Structure, stability, energy barrier and ionization energies of chemically modified DNA-bases: Quantum chemical calculations on 37 favored and rare tautomeric forms of tetraphosphoadenine

Density Functional Theory (BMK and B3LYP) and ab initio complete basis set (CBS-QB3) methods are used to study the stability order and tautomerization processes of all possible 37 tetraphosphoadenine tautomers in the gas phase for the first time. Adiabatic ionization energy (IE) and electron affinity (EA) of all tetraphosphoadenine tautomers were computed at B3LYP/6–311 + G(d,p) level of theory at 0 K. Positive calculated EA values suggest that the tetraphosphoadenine molecule is an attractive candidate for electron capture. It has been found that all tautomers of tetraphosphoadenine exists within the energy range of 0–150 kJ mol−1. Similar to adenine, canonical tautomers PA7 and PA9 of tetraphosphoadenine are shown to be the most stable tautomeric forms as predicted by all of the above computational methods. The transition states for the tetraphosphadenine tautomerization processes were investigated for selected systems at B3LYP/6–311 + G(d,p) level of theory in the gas phase. Energy barrier for the proton transfer processes of tetraphosphadenine, is up to 97 kJ mol−1. Dipole moment, full atomic natural charges and bond orders have been computed and discussed.