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

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. 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.