No abstract

No 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.

Four novel rhenium complexes of formula [ReCl(4)(bpym)] (1), [ReBr(4)(bpym)] (2) PPh(4)[ReCl(4)(bpym)] (3) and NBu(4)[ReBr(4)(bpym)] (4) (bpym = 2,2'-bipyrimidine, PPh(4) = tetraphenylphosphonium cation and NBu(4) = tetrabutylammonium cation), have been synthesized and their crystal structures determined by single-crystal X-ray diffraction. The structures of 1 and 2 consist of [ReX(4)(bpym)] molecules held together by van der Waals forces. In both complexes the Re(iv) central atom is surrounded by four halide anions and two nitrogen atoms of a bpym bidentate ligand in a distorted octahedral environment. The structures of 3 and 4 consist of [ReX(4)(bpym)](-) anions and PPh(4)(+) () or NBu(4)(+) (4) cations. The coordination sphere of the Re(iii) metal ion is the same as in 1 and 2, respectively. However, whereas the Re-X bonds are longer the Re-N bonds are shorter than in 1 and 2. This fact reveals that the bpym ligand forms a stronger bond with Re(iii) than with Re(iv) resulting in a stabilisation of the lower oxidation state. [ReX(4)(bpym)] complexes are easily reduced, chemically and electrochemically, to the corresponding [ReX(4)(bpym)](-) anions. A voltammetric study shows that the electron transference is a reversible process characterized by formal redox potentials of +0.19 V (1) and +0.32 V (2) vs. NHE, in acetonitrile as solvent.

Students of physical chemistry in biochemical disciplines need biochemical examples to capture the need, not always understood, of a difficult area in their studies. The use of thermodynamic data in the chemical reference state may lead to incorrect interpretations in the analysis of biochemical examples when the analysis does not include relevant conditions to the biochemical state. The definition of a new reference state for biochemical reactions in 1994 still contends with the low number of published and consistent compilations of thermodynamic data. To fill the gap in the organized presentation of biochemical thermodynamic data, reliable sources of databases of biochemical thermodynamic data are discussed, along with textbooks and specialized academic sources.