Effect of Sr²⁺association on the tautomerization processes of uracil and its dithio- and diseleno-derivatives

Abstract

The structures and relative stabilities of the complexes formed by uracil and its thio- and seleno-derivatives with the Sr(2+) cation, in the gas phase, have been analyzed by means of G96LYP density functional theory (DFT) calculations. The attachment of the Sr(2+) cation to the heteroatom at position 4 is preferred systematically. Although the enolic forms of uracil and its derivatives should not be observed in the gas phase, the corresponding Sr(2+) complexes are the most stable. The enhanced stability of these tautomers is two-fold, on the one hand Sr(2+) interacts with two basic sites simultaneously, and on the other hand an aromatization of the six-membered ring takes place upon Sr(2+) association. Sr(2+) attachment also has a clear catalytic effect in the tautomerization processes involving uracil and its derivatives. This catalytic effect increases when oxygen is replaced by sulfur or selenium. The Sr(2+) binding energy with uracil and its derivatives is bigger than the tautomerization barriers connecting the dioxo forms with the corresponding enolic tautomers. Consequently, when associated with Sr(2+), all tautomers are energetically accessible and should all be observed in the gas phase.

Conformational, electronic and antioxidant properties of lucidone, linderone and methyllinderone: DFT, QTAIM and NBO studies

“…The large portion of the spin density of the complexes can be ascribed to the Fe d atomic orbitals, although some spin density is delocalised to the ligand. The spin density of the [L···Fe] 2 + complexes Downloaded by [The University of Manchester Library] at 09: 57 19 November 2014 is therefore somewhat spread between the ligand and the Fe ion…”

Modelling peptide–metal dication interactions: formamide–Ca2+ reactions in the gas phase

“…This is however in clear contrast with the catalytic effect exhibited by Ca 2+ on the tautomerization processes of uracil and its thio-and seleno-derivatives. [60][61][62] Whereas in formamide, the enhanced acidity of the amino group in complex 1 is counterbalanced by the significant reduction of the basicity of the carbonyl group, whose electron density is strongly polarized towards the doubly-charged metal ion, disfavoring the proton transfer process, the existence of two carbonyl groups susceptible of undergoing enolization in the case of uracil change completely the scenario, because the process which is actually catalyzed corresponds to the proton transfer towards the carbonyl group which is not interacting with the metal dication, [60][61][62] and whose intrinsic basicity is enhanced rather than reduced.…”

[MLn]2+ doubly charged systems: modeling, bonding, life times and unimolecular reactivity

“…A paradigmatic example is provided by the effects of doubly charged metal ions (Ni 2+ , Cu 2+ , Zn 2+ , Ca 2+ or Sr 2+ ) on the relative stabilities of the different tautomeric forms of DNA bases such as thymine, 135 uracil, 136 and its sulfur 136 and selenium 137 derivatives. In the gas phase these bases exist exclusively in their diketo form because the corresponding enolic tautomers lie much higher in energy.…”

“…On the other hand, the strong perturbation of the electron density of the base results in a significant aromatization of the six-membered ring, which contributes to stabilizing the doubly charged molecular ion. [135][136][137] Also interestingly, although to different degrees, all these metal dications significantly catalyze the keto-enol tautomerization processes, through extra stabilization of the corresponding transition states. [135][136][137] Doubly charged metal ions also catalyze peptide bond formation.…”

“…[135][136][137] Also interestingly, although to different degrees, all these metal dications significantly catalyze the keto-enol tautomerization processes, through extra stabilization of the corresponding transition states. [135][136][137] Doubly charged metal ions also catalyze peptide bond formation. In the first experimental work in which a peptide was successfully isolated, Cu(II) was used as a catalyst.…”