Amino-imino adenine tautomerism induced by the cooperative effect between metal ion and H2O/NH3.

Tautomerization processes of amino-imino adenine isomer (A → A1) in five different environments are studied by the density functional theory (B3LYP) method. The five environments are metal ion (M, M = K(+), Na(+), Cu(+), Zn(+), Ca(2+), Mg(2+), Cu(2+), Zn(2+)) coordinated bidentate system, either monowater (W) or monoammonia (N) attached system, both metal ion and monowater cooperative system (M-W), and both metal ion and monoammonia cooperative system (M-N). Results show that the complexes formed by noncanonical rare imino form A1 are more stable than those formed by the canonical amino one in most of these environments. The tautomerization of A → A1 becomes quite easy in either M-W or M-N system. It is noteworthy that under divalent M-N environment the A → A1 process meets with particularly lower and even free energy barrier, indicating the instability of the amino adenine isomer and probable existence of more stable imino adenine isomer. Expanding studies for the microhydration at the metal ion of the M-N system predict the required number (n) of water molecules to remain the amino adenine isomer A (AMNnW) stable. The number of n is 2, 3, 3, and 4 for M = Ca(2+), Zn(2+), Cu(2+), and Mg(2+), respectively. The present study provides further understanding for the amino-imino tautomerization behavior of the most stable adenine under the influence of several related closely factors, and is useful for rational design of these different environments for the purposes of prevention and control of pyrimidines mispairing, which is responsible for the mutagenic properties of the nucleic acid bases.