No abstract

Quantum-chemical calculation of the energies of the electronic transitions and the electronic structures of the neutral and ionic species of the nucleic acids components in their ground and lower excited singlet and triplet rrr* and nrr* states has been carried out in the all-valence-electron approximation CNDO/.S. The results of the calculation allow one to identify the most photoreactive sites of the molecules and to consider the dependence of the location of these sites on the ionic state of the molecules. The calculated data are compared with our previous results obtained in a rr-electron approximation. The individual absorption spectra of various ionic and tautomeric species of the nucleic acids components obtained by us earlier have been decomposed into bands corresponding to separate electronic transitions. As a rule, there is a good agreement between the calculated data in the two approximations and the experimental results.At the present time study of the electronic structure of various ionic and tautomeric species of the nucleic bases and their analogs attracts increasing attention. Indeed, the structural and functional specificity of the nucleic acids is determined by the reactivity of their components, which in turn is essentially determined by electronic structure. However, until recently attention was paid mainly to the electronic structure of only neutral species of the nucleic bases in their ground state. Only a few ionic species of the nucleic bases were studied in some papers (see Refs. 1-3); a different quantum-chemical method or a parameter system was used in each of them.One of the possible reasons for the inadequate attention to the electronic structure of the ionic species of the pyrimidine and purine bases of the nucleic acids was lack of a sufficiently complete spectroscopic description, which hampered comparison of the calculated data with the experimental results. In this connection in 197 1 a systematic experimental-theoretical study of the uv-spectral properties and of the electronic structure of various ionic and tautomeric pyrimidine and purine nucleic

The individual absorption spectra of various ionic and tautomeric species of some derivatives of nucleic-acid components have been determined by computer handling of the summary spectra of these compounds measured at different pH values. These individual spectra have been decomposed into separate bands represented by log-normal curves and corresponding to different electronic transitions. Using the parameter system calculated earlier for the molecules of the vitamin B6 family, the calculation of the electronic spectra and electronic structure of various ionic and tautomeric species of the molecules studied have been carried out in the r-electron approximation. These properties have also been calculated by the all-valence electronic method CNDOS. The calculated results correlate well with both approximations and with our experimental spectral data. The localization of the molecular electronic excitation studied on the transition to the S1 and TI states is considered in connection with the reactivity in these states.The primary reason for the mutagenic action of hydroxylamine and 0 -methylhydroxylamine on metabolic and nonmetabolic systems is the enzymatic and direct chemical transformation of the cytidine and adenosine residues into N4-hydroxy-or N4-methoxycytidine and N6-hydroxy-or N6-methoxyadenosine, respectively [ 13. In the template synthetic systems these compounds have ambiguous specificity and not only imitate the cytidine and adenosine components, but also uridine-thymidine or guanosine [ 11. Some 5-substituted derivatives of the pyrimidine nucleic-acid components also have a dual specificity [2].Hydroxy-and methoxyderivatives of cytidine and adenosine also have unusual photochemical properties: on uv irradiation, rupture of the N-0 bond with the formation of cytidine and adenosine occurs [3].This led us to study in detail the uv spectroscopic properties, base-acid, and tautomeric equilibria of these compounds and of their fixed analogs, and also the electronic structure of these compounds in the ground and lower-excited singlet and triplet states.