Tautomeric forms of 2-thiobarbituric acid as studied in the solid : in polar solutions

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Tautomeric forms of 2-thiobarbituric acid as studied in the solid : in polar solutions

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FISICA
RESONANCIA MAGNETICA NUCLEAR
ESTRUCTURA MOLECULAR
ESPECTROSCOPIA INFRAROJO
BIBLIOGRAFIA NACIONAL QUIMICA
2007

Abstract

2-Thiobarbituric acid (TBA) coated gold nanoparticles (average diameter ) 5.90 nm) were produced and studied by several experimental and theoretical methods. As part of this study, the molecular structure of TBA tautomers in the solid, in polar solutions, and adsorbed onto gold nanoparticles was studied. The resolution of this complicated system (10 possible isomers) was accomplished with the aid of experimental (IR, UVvis, and NMR) and theoretical (DFT and MP2) methods. The general conclusion is that there are two preeminent isomers, N1 and N10, with different stabilities in different media. N1, the keto-thione tautomer, is the most stable in gas phase (¢Go 298  8-9 kcal/mol lower than the second-most stable isomer, depending on the method of calculation used). However, experimental spectroscopic data supported by the theoretical calculations strongly suggest an equilibrium between the tautomers N1 and N10 in methanol solution, where enolization of one keto group is produced by proton transfer from the methylene group, which is more acidic than the NH groups. With the use of the polarizable continuum method for simulating solvents, N10 is predicted to be even more stable than N1 by ¢Go 298  1 kcal/mol in methanol. On the other hand, the IR spectrum of the solid can be best explained by assuming that only N10 is present, a fact also supported by the observation that the IR spectrum of TBA absorbed onto gold nanoparticles can be explained by a larger ratio of [N10]/ [N1] than that present in methanolic solution. Isomerization of N1H N10 can be explained by intervention of the solvent, proceeding faster in methanol solutions than in DMSO, where it is nevertheless observed after a time, according to the 13C NMR spectra. Our experiments support absorption of TBA onto gold nanoparticles through S-Au and N-Au interactions, with the preeminence of a N10-like enol structure. The experiments also demonstrate that the synthesized TBA-coated gold nanoparticles can autoassociate by hydrogen bonding to form larger structures. This same H-bonding capacity also assures that these coated nanoparticles act as thistles toward proteins in solution, binding them strongly, presumably not by chemical reaction but by a network of hydrogen bonds.

Autor

Méndez, Eduardo
Cerdá, María Fernanda
Kieninger, Martina

Fuente

Journal of Physical Chemistry C v. 111, no. 8, 2007. -- p. 3369-3383

Editor

American Chemical Society

Fecha

2007

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Identificador

DOI: 10.1021/jp0628176
Fecha de agregación
December 1, 2014
Colección
Bibliografía Nacional Química
Tipo de Elemento
Document
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Citación
Méndez, Eduardo, “Tautomeric forms of 2-thiobarbituric acid as studied in the solid : in polar solutions,” RIQUIM - Repositorio Institucional de la Facultad de Química - UdelaR, accessed May 27, 2022, http://riquim.fq.edu.uy/items/show/2102.
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