Variations of the tautomeric preferences and π-electron delocalization for the neutral and redox forms of purine when proceeding from the gas phase (DFT) to water (PCM)

Quantum-chemical calculations were performed for all possible nine neutral tautomers of purine and their oxidized and reduced forms in water {PCM//DFT(B3LYP)/6-311+G(d,p)} and compared to those in the gas phase {DFT(B3LYP)/6-311+G(d,p)}. PCM hydration influences geometries, π-electron delocalization, and relative energies of purine tautomers in different ways. Generally, the harmonic oscillator model of electron delocalization (HOMED) indices increase when proceeding from the gas phase to aequeous solution for the neutral and redox forms of purine. Their changes for the neutral and oxidized tautomers are almost parallel to the relative energies showing that aromaticity plays an important role in the tautomeric preferences. Tautomeric stabilities and tautomeric preferences vary when proceeding from the gas phase to water indicating additionally that intra- and intermolecular interactions affect tautomeric equilibria. The tautomeric mixture of neutral purine in the gas phase consists mainly of the N9H tautomer, whereas two tautomers (N9H and N7H) dominate in water. For oxidized purine, N9H is favored in the gas phase, whereas N1H in water. A gain of one electron dramatically changes the relative stabilities of the CH and NH tautomers that C6H and C8H dominate in the tautomeric mixture in the gas phase, whereas N3H in water. These variations show exceptional sensitivity of the tautomeric purine system on environment in the electron-transfer reactions.