Systematic theoretical investigations on all of the tautomers of guanine: From both dynamics and thermodynamics viewpoint

The tautomers of the guanine are systematically investigated, using density functional theory (DFT) calculations by means of the B3LYP exchange and correlation functions. The 36 isomers were investigated; the order of the relative stabilities of them was established. One-step transition process by proton transfer process or internal rotation between two related isomers was carefully studied, and 116 transition processes were studied in all. We divided these processes into seven groups according to the position and the reaction type. The energy barrier of each transition was calculated and compared with the corresponding reverse process. Some common rules were found in the transitions belonging to each group, and this indicated that the reaction direction was greatly influenced by the geometric property of the local environmental of the reaction position. Comparisons were made between the activation energies of all the producing processes of each tautomer, the most energetically favored pathway was presented. The results showed that the stable tautomers are both thermodynamically and dynamically favored. Additionally, based on our calculations, we found that in gas phase keto and amino forms were generally more stable than the corresponding enol and imino forms, respectively. Furthermore, a linear relationship between the activation energy and the proton transfer distance was observed in guanine.