Gas-phase
chemistry of cation radicals related to ionized nucleic
acids has enjoyed significant recent progress thanks to the development
of new methods for cation radical generation, ion spectroscopy, and
reactivity studies. Oxidative methods based on intramolecular electron
transfer in transition-metal complexes have been used to generate
nucleobase and nucleoside cation radicals. Reductive methods relying
on intermolecular electron transfer in gas-phase ion–ion reactions
have been utilized to generate a number of di- and tetranucleotide
cation radicals, as well as charge-tagged nucleoside radicals. The
generated cation radicals have been studied by infrared and UV–visible
action spectroscopy and ab initio and density functional theory calculations,
providing optimized structures, harmonic frequencies, and excited-state
analysis. This has led to the discovery of stable noncanonical nucleobase
cation radicals of unusual electronic properties and extremely low
ion–electron recombination energies. Intramolecular proton-transfer
reactions in cation radical oligonucleotides and Watson–Crick
nucleoside pairs have been studied experimentally, and their mechanisms
have been elucidated by theory. Whereas the range of applications
of the oxidative methods is currently limited to nucleobases and readily
oxidizable guanosine, the reductive methods can be scaled up to generate
large oligonucleotide cation radicals including double-strand DNA.
Challenges in the experimental and computational approach to DNA cation
radicals are discussed.