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.

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.

No abstract

Heterocycles containing one or more amidine moieties {-NH-C(R) ]N-} such as adenine and its building blocks, imidazole, 4-aminopyrimidine, and purine, are excellent examples of tautomeric systems for which changes of position(s) of labile proton(s) cause parallel changes of geometric and energetic parameters for prototropic tautomers. One-electron oxidation has a slight effect on this relationship. The amino-imine conversions within the amidine group(s) are favored. Well delocalized (aromatic) tautomers, containing labile proton(s) at heteroatom(s), are major, minor, or rare forms. Non-aromatic tautomers with a labile proton at the C atom can be considered as very rare isomers. Dramatic changes take place for reduced heterocycles. Electron delocalization is not the main factor that dictates tautomeric preferences. The HOMED/DE relationship seems to be more complex. The enamino-imine conversions predominate and some very rare forms have the lowest energies. This clearly shows the importance of very rare tautomers, often neglected in proton-transfer, electron-transfer, and ion-radical reactions.

A series of new derivatives of adamantane was synthesised. The new compound 4-(adamant-1-ylethylenoxycarbonyl)phthalanhydride obtained from 1-adamantaneethanol and trimellitic anhydride chloride, as well as 4-(adamant-1-ylmethylenoxycarbonyl)phthalanhydride, appeared very useful for preparation of a number of N-substituted phthalimides. Antimicrobial activity of newly obtained derivatives such as, for example, 4-(adamant-1-ylethylenoxycarbonyl)-N-(L-phenylalanyl)phthalimide or 4-(adamant-1-ylmethylenoxycarbonyl)-N-(L-leucyl)-phthalimide was tested against Staphylococcus aureus, Bacillus sp., Micrococcus flavus and Enterococcus faecium. The minimal inhibitory concentrations for these compounds against Bacillus cereus were 15 and 8 microg/ml, respectively.