The nature of the transition mismatches with Watson–Crick architecture: the G*·T or G·T* DNA base mispair or both? A QM/QTAIM perspective for the biological problem

This study provides the first accurate investigation of the tautomerization of the biologically important guanine*·thymine (G*·T) DNA base mispair with Watson–Crick geometry, involving the enol mutagenic tautomer of the G and the keto tautomer of the T, into the G·T* mispair (∆G = .99 kcal mol−1, population = 15.8% obtained at the MP2 level of quantum-mechanical theory in the continuum with ε = 4), formed by the keto tautomer of the G and the enol mutagenic tautomer of the T base, using DFT and MP2 methods in vacuum and in the weakly polar medium (ε = 4), characteristic for the hydrophobic interfaces of specific protein–nucleic acid interactions. We were first able to show that the G*·T↔G·T* tautomerization occurs through the asynchronous concerted double proton transfer along two antiparallel O6H···O4 and N1···HN3 H-bonds and is assisted by the third N2H···O2 H-bond, that exists along the entire reaction pathway. The obtained results indicate that the G·T* base mispair is stable from the thermodynamic point of view complex, while it is dynamically unstable structure in vacuum and dynamically stable structure in the continuum with ε = 4 with lifetime of 6.4·10−12 s, that, on the one side, makes it possible to develop all six low-frequency intermolecular vibrations, but, on the other side, it is by three orders less than the time (several ns) required for the replication machinery to forcibly dissociate a base pair into the monomers during DNA replication. One of the more significant findings to emerge from this study is that the short-lived G·T* base mispair, which electronic interaction energy between the bases (−23.76 kcal mol−1) exceeds the analogical value for the G·C Watson–Crick nucleobase pair (−20.38 kcal mol−1), “escapes from the hands” of the DNA replication machinery by fast transforming into the G*·T mismatch playing an indirect role of its supplier during the DNA replication. So, exactly the G*·T mismatch was established to play the crucial role in the spontaneous point mutagenesis.


Introduction
The accuracy of the DNA replication in the living cell is a very important biological phenomenon that determines the genetic stability of all living things on the Earth (Kornberg & Baker, 1992). It is well known that the point (gene) mutations play the dual role: evolutionary, on the one side, for maintaining genetic information over many generations and, on the other side, they can be considered as a key cause of different illnesses, in particular cancer in humans (Drake, 1991;Friedberg et al., 2006;Vonborstel, 1994).
Transitions, as events that occur most frequently and, therefore, are often eliminated from the genome by the reparations systems of the cell (Lee, Popodi, Tang, & Foster, 2012;Rogozin and Pavlov, 2003), occupy a prominent place among the point mutations, the underlying cause of which is an accidental formation of the irregular base pairs with geometry close to those of the Watson-Crick base pairs in the base pair recognition pocket of the DNA polymerase (Goodman, 1997;Hwang & Taylor, 2005;Kool, 2002;Minnick, Liu, Grindley, Kunkel, & Joyce, 2002), e.g. of the G·T mismatch in the recognition pocket of the DNA polymerase λ, that is well poised for catalysis (Bebenek, Pedersen, & Kunkel, 2011). Unfortunately, their physicochemical nature remains understandable only in the most general features (Topal & Fresco, 1976). This is obviously due to the lack of the well-grounded from the physicochemical point of view and internally non-contradictable approach or ideology to the successful solution of these problems. In fairness it must be said that only the so-called Watson and Crick's tautomeric hypothesis formulated shortly after the establishment of the spatial structure of the DNA can pretend on the role of this ideology (Watson & Crick, 1953). Nowadays, the intensive accumulation of both experimental and theoretical data continues within the framework of this hypothesis Danilov, Anisimov, Kurita, & Hovorun, 2005;Danilov et al., 2009;Wang, Hellinga, & Beese, 2011).
