Initial Substrate Concentration Research Articles

Interlining Cr(VI) remediation mechanism by a novel bacterium Pseudomonas brenneri isolated from coalmine wastewater

A bioremedial approach was investigated on the removal of Cr(VI) from aqueous solution using a novel chromium reducing bacteria isolated from coalmine wastewater. Cr(VI) removal efficacy of the bacterium was determined in a series of batch studies under the influence of various parameters viz., pH (1–7), temperature (20–40 °C), initial metal concentration (1–150 mg/L), agitation speed (80–150 rpm) and substrate concentration (1–5 mg/L). Oxygen involvement in the removal process was determined by different incubation conditions. Substrate consumption and its resultant biomass generation were considered for determining the viability of the microbe under varied metal concentration. The microbial isolate survived in Cr(VI) tainted solution with initial concentration of 1–140 mg/L, among which maximum remediation was found in 60 mg/L Cr(VI) loaded solution. The bacterial species also survived in other metal solution viz., Fe(II), As(V), Cu(II), Pb(II), Zn(II), Mg(II), Mn(II) apart from Cr(VI). Multiple approaches were tested to facilitate understanding of the bacterial Cr(VI) removal mechanism. The bacteria accumulated metal ions in the exponential growth phase both on and within the cell. Underlying latent factors which governed the bacterial growth and its removal activity was determined with the classical Monod equation. The isolated bacterium also survived in the bimetallic solutions with significant removal of Cr(VI). The microbial species isolated from mining area was identified as Pseudomonas brenneri by 16s rRNA molecular characterization. Hence, the isolated novel bacterium illustrated promising involvement towards bio-treatment of Cr(VI) laden wastewater.

Lab-Scale Biodegradation Study of BTEX under the Unsaturated Condition and Its Effect on Soil Matric Potential

ABSTRACTBenzene, toluene, ethylbenzene, and xylene are collectively known as BTEX which contributes to volatile environmental contaminants. This present study investigates the microbial degradation of BTEX in batch and continuous soil column experiments and its effects on soil matric potential. Batch degradation experiments were performed with different initial concentrations of BTEX using the BTEX tolerant culture isolated from petroleum-contaminated soil. In batch study, the degradation pattern for single substrate showed that xylene was degraded much faster than other compounds followed by ethylbenzene, toluene, and benzene with the highest μmax = 0.140 h−1 during initial substrate concentration of 100 mg L−1. Continuous degradation experiments were performed in a soil column with an inlet concentration of BTEX of about 2000 mg L−1 under unsaturated flow in anaerobic condition. BTEX degradation pattern was studied with time and the matric potential of the soil at different parts along the length of the column were determined at the end of the experiment. In continuous degradation study, BTEX compounds were degraded with different degradation pattern and an increase in soil matric potential was observed with an increase in depth from top to bottom in the column with applied suction head. It was found that column biodegradation contributed to 69.5% of BTEX reduction and the bacterial growth increased the soil matric potential of about 34% on an average along the column height. Therefore, this study proves that it is significant to consider soil matric potential in modeling fate and transport of BTEX in unsaturated soils.

Co-Digestion of Napier Grass with Food Waste and Napier Silage with Food Waste for Methane Production

Enhancement of methane production by co-digestion of Napier grass and Napier silage with food waste was investigated in batch and repeated batch modes. First, the ratios of Napier grass to food waste and Napier silage to food waste were varied at different g-volatile solids (VS) to g-VS at an initial substrate concentration of 5 g-VS/L. The optimum ratios of Napier grass to food waste and Napier silage to food waste were 1:4 and 3:2 (g-VS/g-VS), respectively. This gave maximum methane yields (MY) of 411 and 362 mL-CH4/g-VSadded, respectively. Subsequently, the suitable ratios were used to produce methane at various substrate concentrations. A maximal MY of 403 and 353 mL CH4/g-VS were attained when concentrations of Napier grass co-digested with food waste and Napier silage co-digested with food waste were 15 g-VS/L and 20 g-VS/L, respectively. Under the optimum substrate concentration, the maximum MY from co-digestion of Napier grass with food waste was 1.14 times higher than that of Napier silage with food waste. Thus, co-digestion of Napier grass with food waste was further investigated at various organic loading rates (OLRs) in a 10.25 L horizontal reactor with a working volume of 5 L at an optimal ratio of 1:4 (g-VS/g-VS) and substrate concentration of 15 g VS/L. An OLR of 1.5 g-VS/L∙d gave a maximum methane production rate and MY of 0.5 L CH4/L∙d and 0.33 L-CH4/g-VSadded, respectively. Under the optimum OLR, the predominant methane producers were Methanoregula sp., Methanotorris sp., Methanobacterium sp., Methanogenium sp. and Methanosarcina sp. An energy production of 11.9 kJ/g-VSadded was attained.

