It has been previously reported that resting-cells, non-proliferating cells, of Gordonia alkanivorans RIPI90A can convert dibenzothiophene (DBT) to 2-hydroxybiphenyl (2-HBP) via the 4S pathway in a biphasic system. The main goal of the current work was to study the behaviour of resting-cells of this strain in biphasic organic media. Resting-cells showed strong affinity for sulfurous organic substrates and were able to stabilize water/gas oil emulsions by attaching to the interface without decreasing the surface tension of their environment. This was consistent with the behaviour of the whole cells but not the surfactants, suggesting that microbial cell-mediated emulsification occurs. It was found that the emulsion-stabilizing activity of the resting-cells was influenced by the growth stage, but was not directly influenced by the metabolic activity of the resting-cells. This activity may be related to cell-surface hydrophobicity, which results from the unique chemical structure of the cell surface. In some biphasic biodesulfurization (BDS) bioreactors, emulsions are created without addition of any surfactant. Cell surface-mediated stabilization helps prolong the emulsions and therefore overcomes mass-transfer limitations in bioreactors. The simultaneous occurrence of emulsion-stabilizing and desulfurization activities of resting-cells was observed for what is believed to be the first time. The results suggest that this strain may have potential for the BDS of diesel oils.
Transition metal sulfide semiconductors have achieved significant attention in the field of photocatalysis and degradation of pollutants. MoS 2 with a two dimensional (2D) layered structure, a narrow bandgap and the ability of getting excited while being exposed to visible light, has demonstrated great potential in visible-light-driven photocatalysts. However, it possesses fast-paced recombination of charges. in this study, the coupled MoS 2 nanosheets were synthesized with Zno nanorods to develop the heterojunctions photocatalyst in order to obtain superior photoactivity. the charge transfer in this composite is not adequate to achieve desirable activity. therefore, heterojunction was modified by reduced graphene oxide (RGO) nanosheets and carbon nanotubes (cnts) to develop the RGo/Zno/MoS 2 and cnts/Zno/MoS 2 ternary nanocomposites. the structure, morphology, composition, optical and photocatalytic properties of the as-fabricated samples were characterized through X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), Field Emission Scanning electron Microscopy (feSeM), transmission electron Microscopy (teM), energy-Dispersive X-ray (eDX), elemental mapping, photoluminescence (pL), Ultraviolet-Visible spectroscopy (UV-ViS), and Brunauer-emmett-teller (Bet) techniques. the photo-catalytic performance of all samples was evaluated through photodegradation of aniline in aqueous solution. the combination of RGo or cnts into the Zno/MoS 2 greatly promoted the catalytic activity. However, the resulting RGo/Zno/MoS 2 ternary nanocomposites showed appreciably increased catalytic performance, faster than that of cnts/Zno/MoS 2. charge carrier transfer studies, the Bet surface area analysis, and the optical studies confirmed this superiority. The role of operational variables namely, solution pH, catalyst dosage amount, and initial concentration of aniline was then investigated for obtaining maximum degradation. complete degradation was observed, in the case of pH = 4, catalyst dosage of 0.7 g/L and aniline concentration of 80 ppm, and light intensity of 100 W. According to the results of trapping experiments, hydroxyl radical was found to be the main active species in the photocatalytic reaction. Meanwhile, a plausible mechanism was proposed for describing the degradation of aniline upon ternary composite. Moreover, the catalyst showed excellent reusability and stability after five consecutive cycles due to the synergistic effect between its components. Total-Organic-Carbon concentration (TOC) results suggested that complete mineralization of aniline occurred after 210 min of irradiation. Finally, a real petrochemical wastewater sample was evaluated for testing the catalytic ability of the as-fabricated composites in real case studies and it was observed that the process successfully quenched 100% and 93% of Chemical Oxygen Demand (COD) and TOC in the wastewater, respectively.
In our previous study, we reported an increase in the thermal stability of bacterial laccase from Bacillus sp. HR03 using site‐directed mutagenesis. Three‐dimensional model of this enzyme showed a negative patch in the connecting loop between domains 1 and 2. In the present study, the stability of laccase in organic solvents was improved by introducing nonpolar (E188 → A, I, L, and V) and positively charged (E188 → K and R) residues in this region by site‐directed mutagenesis. Irreversible thermoinactivation, C50 value (organic solvent concentration at which 50% of enzyme activity remains), change in transition‐state stabilization energy, and kinetic parameters of the wild type and its variants were calculated in the presence and absence of various organic solvents (ethanol, methanol, and 1‐propanol). All variants showed higher C50 values when compared to the wild type. Nonpolar amino acid substitutions were found to be the most efficient mutants for their remarkable increase in C50 value and a decrease in thermoinactivation rate in the presence of mentioned solvents. Data showed that replacing a negative residue with hydrophobic residues on the surface of a protein could enhance thermoresistance as well as solvent stability. The stability of the resulting enzymes was dependent on the length of the alkyl chain. Results demonstrated that solvent tolerance was positively correlated with thermal stability.
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