Thus, in our previous work  by applying DFT and MP2 quantum chemical methods and methodology of the scans of the electrontopological, energetic, geometrical, and polar characteristics along the intrinsic reaction coordinate (IRC) of the tautomerization, it was shown that the lifetime (5.76·10 −10 s) of the thermodynamically and dynamically stable A*·C base mispair (ΔG = 2.72 kcal mol −1 , population = 1.02 % obtained at the MP2 level of quantummechanical (QM) theory in the continuum with ε = 4) is by order less than the time (several ns) required for the replication machinery to forcibly dissociate a base pair into the monomers during DNA replication. These data suggest that the enzymatically competent A·C base mispair possessing Watson-Crick geometry, that was experimentally fixed in the recognition pocket of the DNA polymerase (Wang et al., 2011), is in the A·C* tautomeric form (here and below the mutagenic tautomers (Brovarets' & Hovorun, 2010a, 2011aFurmanchuk et al., 2011;Kosenkov et al., 2009;Platonov, Samijlenko, Sudakov, Kondratyuk, & Hovorun, 2005;Samijlenko, Yurenko, Stepanyugin, & Hovorun, 2012) of the canonical DNA bases are marked with asterisks), associated by the three cooperative N6H···N4, N1···HN3, and C2H···O2 H-bonds. Notably, the C2H···O2 specific contact was established to be the real H-bond , which acts as a third point of support, thereby ensuring the similarity of the structurally dynamical properties of this mismatch to the classical Watson-Crick DNA base pairs. This paper is a logical continuation of the previous one . It is aimed to give an unambiguous answer to the biologically important question, issued in its title, and is focused on the thorough elucidation of the physico-chemical mechanism of the G*·T↔G·T* tautomerization through the double proton transfer (DPT) to understand the role of each of these base mispairs in the production of the spontaneous point replication errors. In fact, the incorrect G*·T and G·T* purine-pyrimidine base mispairs with Watson-Crick architecture and mechanism of their tautomerization through the DPT are the subjects of the quantum chemical study both in vacuum and in the continuum with ε = 4, typical for the hydrophobic interiors of proteins (García-Moreno et al., 1997) and the hydrophobic interfaces of specific protein-nucleic acid interactions (Bayley, 1951;Brovarets', Yurenko, Dubey, & Hovorun, 2012;Dewar & Storch, 1985;Mertz & Krishtalik, 2000;Petruska, Sowers, & Goodman, 1986).
In this paper, for the first time we have succeeded in the theoretical survey of the G*·T↔G·T* DPT tautomerization. It was established that this reaction proceeds in the ground electronic state through a single transition state following the concerted mechanism, in which the protontransfer process occurs asynchronously without forming any stable intermediates. The G·T* DNA base mispair with the population equal to 15.8% was found to be the dynamically stable structure with lifetime of 6.39·10 −12 s (in the continuum with ε = 4), that is by three orders less than the time required for the replication machinery to forcibly dissociate a base pair into the monomers during DNA replication. This work also reports the evolution of the energetic, electron-topological, geometric, polar, and natural bond orbital (NBO) parameters (Brovarets' & Hovorun, 2013e, 2014bBrovarets', Zhurakivsky, & Hovorun, 2013bBrovarets', Kolomiets', & Hovorun, 2012), describing the course of the tautomerization via the DPT along the IRC. It was established based on the energetic scans, that the O6H···O4/O4H···O6, N3H···N1/N1H···N3 and N2H···O2 H-bonds in the G*·T/G·T* DNA base mispairs, respectively, are significantly cooperative.
The correspondence of the stationary points to minimum on the potential energy landscape or TS, located by means of Synchronous Transit-guided Quasi-Newton method (Peng, Ayala, Schlegel, & Frisch, 1996;Peng & Schlegel, 1993), has been checked by the absence or the presence, respectively, of one and only one imaginary frequency corresponding to the normal mode that identifies the reaction coordinate.
