Intramolecular proton transfer in adenine imino tautomers

Abstract

The electronic structural impact on intramolecular proton transfer in the cis- and trans-imino N7 and N9 tautomers of adenine (A) has been studied quantum mechanically, using density functional theory (B3LYP/TZVP, SAOP/TZ2P, LB94/TZ2P) and Green function (OVGF/TZVP) models. It is found that proton transfer does not significantly change isotropic properties but has profound impact on electron distributions of the species through anisotropic properties. The relative energies with respect to the canonical A tautomer (amino-9H), ΔE, for imino 7Hcis, imino 7Htrans, imino 9Hcis and imino 9Htrans are calculated as 16.15, 16.43, 18.46 and 13.80 kcal mol− 1 (B3LYP/TZVP model) and some minor changes in perimeters of the purine ring is also observed. The Hirshfeld atomic charges indicate that whether a proton attached to N(7) or N(9) causes a significant local charge redistribution. However, these charges are insensitive to cis–trans proton transfer. Condensed Fukui function reveals N(10) and C(8) as the most electrophilic reactive site among N- and C-atom sites, respectively. We also found that proton transfer significantly alters in-plane σ orbitals, rather than out of plane π orbitals including the frontier orbital 6a″. Moreover, orbital based responses to various proton transfers are presented: the orbital 29a′ (HOMO-1) is a signature orbital differentiating all the four tautomers. Orbital 27a′ is a site (N(7) and N(9)) sensitive orbital, whereas orbital 22a′ is only sensitive to proton orientation on the imino group = N–H.