- values obtained at the MP2/aug-cc-pVQZ level of theory (S. Wang & Schaefer III, 2006);

TS2 - transition state of the amino group rotation toward the N1 atom for Ade, Gua or the N3 atom for Cyt; TS3 - transition state of the amino group rotation toward the N7 atom for Ade, the N3 atom for Gua or the C5-H group for Cyt

Table 1. Relative values of Gibbs free energy (∆∆G) (T=298.15 K) and electronic energy (∆∆E) (in kcal/mol) for the Ade, Gua, and Cyt transition states of amino group interconversion (plane inversion TS1 and anisotropic rotations TS2, TS3) and corresponding vibrational modes (in cm-1) obtained at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of theory in vacuum

We obtained that the deviations from the main geometric parameters of ≥C6-N6H2 amine fragment of Ade are the following: the length of the C6-H6 bond is increased by 0.072 and 0.074 Å, the lengths of the N6-H are decreased on average by 0.011 Å, and the valence angle H-N6-H is decreased from 120.4° up to 105.8° and 105.9° at the transition states TS2 and TS3, respectively, as compared to those in the nonplanar equilibrium configuration of Ade (Brovarets' and Hovorun, 2010b). In the planar transition state TS1 of the ≥C6-N6H2 fragment inversion the exocyclic С6-N6 bond is shortened by 0.005 Å, the N6-H bonds are elongated by 0.002 Å as compared to those in the nonplanar equilibrium configuration, and the valence angle H-N6-H becomes close to 120° and is equal to 120.9° comparatively with the equilibrium state (118.7°).

#### **3.2 Pyramidalization of the amine fragment of the Gua**

It is commonly thought that exactly due to the presence of the neighbouring N1-H group, the pyramidalization of the amino group in guanine is higher than in canonical cytosine and

 1 The result obtained at the MP2/6-311++G(2df,pd)//B3LYP/cc-pVDZ level of theory.

the activation energy of 0.12 kcal/mol1 (Table 1). MP2 complete basis set limit method with the aug-cc-pVTZ → aug-cc-pVQZ (aTZ → aQZ) extrapolation scheme has predicted very small planarization barrier of the Ade amino group, 0.015 kcal/mol (Zierkiewicz et al., 2008), which is in very good agreement with the MP2-predicted planarization barrier of 0.020 kcal/mol reported by Wang and Schaefer III (S. Wang & Schaefer III, 2006). Similar results were calculated using coupled cluster CCSD(T) complete basis set method – 0.125 kcal/mol (Zierkiewicz et al., 2008). Thus, the literature review highlights that the amino group in

Plane inversion (TS1) Rotation (TS2) Rotation (TS3)

cm-1 kcal/

ν, cm-1

mol cm-1 kcal/

309.4\* 14.34 13.30 4652.7 539.5 14.57 13.51 4727.3 539.5

258.9# 542.6 9.14 9.48 3316.4 327.9 5.40 5.35 1872.1 327.9

10.5# 212.2 11.9 11.74 4105.3 524.6 15.85 16.11 5633.7 524.6

∆∆<sup>G</sup> ∆∆<sup>E</sup>

mol cm-1

ν,

∆∆<sup>G</sup> ∆∆<sup>E</sup>

\* - values obtained at the MP2/6-311++G(2df,pd)//B3LYP/cc-pVDZ level of theory (Brovarets' &

TS2 - transition state of the amino group rotation toward the N1 atom for Ade, Gua or the N3 atom for Cyt; TS3 - transition state of the amino group rotation toward the N7 atom for Ade, the N3 atom for Gua or

Table 1. Relative values of Gibbs free energy (∆∆G) (T=298.15 K) and electronic energy (∆∆E) (in kcal/mol) for the Ade, Gua, and Cyt transition states of amino group interconversion (plane inversion TS1 and anisotropic rotations TS2, TS3) and corresponding vibrational modes (in cm-1) obtained at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of

We obtained that the deviations from the main geometric parameters of ≥C6-N6H2 amine fragment of Ade are the following: the length of the C6-H6 bond is increased by 0.072 and 0.074 Å, the lengths of the N6-H are decreased on average by 0.011 Å, and the valence angle H-N6-H is decreased from 120.4° up to 105.8° and 105.9° at the transition states TS2 and TS3, respectively, as compared to those in the nonplanar equilibrium configuration of Ade (Brovarets' and Hovorun, 2010b). In the planar transition state TS1 of the ≥C6-N6H2 fragment inversion the exocyclic С6-N6 bond is shortened by 0.005 Å, the N6-H bonds are elongated by 0.002 Å as compared to those in the nonplanar equilibrium configuration, and the valence angle H-N6-H becomes close to 120° and is equal to 120.9° comparatively with

It is commonly thought that exactly due to the presence of the neighbouring N1-H group, the pyramidalization of the amino group in guanine is higher than in canonical cytosine and

1 The result obtained at the MP2/6-311++G(2df,pd)//B3LYP/cc-pVDZ level of theory.