Once the stationary points were located, the reaction pathway was established by performing the IRC calculations in the forward and reverse directions from the TS G*·T↔G·T* to reach the reactant (G*·T base mispair) and product (G·T* base mispair) using the Hessian-based predictor-corrector integration algorithm (Hratchian & Schlegel, 2004, 2005a, 2005b with tight convergence criteria, that eventually ensure that the proper reaction pathway has been found. By using the IRC data we have established the profiles of the physico-chemical properties of the DNA base mispairs and intermolecular contacts stabilizing them, in particular the electronic energy, the first derivative of the electronic energy with respect to the IRC, the dipole moment of the base pair, the distances and the angle of the intermolecular H-bonds, the electron density, the Laplacian of the electron density, ellipticity and the energy at the (3,−1) BCPs of the intrapair H-bonds, the NBO charges of the hydrogen atoms involved in the intermolecular H-bonds, the glycosidic angles, the distance between the glycosidic hydrogens and dihedral angle along the reaction pathway of the G*·T↔G·T* DPT tautomerization establishing them at the each point along the IRC in vacuum and in the continuum with ε = 4.
The electronic interaction energies in vacuum E int have been computed at the MP2/6-311++G(2df,pd) level of theory for the geometries optimized at the DFT and MP2 levels of theory as the difference between the total energy of the base mispair and the energies of the isolated monomers. In each case the interaction energy was corrected for the basis set superposition error (BSSE) (Boys & Bernardi, 1970;Gutowski, Van Lenthe, Verbeek, Van Duijneveldt, & Chałasinski, 1986) through the counterpoise procedure (Sordo, Chin, & Sordo, 1988;Sordo, 2001). The interaction energies in solution E sol have been calculated as the sum of gas phase interaction energies and a solvation correction term : (1) where E cosmo and E gas phase are the BSSE uncorrected interaction energies in solution and gas phase, respectively, evaluated for the geometries optimized in solution.
The Gibbs free energy G values for all structures was obtained in the following way: where E elthe electronic energy, E corrthermal correction. We applied the standard TS theory (Atkins, 1998) to estimate the activation barriers of the tautomerization reaction. The time τ 99.9% necessary to reach 99.9% of the equilibrium concentration of the G*·T and G·T* DNA base mispairs in the system of reversible first-order forward (k f ) and reverse (k r ) reactions can be calculated by the formula (Atkins, 1998): and the lifetime τ of the G·T* complex is obtained as 1/k r . The equilibrium constants were calculated using the standard equation (Atkins, 1998): where ΔGthe relative Gibbs free energy of the product the G·T* DNA base mispair, Tthe absolute temperature and Rthe universal gas constant.
To estimate the values of the rate constants k f and k r : we applied the standard TS theory (Atkins, 1998), in which quantum tunneling effect are accounted by the Wigner's tunneling correction (Wigner, 1932) that is adequate for the DPT reactions (Arabi & Matta, 2011;Brovarets', Zhurakivsky, & Hovorun, 2014a: where k B -Boltzmann's constant, h -Planck's constant, ΔΔG f,r -Gibbs free energy of activation for the DPT reaction in the forward (f) and reverse (r) directions, ν ithe magnitude of the imaginary frequency associated with the vibrational mode at the TS G*·T↔G·T* that connects reactants and products. Bader's quantum theory "Atoms in molecules" (QTAIM) was applied to analyze the electron density distribution (Bader, 1990), using program package AIM-All (Keith, 2010) with all default options. Wave functions were obtained at the level of theory used for geometry optimization. The presence of a bond critical point (BCP) (Bader, 1990), namely the so-called (3,−1) BCP, and a bond path between the H-bond donor and acceptor, as well as the positive value of the Laplacian at this BCP (Δρ > 0), were considered as criteria for the H-bond formation.
