The role of pyrimidine nucleobase excimers in DNA photophysics and photoreactivity

Quantum chemical studies using the accurate CASPT2//CASSCF procedure show that π-stacked interactions in biochromophores such as pyrimidine (Pyr) DNA/RNA nucleobases pairs yield excimer-like situations which behave as precursors of processes like charge transfer (CT) or photoreactivity and are the source of the emissive properties in DNA. Examples are the CT between adjacent DNA nucleobases in a strand of oligonucleotides and the photodimerization taking place in cytosine (C) pairs leading to cyclobutanecytosine (CBC) mutants. These processes take place through nonadiabatic photochemical mechanisms whose evolution is determined by the presence and accessibility of conical intersections (CIs) and other surface crossings between different electronic states.