Prototropy and π‐electron delocalization for purine and its radical ions – DFT studies

Abstract

Complete tautomeric equilibria and π‐electron delocalization were studied at the B3LYP/6‐311+G** level for neutral purine (P) and its charged radicals (P+• and P−•). All possible nine tautomers (four NH and five CH forms) and all possible 36 tautomeric equilibria (six NiH → NkH, twenty NH → CH, and ten CiH → CkH conversions) were considered. The greatest variations of the tautomeric equilibrium constants (as pKT) were observed for the NH → CH conversions when proceeding from neutral to reduced purine (P + e → P−•). These variations completely change the tautomeric preferences. One‐electron oxidation (P − e → P+•) has considerably smaller effect on the pKT values and does not change the tautomeric preferences. π‐Electron delocalization depends on the position of the moving proton and on the type of the electron transfer. For individual tautomers, some linear relations between the relative stabilities and the HOMA (harmonic oscillator model of aromaticity) indices occur for neutral and oxidized purine. For reduced purine, a scatter plot is found. Copyright © 2010 John Wiley & Sons, Ltd.

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)

“…Although the purine CH tautomers are π-π and σ-π conjugated, their rHOMA indices are negative or close to zero. However, when cyclic compounds of similar electron delocalization (cyclohexane, cyclohexene, benzene, piperidine, 2,3,4,5-tetrahydropyrimidine and 1,3,5-triazine) were applied as the reference molecules for the CC and CN bonds, the HOMA indices for the neutral NH and CH isomers of purine are between 0.3 and 1.0 [33]. …”

“…Radchenko and co-workers [21] concluded on the basis of the IR spectra of isolated purine molecules in argon matrix that the N7H and N9H tautomers coexist in approximately equal proportions, whereas Kwiatkowski and co-workers [22–26] and also Adamowicz and co-workers [27] using the combined matrix-isolation IR spectra and ab initio calculations showed that the N9H tautomer dominates in such conditions. Due to an intramolecular interaction between the N9H group and the lone electron pair of the N3 atom, the N9H tautomer is favored for neutral purine in the gas phase and in non-polar solvents [23–33]. Most studies on the structure of purine in the gas phase are theoretical.…”

“…They are also favored for reduced aromatic imidazole and aniline as well as for reduced aliphatic vinylamine-acetaldimine [39, 40]. For these reasons, the complete tautomeric mixture (including CH tautomers) has been considered for investigations of tautomeric equilibria for purine in the gas phase [32, 33]. …”

“…For calculations, the 6-311+G(d,p) basis set [46] with the diffuse and polarization functions was employed similarly as previously described for the gas phase purine structures [32, 33]. The B3LYP functional and the basis set with the diffuse and polarization functions have been recommended in the literature for charged radicals [47–49].…”

“…Our computations give the possibilities to study the variations of geometric and energetic parameters, spin densities, charges, geometry-based indices of aromaticity, ionization potentials and electron affinities for all individual purine tautomers when proceeding from the gas phase to water. Since the geometry-based harmonic oscillator model of aromaticity (HOMA) index reformulated by Krygowski in 1993 [52] is not appropriate for heteroatomic compounds [6, 33, 39, 40], the new parametrized harmonic oscillator model of electron delocalization (HOMED) index [53, 54] was applied to determine the distribution of π electrons for all neutral and charged NH and CH purine tautomers. The discrepancies of the reformulated HOMA index are also discussed.…”