The energies of the intermolecular H-bonds under the investigation of the scans of their energies were evaluated by the empirical Espinosa-Molins-Lecomte (EML) formula (Espinosa, Molins, & Lecomte, 1998;Mata, Alkorta, Espinosa, & Molins, 2011), which was first successfully applied for the estimation of the individual energetic contributions of the separate H-bonds in the two Watson-Crick DNA base pairs (Matta, Castillo, & Boyd, 2006): where V(r)the value of a local potential energy density at the (3,−1) BCPs. The energies of the O6H···O4 and N2H···O2 H-bonds in the TS G*·T↔G·T* were estimated by the Nikolaienko-Bulavin-Hovorun formulas (Nikolaienko, Bulavin, & Hovorun, 2012): where ρthe electron density at the (3,−1) BCP of the H-bond. The energies of the conventional H-bonds were evaluated by the empirical Iogansen's formula (Iogansen, 1999): where Δνthe magnitude of the redshift (relative to the free molecule) of the stretching mode of the H-bonded groups involved in the H-bonding. The partial deuteration was applied to minimize the effect of vibrational resonances . The period of the intermolecular vibrations T was calculated as: where νthe frequency of vibrations, cthe speed of the light in vacuum. The atomic numbering scheme for the purine and pyrimidine bases is conventional (Saenger, 1984).
The current study found that the electron-topological (the electron density ρ is in the acceptable range of values; the Laplacian of the electron density Δρ at the (3,−1) BCP of the bond is positive), geometric (the d H···N1/N3/N2 and d H···O6/O4/O2 distances are less than the sum of corresponding Bondi's (Bondi, 1964) van der Waals radii; the AH H-bond donating groups elongate upon the formation of the conventional AH···B H-bonds; the angles of the H-bonding are obtuse), and spectroscopic (the frequency of the stretching vibrational mode ν(AH) of the AH donor group is shifted toward the lower frequencies or toward the red end of the spectrum under the formation of all H-bonds) characteristics confirm the existence of the canonical H-bonds in the investigated complexes , 2013e, 2014bBrovarets', Zhurakivsky, & Hovorun, 2013a;Ponomareva, Yurenko, Zhurakivsky, van Mourik, & Hovorun, 2012;Yurenko, Zhurakivsky, Samijlenko, & Hovorun, 2011) (for more details refer to Table 1).
j The dipole moment of the complex (D). Table 2. Electron-topological, structural, and polar characteristics of the intermolecular H-bonds in the nine key points (KPs) revealed along the IRC of the G*•T↔G·T* tautomerization via the DPT obtained at the B3LYP/6-311++G(d,p) (DFT) and MP2/6-311++G(d,p) (MP2) levels of theory in vacuo (ε = 1) and in the continuum with a low dielectric constant (ε = 4). Note: For footnote definitions see Table 1 , 15 and S14). The analysis of the geometries of the G*·T and G·T* base mispairs evidences that the invariant recognition sites in the minor groove of DNA (Poltev, Shulyupina, & Bruskov, 1998;Poltev & Bruskov, 1977) represented by the O2 oxygen atom of the T base and the N3 nitrogen atom of the G base occupy spatial positions, that are very similar to those in the canonical A·T and G·C Watson-Crick DNA base pairs, thus ensuring their recognition as correct base pairs by the base-pair recognition pocket of the DNA polymerase through H-bonding interactions at the incorporation of the nucleotides (Beard & Wilson, 1998Li & Waksman, 2001).
Another geometric requirement for the successful incorporation of the incorrect base pair into the DNA double helix is the mimicry of its glycosidic sizes (Goodman, 1997). So, the comparison of the glycosidic parameters of the G*·T and G·T* DNA base mispairs, namely the distance between the H9 and H1 glycosidic protons R(H 9 -H 1 ) and the ∠N9HH and ∠N1HH glycosidic angles, with those for the canonical Watson-Crick base pairs indicates that they are close to the G·C DNA base pair (Table 4), in such a way adopting a structure that closely mimics that of a natural base pairs and utilizing an appropriate H-bonding potential. This data suggest that the G*·T and G·T* DNA base mispairs from the geometrical point of view would be successfully incorporated into the DNA double helix by the DNA polymerase during DNA biosynthesis, that agree well with the previously expressed assumptions (Danilov et al., 2005;Topal & Fresco, 1976).
