Biopolymers and cell. 1997; 13 (4): 261 - 268

On the new mechanism of mutations induced by UV-light. A theoretical study of the double-prolon phototautomerism in a model base pair of DNA

V. I. Danilov, O. V. Mikhaleva, O. N. Slyusarchuk, J. J. Stewart, J. L. Alderfer

A detailed study of Frank-Condon potential energy curves for the double-proton transfer along hydrogen bonds in the 7-azaindole dimer in S0 and i'( states is presented. Calculations were carried by the semi-empirical AMI method including configuration interaction. It is shown thai in the St state the rare tautomeric form of the dimer is more stable than its normal form and, under certain conditions, it can become responsible for the spectra of long-wave (green) fluorescence. Calculations of the 7-azaindole dimer with full geo¬metry optimization in comparison to ones where the H...N1 distance in the Nl-H...N1 bonds was not optimized provides more realistic parameter values characterizing Frank-Condon potential curves of hydrogen bonds. In particular the height of the potential barrier decreases considerably for the SQ and Sj states. The simplest model system representing 7-azaindole ditner normal and rare forms, which allows to perform calculations for both forms of the dimer in the S0 and St states with full geometry optimization is suggested. It is found that the minimum corresponding to the rare tautomeric form of the dimer is deeper than the normal form. This result supports main conclusion made from Frank-Condon potential curves. Comparison of the calculated maxima of the absorption and fluorescence bands for the rare dimer form shows a. large Stokes shift of green emission which is close to the one observed in experiments. The possibility of biprotonic photolautotnerism in ON A base pairs is discussed.