Modelling degradation kinetics of metformin and guanylurea in soil microcosms to derive degradation end-points

The degradation of metformin (MET) and guanylurea (GUA) fortified separately in freshly collected two top soils (0–10 cm) from New Zealand's pastoral region was studied under controlled laboratory conditions. Incubation studies were carried at 30 °C under aerobic conditions at 60% of maximum water holding capacity and at two (0.5 mg/kg and 5 mg/kg) nominal soil concentrations. Degradation profiles revealed a bi-phasic pattern of both the compounds with an initial rapid degradation followed by slow dissipation rate, resulting in poor fits by simple first order kinetics. However, the use of three non-linear mathematical models sufficiently described the measured data and well supported by an array of statistical indices to judge model's ability to fit the measured datasets. Further evaluation using box-whisker plots showed that double first-order in parallel (DFOP) and first-order two-compartment (FOTC) models best fitted the data points followed by the Bi-exponential (BEXP) model. Mechanistic assumptions from DFOP and FOTC suggest that degradation of MET and GUA proceeds at two different rates, possibly in two compartments. The calculated DT50 using both models were in the range of 2.7–15.5 days and 0.9–4 days, while 90% dissipation time (DT90) varied between 91 and 123 days and 44 and 137 days for MET and GUA, respectively. Degradation of both compounds were dependent on soil types and properties, incubation conditions and initial substrate concentration. Formation of GUA with decrease in MET concentration over time confirmed that GUA is a transformation product concomitantly formed from aerobic degradation of MET in soil.

Optimization of o-cresol degrading microorganism and kinetics of degradation

In the present study, Pseudomonas monteilii CR13 isolated from petroleum contaminated soil demonstrated the highest specific o-cresol degradation rate at all tested o-cresol concentrations and also was not disturbed by the starting substrate concentration used (o-cresol-500 mg/L). After a serial transfer of the isolate into a series of increasing o-cresol level, the organism demonstrated significant improvement on degradation ability up to 3000 mg/L. The optimum condition for the cell mass increase and biodegradation of o-cresol by Pseudomonas monteilii was in the minimal mineral medium of 3 at a pH of 6.5 and temperature 30 °C, stirring velocity of 160 rpm, and the substrate concentration of 500 mg/L. The biodegradation kinetic study was carried out by bacteria in different initial substrate concentrations (500–3000 mg/L). In the present test the μmax, Ks and the μ were found 0.332 h-1, 0.166 mg/L and 0.0282 mg/L for 500 mg/L of o-cresol, respectively. The organism is highly promising and could be used to remove high concentrations of o-cresol from highly polluted aquatic and soil regions. The cells could be immobilized on a suitable matrix and the efficiency of degradation could be effectively improved.

Metabolic Control Analysis of Reaction Catalysed by Chalcone Synthase to Inhibit Complex Lipid Formation in Mycobacterium tuberculosis

Tuberculosis is among the deadliest diseases occurring worldwide killing almost 2 million of people every year and the number increases every year. The drugs and medications used for treatment become ineffective due to the development of resistance in the causative pathogen, M. tuberculosis towards the drugs. This has led to the development of new approaches to study the diseases from different views. Chalcone Synthase is an enzyme involved in the flavonoid biosynthetic pathway occurring in plants and in certain bacterial species including M. tuberculosis. The enzyme is responsible for lipid biosynthesis in bacterial cell wall of M. tuberculosis. This study describes the kinetic behaviour of reaction; Chalcone Synthase catalyzes the first reaction in flavonoid biosynthesis and needs a certain threshold concentration of substrates for reaction to proceed. In this study, the steady state concentration of substrates is predicted by simulation experiment using Gepasi tool. Initial concentrations of substrates in M. tuberculosis are 4.78*10-2 mMol for p-coumaroyl-CoA and 2.23*10-7 mMol for malonyl-CoA, are taken as input which are essential for synthesis of Naringenin chalcone and release of CoA. This concentration was used to simulate the reaction. Simulation results show that after 5.00 seconds, at equilibrium constant J(R1) 0.000000e mM*ml/s, initial and final concentrations of all the five components become 1.000000 mM, at 0.000 mM reaction rate. Low and stable concentrations of products were obtained during steady state analysis which is 1.157721 mM for both Naringenin chalcone and Coenzyme. The work brings a relevant approach about the possibility of the inactivation of mycobacterial growth by reducing mycolic acid synthesis in the cell wall of mycobacterium and provides basis for new strategy to design vaccine or drug against the pathogen.

