The optical properties of adenine cation in different oligonucleotides: a PCM/TD-DFT study

The absorption spectra of several systems containing an adenine (A) cation are computed in water by TD-DFT calculations, using three different functionals and including solvent effects by a mixed discrete-continuum approach. Our calculations well reproduce the experimental absorption spectrum of deoxyadenosine nucleoside cation (dA), which provides an intense peak at 350 nm and a weaker one at 600 nm, allowing its assignment. M052X and CAM-B3LYP predict that the hole is essentially localized on a single A base also in the cationic forms of dApdA dinucleotide, both in single and in double strand, and of (dApdT) duplex, exhibiting absorption spectra similar to that of dA. For all these compounds, B3LYP predicts instead the delocalization of the hole over two A bases. On this ground, it is possible to propose which would be the spectral signature of partial hole delocalization. Vertical and adiabatic ionization potentials for the compounds under investigation are also computed, providing values in good agreement with the available experimental results.