AUTHOR=Brovarets' Ol'ha O. , Tsiupa Kostiantyn S. , Dinets Andrii , Hovorun Dmytro M. TITLE=Unexpected Routes of the Mutagenic Tautomerization of the T Nucleobase in the Classical A·T DNA Base Pairs: A QM/QTAIM Comprehensive View JOURNAL=Frontiers in Chemistry VOLUME=6 YEAR=2018 URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2018.00532 DOI=10.3389/fchem.2018.00532 ISSN=2296-2646 ABSTRACT=

In this paper using quantum-mechanical (QM) calculations in combination with Bader's quantum theory of “Atoms in Molecules” (QTAIM) in the continuum with ε = 1, we have theoretically demonstrated for the first time that revealed recently highly-energetic conformers of the classical A·T DNA base pairs – Watson-Crick [A·T(w_WC)], reverse Watson-Crick [A·T(w_rWC)], Hoogsteen [A·T(w_H)] and reverse Hoogsteen [A·T(w_rH)] – act as intermediates of the intrapair mutagenic tautomerization of the T nucleobase owing to the novel tautomerisation pathways: A·T(w_WC)↔A·T^*(w^⊥_WC); A·T(w_rWC)↔A·TO2*(w^⊥_rWC); A·T(w_H)↔A·T^*(w^⊥_H); A·T(w_rH)↔A·TO2*(w^⊥_rH). All of them occur via the transition states as tight ion pairs (A⁺, protonated by the N6H₂ amino group)·(T⁻, deprotonated by the N3H group) with quasi-orthogonal geometry, which are stabilized by the participation of the strong (A)N6⁺H···O4⁻/O2⁻(T) and (A)N6⁺H···N3⁻(T) H-bonds. Established tautomerizations proceed through a two-step mechanism of the protons moving in the opposite directions along the intermolecular H-bonds. Initially, proton moves from the N3H imino group of T to the N6H₂ amino group of A and then subsequently from the protonated N6⁺H₃ amino group of A to the O4/O2 oxygen atom of T, leading to the products – A·T^*(w^⊥_WC), A·TO2*(w^⊥_rWC), A·T^*(w^⊥_H), and A·TO2*(w^⊥_rH), which are substantially non-planar, conformationally-labile complexes. These mispairs are stabilized by the participation of the (A)N6H/N6H'···N3(T) and (T)O2H/O4H···N6(A) H-bonds, for which the pyramidalized amino group of A is their donor and acceptor. The Gibbs free energy of activation of these mutagenic tautomerizations lies in the range of 27.8–29.8 kcal·mol⁻¹ at T = 298.15 K in the continuum with ε = 1.