A. P. Sadkov

Methane Oxidation Catalyzed by the Au-Protein from Micrococcus luteus

Earlier, we showed that the Au-protein from the aurophilic bacteria Micrococcus luteus displays activity towards methane [1]. With the use of gas chromatography, it was shown that the amount of methane decreases in Au-protein-containing incubation medium in the presence of NADH, air, K 3 Fe ( CN ) 6 , and TrisHCl buffer. In addition, using the ESR method, we demonstrated a change in the redox state of the Au-protein induced by methane. Based on the results obtained, we assumed that gold incorporated in the active site of the enzyme involves methane into its oxidative cycle. Presumably, the first oxidation product could be methanol. In the present study, we sought to verify the above assumption using the isotopic-dilution approach for quantitative analysis of methanol with the use of 14 C-labeled methane as a substrate for the Au-protein.

Electrochemical and ESR studies of au-protein from Micrococcus luteus

Au-protein from Micrococcus luteus, with and without Au inactive center, and chloroauric acid (HAu IIICl4·4H2O) with the addition of rutin, catechol, and riboflavin have been studied by means of electrochemistry and ESR. The redox potentials for Au-protein, as well as for the complexes Au-rutin and Au-catechol, have been measured, and ESR spectra of complexes Au-rutin and Au-catechol have been recorded. It has been shown that the Au atom binds to Au-protein via OH-groups of rutin. Flavin does not participatein gold binding. Au-protein is characterized by two peaks of cyclic voltammogram, −0.37 and −0.54 V. Au-protein with these potentials is able to function in the electron-transport chain of membranes between flavoproteins and quinones.

Deep oxidation of rutin and quercetin during their reaction with HAuCl4 in aqueous solutions

The processes of deep oxidation during the reaction of rutin and quercetin with HAuCl4 under anaerobic conditions at temperatures of 30, 60, and 100 °C were studied. The formation of CO and CO2 was revealed by mass spectrometric analysis to occur at temperatures ≥30 °C. Since the processes of oxidation of quercetin and rutin are similar, it was concluded that, on the one hand, the sugar residue of rutin is not mainly subjected to deep oxidation and, on the other hand, the hydrolysis of the primary product of rutin oxidation occurs to form the primary product of quercetin oxidation. The analysis of the optical absorption spectra of the systems studied shows that gold nanoparticles are formed at the reduction of AuIII to Au0. In a large excess of AuIII ions, some portion of them remains non-consumed and the IR spectrum of an Au: rutin (40: 1) system after water sublimation mainly exhibits vibrations of the sugar residue. A possible mechanism for CO formation due to the decomposition of the hydrated isomer of the product of two-electron oxidation of quercetin containing three consecutively bonded carbonyl groups was proposed on the basis of the PBE density functional quantum chemical calculations.

A study of the molecular structure of bacterial lysodektose by quantum-chemical calculations and EPR spectroscopy

The standard redox potentials of the sequential oxidation of lysodektose to the corresponding nitrone were estimated by quantum chemistry methods. It follows from these estimates that the experimentally observed accumulation of the intermediate nitroxyl radical in substantial amounts during the oxidation of lysodektose can be explained by high medium reorganization energy in the oxidation of the nitrosyl radical with simultaneous proton abstraction. The EPR spectra of the radical lysodektose form were modeled. Arguments in favor of the suggestion that one nonequivalent proton appeared in the formation of an intramolecular H-bond were presented. Quantum-chemical calculations of the hyperfine structure constants were in satisfactory agreement with experiment.

Electrochemical study of Au(III)-luteolin flavonoid system in tris-buffer

The Au(III)-luteolin system was studied by means of cyclic voltammetry, spectrometry, and quantum chemical simulation. The mutual effect of luteolin to Au(III) reduction and Au(III) to luteolin oxidation was studied by means of cyclic voltammetry on Pt and carbon glass electrodes in 0.05 M tris-buffer solution (pH 8) containing ethyl alcohol. The absorption spectra of luteolin were recorded with and without Au(III) in 0.05 M tris-buffer solution (pH 8) containing ethyl alcohol. The quantum chemical simulation of Au(III)-tris, Au(III)-luteolin, and Au(III)-tris-luteolin systems was carried out. On the basis of the collected data, formation of Au(III)-tris-luteolin complex in 0.05 M tris-buffer solution (pH 8) in the presence of ethanol was suggested.

Gold(III) reduction in a tris-HCl buffer: Effect of riboflavin, rutin, 1,1-dipyridyl, and 1-naphthol

Reduction of chloroauric acid on platinum and gold electrodes in a 0.1 M tris-HCl buffer of pH 8 containing riboflavin, rutin, 1,1-dipyridyl, or 1-naphthol is studied by cyclic voltammetry and in situ ESR methods. On the basis of the obtained data it is assumed that in the buffer there occurs the reduction of Au(III) to Au(I). In the presence of 1,1-dipyridyl, there occurs the reduction of complex [Au(III)-1,1-dipyridyl]. The reduction of Au(III) in the presence of 1-naphthol is realized in the composition of complex [Au(III)-tris-1-naphthol]. The hampering of the electrode process of the Au(III) reduction in the presence of 1-naphthol is caused by the adsorption of the [tris-1-naphthol] associates at the electrode surface. The presence of Au(III) does not exert any influence on the process of electroreduction of riboflavin. The obtained results make it possible to presume that the resistance of gold-accumulating cells Micrococcus luteus toward toxic compounds that are inhibitors of the respiratory chain, such as 1,1-dipyridyl and 1-naphthol, is caused by their binding in gold-containing complexes in the composition of Au-protein.