Calculations and characterization of the electronic spectra of DNA bases based on ab initio MP2 geometries of different tautomeric forms

The absorption spectra and transition moment directions of the four DNA bases and uracil were calculated, utilizing the semiempirical INDO/S-CI method and the ab initio CIS/6-31G* method. A linear scaling of the CIS/6-31G* transition energies was necessary to get good agreement with the observed energies. CASPT2 pi --> pi* transition energies and polarizations of adenine were also calculated. Fully MP2/6-31G*-optimized geometries were used in the calculations of the electronic spectra. The effect of possible tautomerism was investigated by calculation of the spectrum of several tautomers of guanine and cytosine. Specific and nonspecific solvent effects were considered in the calculations. Nonspecific solvent effects were modeled by a continuum model. Specific solvent interactions were modeled by inclusion of one water molecule in ''supermolecule'' calculations of the geometry and the spectrum of cytosine and guanine. Experimental transition energies were reproduced with an average error of less than 0.3 eV, and in most cases even better. Transition moment directions were calculated in good agreement with observations and other theoretical results in most cases. In some cases directions do not agree with observations. In these cases we have shown that the observed directions might be affected by the presence of more than one tautomeric form or by solvent or crystal effects. We propose a new interpretation of the pi --> pi* absorption spectrum of cytosine and guanine due to solvent effects and tautomerism. A simple model, based on symmetry and geometry arguments, was used to rationalize the computed transition moment directions.