Thermodynamic and dynamic stability of the G*·T and G·T* DNA base mispairs From the data presented in Table 3 we can come to the conclusion that the G*·T and G·T* DNA base mispairs are thermodynamically  stable structures, since the values of their Gibbs free energies of the interaction between the bases within the base pair are less than zero. The intermolecular H-bonds in the G*·T and G·T* DNA base mispairs make a significant contribution into the electronic energy of the interaction between the bases, that corroborates well with the data obtained in previous observations   (Table 3).
It should be noted that the electronic and Gibbs free energies of the interaction between the G and T* bases in the G·T* base mispair exceed the same values for the canonical G·C Watson-Crick base pair (Brovarets' & Hovorun, 2013e) that can be considered as a unique case among those that were previously considered . Based on the obtained results the following order of stability can be derived both in vacuum and in the continuum with ε = 4: A·T < G*·T < G·C < G·T* (Table 3). In view of this, the G·T* DNA base mispair is not suitable candidate to cause spontaneous point mutations in DNA, since it acts as an impediment to work of the DNA polymerase, designed for a range of values of the canonical A·T and G·C DNA base pairs.
It is interestingly to note, that the G·T* base mispair is dynamically unstable structure in vacuum (the zero-point energy E ZPE = 1511.8 cm −1 or 4.32 kcal mol −1 of the corresponding vibrational mode, which frequency becomes imaginary at the TS G*·T↔G·T* , exceeds the value of the reverse barrier ΔΔE = 1334.7 cm −1 or 3.82 kcal mol −1 ), but it aquires dynamical stability in the continuum with ε = 4 (the zero-point energy E ZPE = 1485.9 cm −1 or 4.25 kcal mol −1 of the corresponding vibrational mode is less than the value of the reverse barrier ΔΔE = 1744.5 cm −1 or 4.99 kcal mol −1 ) (Table 5).
However, the lifetime of the formed G·T* base mispair involving T* mutagenic tautomer is by three orders less than the time required for the replication machinery to forcibly dissociate a base pair into the monomers during DNA replication (several ns (Brovarets', Kolomiets', et al., 2012)) both in vacuum (τ = 2.41·10 −12 s) and in the continuum with ε = 4 (τ = 6.39·10 −12 s) ( Table 5). These results highlight that the short-lived G·T* base mispair plays the role of the supplier of the G*·T base mispair, that is the G·T mismatch with Watson-Crick architecture decays into the G* and T monomers at the dissociation by the DNA polymerase.
This, in addition to all of the above, clearly shows that exactly the G*·T base mispair, associated by three cooperative intermolecular O6H···O4 (9.50), N3H···N1 (7.11), and N2H···O2 (4.01) H-bonds (the energy of the each H-bond obtained in the weakly polar surrounding with ε = 4 is presented in kcal·mol −1 in brackets after each of them), is experimentally registered in the base pair recognition pocket of the DNA polymerase (Bebenek et al., 2011) (see detailed discussion below).
The time necessary to reach 99.9% of the equilibrium concentration between the G*·T and G·T* DNA base mispairs (τ 99.9% = 4.95·10 −11 s (Table 5)) was established to be less by eight orders than the time given by the replication machinery to incorporate one nucleotide into the synthesized DNA double helix (10 −3 s (Brovarets', Kolomiets', et al., 2012;Kornberg & Baker, 1992)).
