Geometrical structures and energetic properties for different tautomers of adenine are calculated in this study,
using multi-configurational wave functions. Both the ground and the lowest singlet excited-state potential
energy surface are studied. Four tautomeric forms are considered, and their energetic order is found to be
different on the ground and the excited state potential energy surfaces. Minimum energy reaction paths are
obtained for hydrogen atom transfer (tautomerization) reactions in the ground and the lowest excited electronic
state. It is found that the barrier heights and the shapes of the reaction paths are different for the ground and
the excited electronic state, suggesting that the probability of such tautomerization reaction is higher on the
excited state potential energy surface. This tautomerization process should become possible in the presence
of water or other polar solvent molecules and may play an important role in the photochemistry of adenine.
Methylation of DNA occurs most readily at N(3), N(7), and O(6) of purine bases and N(3) and O(2) of pyrimidines. Methylated bases are continuously formed through endogenous and exogenous mechanisms. The results of a theoretical ab initio study on the methylation of G:C base pair components are reported. The geometries of the local minima were optimized without symmetry restrictions by the gradient procedure at DFT level of theory and were verified by energy second derivative calculations. The standard 6-31G(d) basis set was used. The single-point calculations have been performed at the MP2/6-31G(d,p), MP2/6-31++G(d,p), and MP2/6-311++G(2d,2p) levels of theory. The geometrical parameters, relative stability and counterpoise corrected interaction energies are reported. Also, using a variation-perturbation energy decomposition scheme we have found the vital contributions to the total interaction energy.
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