Tautomeric Forms of 2-Thiobarbituric Acid As Studied in the Solid, in Polar Solutions, and on Gold Nanoparticles

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, UV−vis, 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 (ΔG o 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 ΔG o 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 N1⇋ 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.

Photoinduced Electron Transfer Reaction of 2-Thiobarbituric Acid and Methylene Blue: Mechanism and Kinetics

“…In agreement with literature 10 , the UV spectrum of the 2TBA (5х10 -5 M) recorded by us in aqueous solution exhibits two well-defined maxima one at 285 nm (ε = 24,000 L mol -1 cm -1 ) and a second at 235 nm (ε = 9,000 L mol -1 cm -1 ) at pH 2. The cationic and neutral species of 2TBA are in equilibrium between the pH value 2 to 6.…”

Calcium and strontium thiobarbiturates with discrete and polymeric structures

“…The IR spectra of 1-3 show distinct vibrational bands between 1539-1540 and 1362-1392 cm −1 , which are assigned to the (CN) vibrations (thioamide I and II bands), and the bands between 1173-1182 and 775-811 cm −1 can be attributed to the (CS) vibrations (thioamide III and IV bands). The corresponding thioamide I and II bands in crystalline H 2 TBA appear at 1527 and 1350 cm −1 , while the thioamide bands III and IV are observed at 1154 and 801 cm −1 [11]. The shift of the thioamide III band in 3 to lower frequencies from that in H 2 TBA supports the S-coordination of the ligand.…”

“…It has many possible tautomeric structures [10,11] and very desirable properties for forming supramolecular structures [3]. For oxophilic s-block metals, the HTBA − ligand containing O and O′ donors could be a useful linker for the preparation of coordination polymers, since the metals can be linked into an extended chain with bridging HTBA − .…”

Synthesis, structural characterization and biological studies of the triphenyltin(IV) complex with 2-thiobarbituric acid

“…General aspects 2-Thiobarbituric acid (H 2 TBA) is a hydroxyl substituted thiouracil with 3 out of 4 mobile H atoms and many possible neutral tautomeric forms [21]. Thus, a variety of possible coordination modes lead to possible structural diversity.…”