Abstract:
Oxidation of the nucleobases is of great concern for the stability of DNA strands and is considered as a source of mutagenesis and cancer. Ionizing radiation may lead to the formation of the radical cations of the DNA bases in their electronic excited states. Therefore, to have a complete knowledge of the effect of ionizing radiation on DNA, it is important to understand the underlying molecular mechanisms when light interacts with DNA bases, its building-blocks, as well as their spectroscopic and molecular properties. However, precise spectroscopy data, in particular in their electronic excited states are scarce if not missing. We here report an original way to produce isomer-selected radical cations of nucleic acids, exemplified in the case of pyrimidine bases (Cytosine, Thymine and Uracil for RNA), through the photodissociation of cold silver complex containing nucleic acids. UV photodissociation (UVPD) of the isomer-selected silver complexes produces the nucleic acid radical cations (C•+, T•+ or U•+) without isomerization.[1] IR-UV cryogenic ion spectroscopy of the radical cations allows for their unambiguous structural assignments. These benchmark spectroscopic data of the electronic excited states of nucleic acid radical cation are used to assess the quantum chemistry calculations performed at the TD-DFT, CASSCF/CASPT2 and CASSCF/MRCI-F12 levels.