Taking into account the above arguments, we summarized that the extremely stable G·T* DNA base mispair "slips away from the hands" of the replication machinery into the G*·T DNA base mispair with the lower energy, that further dissociates into the G* and T monomers. Table 3. Interbase interaction energies (in kcal mol −1 ) for the G*·T/G·T* base mispairs and A·T/G·C Watson-Crick DNA base pairs obtained at the B3LYP/6-311++G(d,p) (DFT) and MP2/6-311++G(d,p) (MP2) levels of theory in vacuo (ε = 1) and in the continuum with a low dielectric constant (ε = 4).  Notes: R(H-H)distance between the H1 and H9 glycosidic protons, Å; α 1 (∠N9HH) and α 2 (∠N1HH)glycosidic angles, degree. b Data are taken from Hovorun (2013e, 2014b).

Foreseen reaction pathway of the G*·T↔G·T* tautomerization via the DPT
The spacings of the χ-like crossings on the calculated profiles of the d O6H/HO4 , d N1H/HN3 , ρ O6H/HO4 , ρ N1H/HN3 , Δρ O6H/HO4 and Δρ N1H/HN3 values of the intermolecular Hbonds along the IRC of the tautomerization (Figures 5, 6, 11, S4, S5, and S10) evidence that this tautomerization reaction occurs through the asynchronous concerted mechanism. Furthermore, we have established the nine key points ( (Politzer, Murray, & Jaque, 2013), and the KP 8, coinciding with the second extremum (minimum) of the dE/dIRC or the maximum of the reaction force (Politzer et al., 2013) (Figures 2, 3, S1 and S2). It should be noted that all intermolecular H-bonds revealed in the nine key points, the detailed physico-chemical characteristics of which are presented in Table 2, meet all requirements for the canonical H-bonding (see Ponomareva et al., 2012;Yurenko et al., 2011 and bibliography therein). Obtained data suggest that the energy equal to 4.84/ 5.66/4.81/4.73 kcal·mol −1 , representing 79.3/77.3/7.0/ 37.9% of the TS G*·T↔G·T* electronic energy relatively to the starting G*·T DNA base mispair, is spent at the reactant region on the mutual adjustment of the G* and T DNA bases within the G*·T base mispair getting the donor and acceptor atoms closer to each other in order to initiate the DPT reaction at the TS region, whereas the energy equal to 3.43/4.13/5.53/5.93 kcal·mol −1 , representing 56.2/56.3/80.5/47.5% of the TS G*·T↔G·T* electronic energy relatively to the starting G*·T DNA base mispair, is spend at the product region on the structural relaxation of the KP 8 to the final G·T* DNA base mispair. These data suggest that almost equal amount of the electronic energy is spent at the reaction region just before the beginning of the chemical reaction and at the product region immediately after the ending of the chemical reaction.
The obtained reaction path enable us to track in detail the atomistic mechanism of the G*·T↔G·T* DPT tautomerization. This reaction starts from the transfer of the proton localized at the N3 nitrogen atom of the T base in the G*·T DNA base mispair (the KP 1) along the N3H···N1 H-bond in the direction of the N1 nitrogen atom of the G* enol tautomer, reaching the KP 2 (Δρ N1···H = 0), at which the H-N3 covalent bond of the T base is significantly weakened and the N1···H H-bond becomes the N1-H covalent bond. Remarkably, exactly at this point the chemical identity of the G* and T bases is lost and in fact the chemical reaction is initiated. Further, bypassing the KP 3 (ρ N1-H = ρ H-N3 ), characterized by the equivalent N1-H/H-N3 covalent bonds involved in the N1-H-N3 covalent bridge, the complex reaches the TS G*·T↔G·T* (the KP 4). After this the base mispair transforms into the KP 5 (Δρ H···N3 = 0), at which the H-N3 covalent bond becomes the H···N3 H-bond thereby arriving at the G + ·T − zwitterionic non-stable intermediate. Then after the first proton is already transferred, the second mobile proton, localized at the O6 + oxygen atom of the G + protonated base, begins to move towards the O4 − oxygen atom of the T − deprotonated base that is accompanied by the breaking of the old O6-H covalent bond at the KP 6 (Δρ H···O4 = 0), by the formation of the equivalent O6-H/H-O4 covalent bonds involved in the O6-H-O4 covalent bridge at the KP 7 (ρ O6-H = ρ H-O4 ) and eventually by the creation of the new H-O4 covalent bond at the KP 8 (Δρ O6···H = 0). In this way ultimately the final G·T* DNA base mispair is achieved (the KP 9). For more details, see Figure 1 and Table 2.
