Comparison of the stability of thymine tautomers and the interaction of its tautomers with Na+, K+, Mg2+ and Ca2+ in gas and solvent phases

The present study compared the interactions among Na ⁺, K ⁺, Mg²⁺ and Ca²⁺, thymine and its tautomers in the gas and solvent phase, an interaction dependent upon the electronic construction of the tautomers. Three types of cation interaction with thymine and its tautomers were observed. In the first one, the metal cations interacted with a lone pair of nitrogen or oxygen of the tautomers. In the second type, there was an interaction among the cations, nitrogen and oxygen at the same time; the last one was that of cations with the electron density of thymine π-system, where the cations were perpendicular to the ring of thymine. The interaction of metals cation with tautomers was studied in the gas and solvent phases; a comparison was then made between interactions in two phases. The interaction energy for all complexes indicated the stability of complexes, an energy which was higher in Ca²⁺ and Mg²⁺ compared with Na⁺ and K⁺. Concerning K⁺ and Na⁺, the stability of all complexes of tautomers was greater than that of thymine complexes; however, the stability of certain Ca²⁺ and Mg²⁺ complexes was lower than the complexes of thymine.     

Dynamic 1H NMR investigation along with a theoretical study around the C–C and C = C bonds in a particular phosphorus ylide

In the present work, the dynamic ¹H NMR effects were investigated at variable temperatures within a particular phosphorus ylide involving a 2‐benzoxazolinone around the carbon–carbon single bond and also partial carbon–carbon double bond in two Z‐ and E‐rotational isomers. Activation and kinetic parameters including ΔH^≠, ΔG^≠, ΔS^≠ and E_a were determined in accord with the dynamic ¹H NMR data for three rotational processes. In addition, theoretical studies based upon rotation around the same bonds were investigated using ab initio and DFT methods at the HF/6‐31G(d,p) and B3LYP/6‐31G(d,p) levels of theory. Theoretical activation and kinetic parameters including ΔH^≠, ΔG^≠, ΔS^≠ and E_a were calculated at 298 K and experimental temperatures for five rotational processes. These results (experimental and theoretical), taken together, indicate that the rotational energy barrier around the C = C double bond was considerably high and the observation of the two rotational isomers was impossible (seen as a single isomer) at the high temperatures, in this case rotation around the C = C bond was too fast on the NMR time scale. When the temperature was relatively low, the rate of rotation was sufficiently slow; therefore, observation of the two Z‐ and E‐isomers was possible. In addition, calculations at the HF/6‐31G(d,p) level of theory showed very favorable results in agreement with the experimental data on rotation around the C = C bond. While, B3LYP level using the 6‐31G(d,p) basis set was provided the reasonable data for the restricted rotations around the C–C and C–N single bond. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical kinetics study of thymine tautomerism and interaction of Na+ with its tautomers

The current work is a theoretical study of the tautomerism of thymine in the gas phase. Eighteen structures were found in the isomerisation reaction of thymine, some of which are reported for the first time. Thirty hydrogen transfer reactions were carried out. In 24 of the reactions, the hydrogen abstractions N―H→O, N―H→C and C―H→O were considered. The potential energy surface for all trajectories was determined for 18 tautomers and 40 transition states. The RRKM-TST model was used to calculate the rate constants of the reactions to examine their kinetics. Nonlinear least-squares fitting was used to calculate the rate constants expressions. The interaction of sodium ion and tautomers in the gas phase was also investigated. Three types of interaction of metal cations with thymine were found. In the first, metal cations interact with a lone pair of nitrogen or oxygen tautomers. The second type is the interaction of metal cations with two nitrogen and oxygen of tautomers. The last type is the interaction of metal cations and the electron density of the π-system of thymine in which the metal ion is perpendicular to the ring of tautomers. The stability ranking of the thymine tautomers and their complexes was also determined.     

Taxezopidine G and 10-deacetyltaxezopidine G from Taxus baccata growing in Iran

A new taxoid, 10-deacetyltaxezopidine G, and a previously reported compound (Taxezopidine G) have been isolated from the needles and young stems of Taxus baccata L. growing in Iran, and the structures were determined on the basis of spectroscopic data. These two compounds were not previously encountered in Taxus baccata L. species. Published in Khimiya Prirodnykh Soedinenii, No. 4, pp. 321–322, July–August, 2006.