Tautomerism and Proton Transfer in 6-Selenoguanine: A Post Hartree−Fock Level ab Initio SCF-MO Investigation

Tautomerism and proton transfer in 6-selenoguanine (6SeG) have been investigated using high level ab initio calculations. Full geometry optimizations were carried out in the gas phase at the HF and MP2 levels using the 6-31G(d,p) basis set. Furthermore, the single point energies were evaluated using larger basis sets augmented with diffuse and polarization functions. At all applied levels of theory, the N 7 protonated form is shown to be the most stable one in the gas phase and is 3.1 kcal/mol more stable than the N 9 protonated tautomer at the MP2/6-311++G(d,p)//MP2/6-31G(d,p) level. However, aqueous solvation studies using the SCI−PCM continuum models show a different trend for energetic preference for selenoguanines. Estimated free energies of tautomerization in an aqueous medium indicate that the N 9 protonated form is more stable than the N 7 protonated form although the energies of these two tautomers are very close. Our calculations suggest that the Se 6 protonated form of N 9 -selenoguanine is the most hydrated one while the selenoic form of N 9 -selenoguanine is the least hydrated one. The following stability order may be established for 6-selenoguanine in the gas phase, 6SeG4 > 6SeG3 > 6SeG2 > 6SeG1 > 6SeG5, while in an aqueous solution, a stability order as such is established, 6SeG1 > 6SeG4 > 6SeG3 > 6SeG2 > 6SeG5. The proton transfer from the N 1 to the Se 6 site involves an energy barrier of about 39 kcal/mol for the N 9 protonated tautomer and 46 kcal/mol for the N 7 protonated tautomer at the HF level and 31.8 and 36.6 kcal/mol, respectively, at the MP2/6-311++G(d,p)//HF/DZP level.