Evolution of the major physicochemical properties of the G*·T and G·T* DNA base mispairs along the IRC of the G*·T↔G·T* DPT tautomerization It should be mentioned that we did not reveal principal differences at the analysis of the graphs of the electrontopological characteristics obtained at the DFT and MP2 levels of QM theory (Figures 2-15 and S1-S14). The calculated electronic energy profile of the DPT tautomerization is asymmetric with shoulder (Figures 2  and S1). It is noteworthy that sharp peak on this profile that corresponds to the TS G*·T↔G·T* is observed at the MP2 level of theory in the continuum with ε = 4 ( Figure 2(b)).
The graph of the changes of the dipole moment μ has an U-like shape with plateau changing at this within the range of values 3.29-5.77/4.35-7.20/2.99-4.98/3.94-6.66 D along the IRC of the G*·T↔G·T* tautomerization (Figure 4 and S3).
The results of the present study show the "breathing" of the G*·T DNA base mispair in the course of the tautomerization, in particular, significant compression is observed at the TS region, as is evidenced by the changes in the distance between the O6 and O4 oxygen atoms ( (Figures 10 and S9). Moreover, the changes of the distance R(H N9 -H N1 ) between the H1 and H9 glycosidic hydrogens (10. 026-10.293/10.049-10.335/9.960-10.218/ 9.987-10.278 Å) were also found to cause this effect (Figures 13 and S12). The variations of the ∠O6H···O4 (a) (b) Figure 8. Profiles of the energy of the H-bonds E HB , estimated by the EML formula (Espinosa et al., 1998;Mata et al., 2011), at the (3,−1) BCPs of the AH···B H-bonds along the IRC of the G*·T↔G·T* tautomerization via the DPT obtained at the MP2 level of theory (a) in vacuo and (b) in the continuum with a low dielectric constant (ε = 4).
What tautomeric form possesses the G·T mismatch with Watson-Crick geometry in the recognition pocket of the high-fidelity DNA polymerase?
In order to answer on this biologically important question, we have compared the experimental structural data obtained by X-ray analysis (Bebenek et al., 2011) with similar theoretical data presented in this paper for the G*·T and G·T* base mispairs in the continuum with ε = 4 at the MP2/6-311++G(d,p) level of theory (Table 6).
Satisfactory coincidence of the experimental data with theoretical one leaves no doubt that the enzymatically competent G·T DNA mismatch exactly possesses electroneutral, energetically favorable G*·T tautomeric form and is stabilized by three intermolecular O6H···O4, N1···HN3, and N2H···O2 H-bonds. The corresponding anionic (deprotonated) G − ·T or G·T − forms are excluded: we have previously shown (Brovarets' et al., 2010) that they have significantly non-planar structure, contradicting to the experimental data (Bebenek et al., 2011), which clearly demonstrate the near-planar structure of the G·T base mispair with Watson-Crick geometry.
Unfortunately, the numerical structural characteristics of the enzymatically competent A·C base mispair with Watson-Crick geometry, that was experimentally registered in the recognition pocket of the DNA polymerase, have not been provided in the work (Wang et al., 2011). However, based on the previously obtained theoretical results , it can be argued by analogy with the G·T base pair, that exactly the A·C* tautomer with Watson-Crick geometry presents an enzymatically competent A·C base mispair, which is experimentally recorded.

