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)

Abstract

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.Electronic supplementary materialThe online version of this article (doi:10.1007/s00894-013-1926-5) contains supplementary material, which is available to authorized users.

Geometric consequences of electron delocalization for adenine tautomers in aqueous solution

“…The HOMED10 values for the purine isomers were taken from ref. [25]. A similar tendency was found for correlations between the relative energies (Δ E ) in the gas phase and in water solution for the neutral adenine and purine tautomers (Fig.…”

DFT studies on the favored and rare tautomers of neutral and redox cytosine

“…This relation has been recently discussed for some simple tautomeric systems [21]. Good linear relationships have been found for the neutral aromatic NH and nonaromatic CH tautomers of imidazole and purine, which have no substituent, and which possess solely the endo functional groups [52,53,[80][81][82][83]. Prototropy is also well related to electron delocalization for the neutral NH and CH tautomers of aminoazines and for the NH-NH and NH-CH tautomers of adenine [52,53,81].…”

Geometric and energetic consequences of prototropy for adenine and its structural models – a review

“…23,137,138 Fig. The HOMED index is close to zero for the hypothetical Kekulé structure (e.g., cyclohexatriene and its 1,3,5-triaza derivative) with localized single and double bonds, which lengths are equal to those for the reference molecules (ethane and ethene for cyclohexatriene, and methylamine and methylimine for its triaza derivative).…”

“…15 However, not all of the aromatic descriptors can be used to polycyclic systems. 8,9,[18][19][20]22,23 Amongst the structural indices, proposed in the literature, 13 the recently extended geometry-based HOMED (Harmonic Oscillator Model of Electron Delocalization) index 137,138 was employed to geometries of individual tautomers optimized by quantum-chemical methods. 14 They describe aromatic character of entire cyclic molecules.…”

“…The reasons are as follows. 23,138 In the HOMHED procedure, the statistical reference bond lengths were applied. 23,138 In the HOMHED procedure, the statistical reference bond lengths were applied.…”