Computational Studies on Photoinduced Charge Transfer Processes in Nucleic Acids: From Watson–Crick Dimers to Quadruple Helices

Absorption of ultraviolet light by nucleic acids can lead to the population of excited electronic states with significant inter-base charge transfer character, which have been recently shown to be involved in their photophysics and photochemistry. In this chapter, we shall concisely review the most recent computational papers on this topic, in oligonucleotide systems of different size and complexity, trying to highlight the most important chemical-physical effects into play. In Watson–Crick hydrogen bonded guanine–cytosine dimers, charge transfer states are populated with a large yield and can undergo inter-base proton transfer. The outcome is a proton-coupled electron transfer process which provides a very effective route for ground state recovery. This mechanism, which does not occur in adenine–thymine Watson–Crick dimer, is significantly affected by environmental effects. In single strands, inter-base charge transfer states are effectively populated for most of the possible stacked pairs. Within duplexes, both intra-strand and inter-strand charge transfer routes can be active and can lead to inter-strand proton transfer. The interplay between these two possible paths depends on the sequence. Finally, charge transfer states are populated also in non-canonical DNA structures as guanine-rich quadruple helices and cytosine-rich I-motifs.