CHAPTER 2: Photoinduced Dimerization in DNA: Singlet Excited State Reaction Paths

This chapter describes the processes of base/base dimerization that ultraviolet (UV) absorption induces in nucleic acids. In the absence of any sensitizer, dimerization principally involves singlet excited electronic states. Focussing on bipyrimidine dimers, which are those produced with the largest yield, we describe which are the major photodimers formed for each sequence, the underlying basic molecular mechanisms and the main experimental features associated with these reactions. We show that different electronic states are associated with different photoproducts and we highlight the importance of the local effects in determining the yield found for the different sequences. In particular, the structure adopted by the reactive dinucleotide step when it absorbs UV light plays a major role.