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The results of calculations of the transition energies and polarization in nucleotide bases performed by the CNDO/S CI method are compared with experimental data for long-wavelength and vacuumabsorption bands. The calculations and the analysis of experimental data testify to the existence of n -T* transitions in the first absorption bands of the bases. The study of double-stranded polynucleotides and DNA hypochromism based on the theoretical electronic characteristics of the bases and perturbation theory is performed. The role of stacking and complementary interactions within the hypochromic effect is cleared up. Two mechanisms for the shift of the fluorescence band maximum are investigated: two-proton transfer along H bonds and excimer formation. The study of H-bond potential curves shows the disadvantage of two-proton phototautomerism in the nucleotide base pairs in contrast to model systems. The possibility of excimer state formation in stacked homo-and heterodimers of nucleotide bases is shown within the extended-Huckel treatment. The nature of excimer minimum for the excited-state potential curve is analyzed.To understand the absorption and luminescence spectra of DNA and also the nature of the related phenomena (hypochromism, energy migration, excimer states formation, etc.), a quantum-mechanical study of electronic structure of nucleotide bases is necessary. A series of important spectroscopic features points out that the knowledge of velectronic wave functions is not sufficient. In this connection the study of the excited singlet and triplet states of the bases with the help of the C N D O /~ method, which permits us to directly take into account the interaction of all valence electrons, has recently been undertaken [l, 21. To obtain detailed information on the electronic structure of the bases, the calculations of the ground and excited states of adenine (A), thymine (T), uracil (U), guanine (G), and The calculated energies of electronic transitions into singlet excited states A E and their oscillator strengths (f) are given in Tables I and 11. Experimental data for the absorption band maxima of the bases in the water solutions [5-81 and sublimated films of the bases [9,10] are also given. In can be seen that the *

Theoretical modeling of initial steps of the photodimerization mechanism of uracil, 5-methyl-and 5-halogen derivatives was performed. The interaction energy of bases in stacked dimers in the ground and lowest excited states was calculated as a function of the distance between the base planes and of the rotation angles within the perturbation theory for the extended Hückel treatment. The existence of excimer and exciplex region on the potential surface of the excited state was revealed. The excimer (exciplex) geometry has the planes nonparallel with more close contact of the C5-C6 bonds as compared to the ground state of dimers. The results provide new information useful for understanding the photodimerization mechanism of bases and testifies that the singlet excimer state can be a precursor of the photodimerization reaction.

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