Cryogenic IR and UV spectroscopy of isomer-selected cytosine radical cation

Oxidation of the nucleobases is of great concern for the stability of DNA strands and is considered as a source of mutagenesis and cancer. 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 DNA bases, exemplified in the case of cytosine, through the photodissociation of cold cytosine-silver (C-Ag+) complex. IR-UV dip spectroscopy of C-Ag+ features fingerprint bands for the two keto-amino cytosine tautomers. UV photodissociation (UVPD) of the isomer-selected C-Ag+ complexes produces the cytosine radical cation (C+) without isomerization. IR-UV cryogenic ion spectroscopy of C+ allows for the unambiguous structural assignment of the two keto-amino isomers of C+. UVPD spectroscopy of the isomer-selected C+ species is recorded at a unique spectral resolution. These benchmark spectroscopic data of the electronic excited states of C+ are used to assess the quantum chemistry calculations performed at the TD-DFT, CASSCF/CASPT2 and CASSCF/MRCI-F12 levels.

Automated summary

By photodissociating cold cytosine-silver complexes, isomer-selected radical cations of DNA bases have been successfully generated, leading to unambiguous structural assignment of cytosine and providing important data for studying the stability of DNA strands.