Benchmark studies on the building blocks of DNA. 2. Effect of biological environment on the electronic excitation spectrum of nucleobases.

In the first paper of this series (Szalay; et al. J. Phys. Chem. A, 2012, 116, 6702) we have investigated the excited states of nucleobases. It was shown that it is only the equation of motion excitation energy coupled-cluster (EOMEE-CC) methods, which can give a balanced description for all type of the transitions of these molecules; if the goal is to obtain accurate results with uncertainty of about 0.1 eV only, triples corrections in the form of, e.g., the EOMEE-CCSD(T) method need to be included. In this second paper we extend this study to nucleobases in their biological environment, considering hydration, glycoside bond, and base pairing. EOMEE-CCSD and EOMEE-CCSD(T) methods are used with aug-cc-pVDZ basis. The effect of surrounding water was systematically investigated by considering one to five water molecules at different positions. It was found that hydration can modify the order of the excited states: in particular, nπ* states get shifted above the neighboring ππ* ones. The glycoside bond's effect is smaller, as shown by our calculations on cytidine and guanosine. Here the loss of planarity causes some intensity shift from ππ* to nπ* states. Finally, the guanine-cytosine (GC) Watson-Crick pair was studied; most of the states could be identified as local excitations on one of the bases, but there is also a low-lying charge-transfer state. Significant discrepancy with earlier CASPT2 and TDDFT studies was found for the GC pair and triples effects seem to be essential for all of these systems.