Concluding remarks
In this paper we have discovered the physico-chemical picture of the G*·T↔G·T* tautomerization via the DPT in vacuum and in the continuum with a low dielectric constant (ε = 4), typical for the hydrophobic interfaces of specific protein-nucleic acid interactions, within the framework of the QM/QTAIM approach. The calculations revealed that the G*·T DNA base mispair joined by the cooperative O6H···O4, N1···HN3, and N2H···O2 H-bonds tautomerizes into the G·T* base mispair (ΔG = .99 and ΔE = 1.31 kcal mol −1 in the continuum with ε = 4) stabilized by the cooperative O6···HO4, N1H···N3, and N2H···O2 H-bonds through the asynchronous concerted DPT without any intermediates along two antiparallel O6H···O4 and N1···HN3 H-bonds and is accompanied by the electronic and structural reconstruction maintaining at this its Watson-Crick-like geometry.     As a result, for the first time we have obtained the scans of the energetic, electron-topological, geometrical, polar, and NBO parameters along the IRC. The current findings add substantially to our understanding of the atomistic details of the G*·T↔G·T* DPT tautomerization enabling us to distinguish the nine key points that illustrate in detail the progress of the tautomerization.
By providing a comprehensive investigation and analysis of the barrier heights and kinetic parameters of this tautomerization, it was found that the thermodynamically stable G·T* DNA base mispair is dynamically unstable structure in vacuum (τ = 2.41·10 −12 s) and dynamically stable structure in the continuum with ε = 4 (τ = 6.39·10 −12 s), which lifetime enables all six low-frequency intermolecular vibrations to develop; however, it is by three orders less than the period of time necessary for the replication machinery to forcibly dissociate a base pair into the monomers during DNA replication (several ns).
Returning to the question posed in the title of this study, it is now possible to state that the G·T* base mispair is too short-lived structure to cause transition mutations in the course of the DNA replication. Taken together, these results suggest that the replication machinery as a part of the replisome deals only with the G*·T base mispair, effectively dissociating it on the G* and T monomers (at the next act of replication the G* enol mutagenic tautomer complementary binds with the T keto canonical tautomer, thereby inducing the appropriate point mutations), in such a way substantially altering the Löwdin's scheme (Löwdin, 1963(Löwdin, , 1966 of the fixation of the replication point errors in DNA, that suggests that both the G*·T and G·T* DNA mismatches are equivalent participants in the mutation process. It is logically to suggest that the enzymatically competent G*·T DNA base mispair can be formed in two ways in the base pair recognition pocket of the highfidelity DNA polymerase during the DNA biosynthesis: as a result of the complementary interaction between the G* and T bases (G* + T → G*·T) or through the transformation of the formed G·T* DNA base mispair (G + T* → G·T* → G*·T). These data mean that the short-lived G·T* DNA base mispair serves as a mediated provider of the G*·T transition mismatch during DNA replication and therefore it is important from the biological point of view.

Supplementary material
The supplementary material for this paper is available online at http://dx.doi.10.1080/07391102.2014.924879.

Supporting information
Sweeps of the electronic energy, the first derivative of the electronic energy with respect to the IRC, the dipole moment of the base pair, the distances and the angle of the intermolecular H-bonds, the electron density, the Laplacian of the electron density, ellipticity and the energy at the (3,−1) BCPs of the intrapair H-bonds, the NBO charges of the hydrogen atoms involved in the intermolecular H-bonds, the glycosidic angles, the distance between the glycosidic hydrogens, and the dihedral angle along the IRC of the G*·T↔G·T* tautomerization via the DPT obtained at the B3LYP/6-311++G(d,p) level of theory in vacuum and in the continuum with ε = 4. This material is available free of charge via the Internet at http://www.tandfonline.com/toc/tbsd. Table 6. Comparison of the geometrical parameters of the intermolecular H-bonds in the G·T transition mismatch with Watson-Crick geometry, obtained experimentally by X-ray analysis and theoretically by ab initio method (MP2/6-311++G (d,p) level of theory in the continuum with a low dielectric constant (ε = 4)).