Tautomeric Forms of 2-Thiobarbituric Acid As Studied in the Solid, in Polar Solutions, and on Gold Nanoparticles
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- 作者：Eduardo Mé ; ndez ; Marí ; a F. Cerdá ; Jorge S. Gancheff ; Julia Torres ; Carlos Kremer ; Jorge Castiglioni ; Martina Kieninger ; Oscar N. Ventura
- 刊名：Journal of Physical Chemistry C
- 出版年：2007
- 出版时间：March 1, 2007
- 年：2007
- 卷：111
- 期：8
- 页码：3369 - 3383
- 全文大小：344K
- 年卷期：v.111,no.8(March 1, 2007)
- ISSN：1932-7455
文摘
2-Thiobarbituric acid (TBA) coated gold nanoparticles (average diameter = 5.90 nm) were produced andstudied by several experimental and theoretical methods. As part of this study, the molecular structure ofTBA tautomers in the solid, in polar solutions, and adsorbed onto gold nanoparticles was studied. The resolutionof this complicated system (10 possible isomers) was accomplished with the aid of experimental (IR, UV-vis, and NMR) and theoretical (DFT and MP2) methods. The general conclusion is that there are two preeminentisomers, N1 and N10, with different stabilities in different media. N1, the keto-thione tautomer, is the moststable in gas phase (
G<SUP>o298 
8-9 kcal/mol lower than the second-most stable isomer, depending on themethod of calculation used). However, experimental spectroscopic data supported by the theoretical calculationsstrongly suggest an equilibrium between the tautomers N1 and N10 in methanol solution, where enolizationof one keto group is produced by proton transfer from the methylene group, which is more acidic than theNH groups. With the use of the polarizable continuum method for simulating solvents, N10 is predicted tobe even more stable than N1 by Go298 1 kcal/mol in methanol. On the other hand, the IR spectrum of thesolid can be best explained by assuming that only N10 is present, a fact also supported by the observationthat 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 N1
N10 can be explained by interventionof the solvent, proceeding faster in methanol solutions than in DMSO, where it is nevertheless observed aftera time, according to the 13C NMR spectra. Our experiments support absorption of TBA onto gold nanoparticlesthrough S-Au and N-Au interactions, with the preeminence of a N10-like enol structure. The experimentsalso demonstrate that the synthesized TBA-coated gold nanoparticles can autoassociate by hydrogen bondingto form larger structures. This same H-bonding capacity also assures that these coated nanoparticles act asthistles toward proteins in solution, binding them strongly, presumably not by chemical reaction but by anetwork of hydrogen bonds.
G<SUP>o298 8-9 kcal/mol lower than the second-most stable isomer, depending on themethod of calculation used). However, experimental spectroscopic data supported by the theoretical calculationsstrongly suggest an equilibrium between the tautomers N1 and N10 in methanol solution, where enolizationof one keto group is produced by proton transfer from the methylene group, which is more acidic than theNH groups. With the use of the polarizable continuum method for simulating solvents, N10 is predicted tobe even more stable than N1 by Go298 1 kcal/mol in methanol. On the other hand, the IR spectrum of thesolid can be best explained by assuming that only N10 is present, a fact also supported by the observationthat 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 N1 N10 can be explained by interventionof the solvent, proceeding faster in methanol solutions than in DMSO, where it is nevertheless observed aftera time, according to the 13C NMR spectra. Our experiments support absorption of TBA onto gold nanoparticlesthrough S-Au and N-Au interactions, with the preeminence of a N10-like enol structure. The experimentsalso demonstrate that the synthesized TBA-coated gold nanoparticles can autoassociate by hydrogen bondingto form larger structures. This same H-bonding capacity also assures that these coated nanoparticles act asthistles toward proteins in solution, binding them strongly, presumably not by chemical reaction but by anetwork of hydrogen bonds.