UV Absorption and Magnetic Circular Dichroism Spectra of Purine, Adenine, and Guanine: a Coupled Cluster Study in Vacuo and in Aqueous Solution

The absorption and magnetic circular dichroism (MCD) spectra of purine and of the purine nucleobases adenine and guanine have been calculated in gas phase at the Coupled Cluster Singles and Doubles (CCSD) and Resolution-of-Identity Singles and Approximate Doubles (RI-CC2) levels of theory. Exploiting a new development in the Turbomole program package for computing vertical excitation energies and Faraday B terms in an implicit solvent approximated by the conductor-like screening model (COSMO) at the CC2 level, we have investigated the solvent effects on the relative positions of the ππ∗ and nπ∗ electronic transitions in these three molecules and compared them to the corresponding vacuum results. In the case of adenine we included also specific solvent effects with a small water cluster. The spectra obtained with the implicit model COSMO are in qualitative agreement with those obtained with explicit water molecules both with and without inclusion of the bulk solvent effects via the continuum solvent model. This suggests that the inclusion of the electrostatic contributions of the solvent can provide a sufficiently accurate description of the absorption spectra for adenine. The results for purine, adenine, and guanine show that after the inclusion of bulk solution the ππ ∗ states shift to lower energies while at the same time nπ∗ states show a reversed behavior. The computed MCD spectra show the characteristic bi-signate profile found experimentally in all cases, despite, for adenine, remarkable differences in the origin of the individual peaks for different computational methods. Therefore, the ability (or inability) of MCD for determining the relative stability of the La and Lb states is critically re-assessed. According to our best estimate for adenine in aqueous solution the L_a state is more stable than L_b.