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Quantum-Chemical Studies on the Favored and Rare Tautomers of Neutral and Redox Adenine
All possible twenty-three prototropic tautomers of neutral
and
redox adenine (nine amine and fourteen imine forms, including geometric
isomerism of the exo NH group) were examined in vacuo {DFT(B3LYP)/6-311+G(d,p)}.
The NH → NH conversions as well as those usually omitted, NH
→ CH and CH → CH, were considered. An interesting change
of the tautomeric preference occurs when proceeding from neutral to
reduced adenine. One-electron reduction favors the nonaromatic amine
C8H–N10H tautomer. This tautomeric preference is similar to
that (C2H) for reduced imidazole. Water molecules (PCM model) seem
to not change this trend. They influence solely the relative energies.
The DFT vertical detachment energy in the gas phase is positive for
each tautomer, e.g., 0.03 eV for N9H–N10H and 1.84 eV for C8H–N10H.
The DFT adiabatic electron affinity for the favored process, neutral
N9H–N10H → reduced C8H–N10H (ground states),
is equal to 0.18 eV at 0 K (ZPE included). One-electron oxidation
does not change the tautomeric preference in the gas phase. The aromatic
amine N9H–N10H tautomer is favored for the oxidized molecule
similarly as for the neutral one. The DFT adiabatic ionization potential
for the favored process, neutral N9H–N10H → oxidized
N9H–N10H (ground states), is equal to 8.12 eV at 0 K (ZPE included).
Water molecules (PCM model) seem to influence solely the composition
of the tautomeric mixture and the relative energies. They change the
energies of the oxidation and reduction processes by ca. 2 eV