine, and guanosine were generated in the gas phase, and their isolated structures and intrinsic properties were probed using UVPD action spectroscopy and excited state computations. The DNA cation radicals generated in the gas phase via MS2-CID of their doubly-charged Cu(II)-complexes were isolated and subjected to MS3-UVPD at incremental wavelengths from 210-700 nm. Experimental action spectra were acquired for the isolated cation radicals and were found to closely match the theoretical spectra generated by time-dependent density functional theory (TD-DFT) calculations, which considered various functionals and basis sets. The results also suggested that the guanine, 9-methylguanine, and guanosine canonical structures were the predominant cation radical conformations in the gas-phase, and were in agreement with past infrared multiphoton photodissociation (IRMPD) action spectroscopy studies. The DNA nucleobase cation radical of thymine was also closely investigated using UVPD action spectroscopy in conjunction with other complementary methods. In this case, thymine cation radicals were generated via MS2-CID of their doubly-charged Cu(II)-complexes and were characterized via UVPD action spectroscopy, hydrogen-deuterium exchange experiments, ion-molecule reactions, and theoretical calculations. The experimental results suggested a mixture consisting chiefly of non-canonical thymine cation radical tautomers (77%), with the canonical isomer as a minor component (23%). Theoretical calculations predicting the low-energy isomers also supported the experimental results. Lastly, the peculiar discovery of gas-phase water splitting involving simple transition metal complexes is addressed. For decades, immense efforts have been directed at achieving catalytic water splitting using sophisticated water oxidation complexes and sunlight to generate carbon-neutral fuels. Here, a novel method utilizing transition metal complexes and near-UV light to split water into hydroxyl radical is explained. (2,2’-bipyridine)Metal-O+ ions (Metal = Cu, Ni, Co) were first produced by near-UV photodissociation of [(2,2’-bipyridine)MetalIINO3]+ ions in the gas phase. Upon storage in an ion trap, the (2,2’-bipyridine)Metal-O+ ions spontaneously added water, and the newly-formed [(2,2’-bipyridine)Metal-O + H2O]+ complexes eliminated OH after subsequent near-UVPD (i.e. 260-340 nm) to achieve stoichiometric homolytic cleavage of gaseous water. Ion-molecule reactions, isotope-labeling experiments, and DFT calculations were further conducted to better characterize the reactions.
- Chemistry