The influence of metal alkali binding on the properties of hydrogen bonds in AsynGanti mispair
The O6 of guanine and N7 atom of adenine in the AsynGanti mispair directly contributes in the formation of hydrogen bonds. Thus, N3 and N7 of guanine and N1 and N3 adenine to be principal acceptors of cations. The G(3), G(7), A(1) and A(3) symbols are used to display these sites, respectively. The schematic representation of optimized structures are shown in Figure 5. In agreement with previous investigations [49,34], a simultaneous coordination of the cation to both the N7 and O6 sites of guanine is observed.
The values of ∆E, ∆Eint and ∆Edef are reported in Table 5. The results indicate the cations binding to A(1), A(3) and G(3)-positions amplified the absolute values of binding energy. Unlike Li+ cation, the ∆E values diminished in the presence of Na+ and K+ cations in N7 site of guanine. For each ion type, the A(1) tautomer is the most stable among all. Contrary to ΔEdef values, ΔEint makes a positive contribution to the ΔE. The contribution of ΔEint is dominant in all considered systems. The results in Table 5 indicate that a direct correlation exists between ∆Eint and ∆E values with coefficient of determination R2= 0.9967 in AsynGanti systems involving cations. It is observed that the reduction in ion charge density is associated with decreasing the absolute values of ∆Eint and ∆E values.
Also, hydrogen bonding energies are calculated by the EML formula (see Table 5). Unlike G(7) and G(3)-sites, the EEML,HB1 in A(1) and A(3)-sites upon interactions with alkali ions increases. A reverse behavior is observed for EEML,HB2. Contrary to G(7) and G(3)-positions, the EEML,HB1values in A(1) and A(3)-positions are in agreement with the charge/radius (q/rad) ratio of cations. This opposite order is reversed for EEML,HB2values. For each ion, the EEML,HB1 and EEML,HB2 values in AsynGanti mismatch increase respectively as following:
A(1) > A(3) > G(3) > G(7)
G(7) > G(3) > A(1) > A(3)
As evident from Tables 1 and 5, the AantiGanti configuration is higher in the absolute values of binding energy than the AsynGanti configuration, indicating that AantiGanti configuration is more stable than AsynGanti one. Also, the absolute values of hydrogen bonding intermolecular energies (EEML,HB ) in AantiGantimismatch are higher than AsynGanti ones.
The most geometrical parameters of the considered systems are given in Table 6. The results indicat that the length of HB1 and HB2 in AsynGanti are greater than corresponding in AantiGanti mispair. One can see that dHB1 in A(1) and A(3)-sites decreases, whereas dHB2 increases upon interactions with alkali ions. An opposite behavior is observed in G(7) and G(3)-sites. The highest contraction in the HB1 and HB2 bond lengths correspond to Li+ cation in A(1) and G(7)- positions, respectively. Also, the highest expansion in the HB1 and HB2 bond lengths correspond to Li+ cation in G(7) and A(3)- positions, respectively. The results indicate that unlike A(1) and A(3)- positions, increase in ion charge density in G(7) and G(3)-positions is accompanied by increasing in HB1 bond lengths. A reverse order are found for HB2 bond lengths. The results indicate that the increasing in dHB is accompanied by decreasing of corresponding EEML,HB values and vice versa.
Unlike A(1) and A(3)-sites, the interaction of cations with G(7) and G(3)-sites leads to increases q1and q2 values corresponding HB1. A reverse order are found for q1and q2 values corresponding HB2. Contrary to A(1) and A(3)- positions, the q1and q2 values corresponding HB1 in G(7) and G(3)-positions are in agreement with the charge/radius (q/rad) ratio of cations. This behavior is reversed for HB2. There are good relationships between |q1|, q2 and corresponding bond length values. The maximum value of dHB is accompanied by the highest |q1 | and q2 values for any type of hydrogen bond and vice versa. Figure 6 shows that there are second order polynomial relationships between the absolute values of q1 and q2 and corresponding EEML for each hydrogen bond in considered systems.
The results of NBO analysis are gathered in Table 7. The results indicate that the cation binding to N3 and N1 atoms of isolated adenine increases positive charge of H atom participating in HB1and decreases negative charge of N atom participating in HB2. One can expected that the strength of HB1 increases due to interaction cations with adenine in these positions. An opposite behavior is expected for HB2. The changes in atomic charge on N3 and N7 atoms in isolated guanine in the presence of cations are discussed in previous section. The most important donor–acceptor interactions connected to HB1 and HB2 are LpOσ *N–H and LpNσ *N–H, respectively. The results indicate that the E(2) values of HB1 and HB2 in AantiGanti are greater than corresponding in AsynGanti mispair. Unlike G(3) and G(7)-sites, interaction of cations with A(1) and A(3)-sites increase the E(2) values of HB1. This behavior is reversed for E(2) values of HB2. Contrary to G(7) and G(3)-positions, increase in ion charge density in A(1) and A(3)- positions is accompanied by increasing in E(2) values corresponding HB1. An opposite behavior are found for HB2. The results indicate that second order polynomial relationship exists between the E(2) values and corresponding dHB with coefficient of determination R2=0.9992 and R2=0.9982 respectively for HB1 and HB2 in studied systems. For each hydrogen bonds, there is a good linear relationship between EEML values and corresponding E(2) values of these interaction. These correlations are shown in Figure 7(a).
As given in Table 8, the ρHB1 and ρHB2values obtained using AIM analysis show that cations binding to N3 and N1 atoms of adenine strengthens HB1 and weakens HB2 in AsynGantimispair. Contrary to HB2, the strength of HB1 decreases in the presence of cations in G(7) and G(3)- sites. Unlike G(7) and G(3)- positions, the ρHB1values in A(1) and A(3)-positions are in agreement with the charge/radius (q/rad) ratio of cations. This behavior is reversed for ρHB2 values. The results indicate that the ρHB1 and ρHB2 values in AantiGanti are greater than corresponding in AsynGanti mispair.
The nature of hydrogen bonds in AsynGanti mispair involving cation is dependent on position of ions. In the presence of cations in N1 and N3 atoms of adenine, HB1 has medium strength while HB2 of weak strength is observed. Also, AsynGanti mispair involving cations in G(7) and G(3)- positions are characterized by the positive values of ∇2ρ(r) and H(r) in the BCP of the HB1 showing that this interaction may be classified as weak bonds. Although ∇2ρ(r) at the BCP of the HB2 in systems involving cations in N3 and N7 atoms of guanine is positive, H(r) is negative, indicating that HB2 has medium strength. There are good linear relationships between E(2) values and corresponding ρ(r) at the BCP of hydrogen bonds of considered systems. The linear coefficients of determination between ρHB1, ρHB2 and E(2) values are equal to 0.9994 and 0.9981, respectively. Also, there are good linear correlations between ρ(r) and corresponding EEML values in studied systems. This correlation is displayed in Figure 7(b).