11. Valence Anions of DNA-Related Systems in the Gas Phase: Computational and Anion Photoelectron Spectroscopy Studies

Formation of stable radical anion is one of the most apparent event resulting from the interaction of a biomolecule with an electron. Although, the dipole-bound (DB) anions of nucleobases (NBs) prevail in the gas phase as indicated by negative ion photoelectron spectroscopy (PES), even relatively weak interactions such as those present in the uracil-water complexes are sufficient to render the valence bound (VB) anions adiabatically stable. Moreover, since the electron clouds of dipole bound anionic states are much more diffuse than those of valence bound anions, they are strongly destabilized with respect to the latter in condensed phase. This is why VB anions rather than DB anions of nucleobases are more relevant for biological systems, i.e., in particular for DNA. In this review article, we discuss molecular factors governing the stability of valence anions of nucleobases. On the basis of PES measurements and quantum chemical calculations, we demonstrate that tautomerisation leading to the very rare tautomers of NBs renders the valence anions of nucleobases adiabatically stable. Moreover, we present how the stability of VB anions increases on the transition from NBs to nucleotides. On the other hand, studying anionic complexes of nucleobases with inorganic and organic proton donors, other nucleobases and nucleosides, we emphasize the importance of interactions within double stranded DNA as well as with species present in the environment in which DNA is always immersed under biological conditions. We show that in the complexes of NBs with sufficiently acidic proton donors, electron attachment frequently induces barrier-free proton transfer (BFPT) leading to a significant stabilization of anionic states in nucleobases. Our discussion is closing with a summary, including open questions on the influence of interactions between DNA and proteins on the stability and fate of anionic species induced by electrons in DNA.