The prototropic tautomerism of 8-azaadenine (azaade) was studied theoretically by means of ab initio methods, in both the gas phase and aqueous solution. A number of tautomeric forms were not included in the calculations after applying a stepwise elimination procedure based on both AM1 and HF/6-31G* energy values. The tautomers 9H-azaade, 8H-azaade and 7H-azaade survived to this elimination and their optimized geometries and energies were calculated at the MP2/6-31*//HF/6-31G* level. To include the solvent effects, two self-consistent reaction field method were used: (1) Onsager's SCRF with multipolar expansion up to the hexadecapolar term and (2) the isodensity polarizable continuum method (IPCM). Both methods produce similar results, although the latter represents better the situation in aqueous solution. The stability order in solution, 8H-b 9H-b 7H-azaade, differs slightly from that found in the gas phase, implying that in general the electrostatic effects in solution are important, but the intrinsic stability of these species in the gas phase overcomes the solvent effect.
The enol keto and thiol thione prototropic tautomerisms in 2-imidazolone and 2-thioimidazolone were studied by ab initio methods in the frame of MO theory. The energetics of these reactions were derived using various basis sets and levels including both polarization and diffuse functions. To account for electron correlation, second-, third-and fourth-order Møller-Plesset perturbation theory was applied. The thermodynamics for both reactions indicate that the keto and thione are the predominant species in the gas phase, whereas in solution the enol and keto forms are present in comparable concentrations. For both tautomerization reactions the transition states for the intramolecular mechanism were found and fully characterized. The calculated energetic barrier for the enol-keto system is 41.6 kcal mol À1 , whereas that for the thiol-thione system is 32.5 kcal mol À1 (1 kcal = 4.184 kJ). The study of the solvent effect, using the PCM model, on E a reveals that solvents lower the barrier by ca 11 and 7 kcal mol À1 for the enol-keto and thiol-thione systems, respectively.
A theoretical study of the non canonical base pair, Guanine-Guanine (G-G) has been carried out in the frame of molecular orbital theory using density functional theory (DFT). The Becke three parameter hybrid including correlation functional that contains local and non-local terms (B3LYP) was used thoroughly. The 6-31G(d,p) basis set was employed to obtain the optimized geometry and energy of the non-Watson-Crick pair formed between two molecules of guanine. The results compare well with the properties of other base pairs of biological interest. At the DFT level the non-planarity of the pair is evident. The two guanines molecules are bent by ca 12 o and buckled. The interaction energy corrected by BSSE is equal to-10.7 kcal / mol, similar to that calculated for other pairs involving bases of nucleic acids.
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