Modeling Power Generation and Energy Efficiencies in Air-Cathode Microbial Fuel Cells Based on Freter Equations

The model proposed in this study was based on the assumption that the biomass attached to the anode served as biocatalysts for microbial fuel cell (MFC) exoelectrogenesis, and this catalytic effect was quantified by the exchange current density of anode. By modifying the Freter model and combining it with the Butler–Volmer equation, this model could adequately describe the processes of electricity generation, substrate utilization, and the suspended and attached biomass concentrations, at both batch and continuous operating modes. MFC performance is affected by the operating variables such as initial substrate concentration, external resistor, influent substrate concentration, and dilution rate, and these variables were revealed to have complex interactions by data simulation. The external power generation and energy efficiency were considered as indices for MFC performance. The simulated results explained that an intermediate initial substrate concentration (about 100 mg/L under this reactor configuration) needed to be chosen to achieve maximum overall energy efficiency from substrate in the batch mode. An external resistor with the value approximately that of the internal resistance, boosted the power generation, and a resistor with several times of that of the internal resistance achieved better overall energy efficiency. At continuous mode, dilution rate significantly impacted the steady-state substrate concentration level (thus substrate removal efficiency and rate), and attached biomass could be fully developed when the influent substrate concentration was equal to or higher than 100 mg/L at any dilution rate of the tested range. Overall, this relatively simple model provided a convenient way for evaluating and optimizing the performance of MFC reactors by regulating operating parameters.

Fed-batch ethanol fermentation at low temperature as a way to obtain highly concentrated alcoholic wines: Modeling and optimization

The effect of product inhibition on yeast hinders the production of wine with ethanol concentration above 80.0 g L−1 (∼10°GL) in the industrial ethanol production process commonly performed at 34 °C. Lowering the fermentation temperature is a way to produce wines with higher ethanol contents. In this work, batch fermentations were carried out at temperatures of 28, 30, 32, and 34 °C, with initial substrate concentration of 180.0 g L−1, using industrial yeast under conditions reproducing those found in industry. The Andrews-Levenspiel hybrid kinetic model, considering viable cells, showed an excellent fit to the experimental data. Kinetic parameters were determined for the different temperatures. The model was used to simulate fed-batch fermentations at different temperatures, with the same total substrate concentration, resulting in satisfactory descriptions of the process behaviors. A new optimization strategy to obtain the maximum possible ethanol production, based on the CEmax parameter of the Andrews-Levenspiel kinetic model, provided ethanol production of up to 134.7 g L-1 (17.1°GL) at 28 °C and 305.4 g L−1 substrate. A modification was made to the Andrews-Levenspiel kinetic model, relating the n parameter to the final ethanol concentration (CEf), in order to enable the model to describe the behavior of fed-batch fermentations performed with high substrate concentration and at temperatures from 28 to 34 °C.

Computational modeling of batch stirred tank reactor based on spherical catalyst particles

This paper presents a model of a batch stirred tank reactor with spherical catalyst particles as microreactors. The model involves three regions: an array of porous enzyme-loaded microreactors where enzyme reaction as well as mass transfer by diffusion take place, a diffusion limiting region surrounding the particles and a convective region where the substrate is of uniform concentration. The microbioreactors are mathematically modeled by a two-compartment model based on reaction–diffusion equations containing a nonlinear term related to the Michaelis–Menten enzyme kinetics. The influence of the physical and kinetic parameters of the microbioreactors on the transient effectiveness of the bioreactor system is numerically investigated in a wide range of model parameters. The numerical simulation was carried out using the finite difference technique. The simulation results show non-monotonic effect of the initial substrate concentration and nonlinear effects of the internal and external diffusion limitations as well as adsorption capacity of the microreactors on the transient effectiveness.

Characterization and application of a 1700-kHz acoustic cavitation field for water decontamination: a case study with toluidine blue

This work aimed at the characterization and application of a cavitation field induced in water by an ultrasonic reactor operating at 1700 kHz and 15 W. It was found that the size of active bubbles varied from 0.23 to 3 µm. The number of active bubbles increased from 6.1142 × 108 s−1 L−1 at 25 °C to 4.4684 × 109 s−1 L−1 at 55 °C. The most active bubbles were those achieving temperature of 4000 K and pressure of 1000 atm at the collapse. The characterized cavitation field removed efficiently toluidine blue (TB), an emerging organic contaminant, through reaction with hydroxyl radical. The best TB-removal rate was obtained under argon saturation, but CO2 completely suppressed the process. TB degradation rate sensitively enhanced with increasing initial substrate concentration and solution pH, whereas the liquid temperature did not affect the degradation rate. Formic acid, as an organic competitor, reduced considerably the degradation of the pollutant.

Kinetic, Thermodynamic and Optimization Studies of Dilute Acid Hydrolysis of Manihot esculenta Peels for Glucose Production

Hydrolysis of various cellulosic and lignocellulosic materials has been a focus of many researches, with sole aim of large scale conversion of these materials to the benefit of mankind. However, there are few reports on the hydrolysis of materials from agricultural wastes compared with commercially available products. This study therefore investigates the hydrolysis of cellulosic materials from the peels of Manihot esculenta (cassava) using dilute hydrochloric acid. The effects of initial substrate and acid concentrations as well as temperature were investigated. The morphological changes upon hydrolysis were also studied using Fourier-transform infrared spectroscopy, scanning electron microscope and X-ray diffraction analysis. Optimization study was carried out by using 3-levels full factorial experimental design to test the combine effect of acid concentration, temperature and time on glucose yield. The kinetics study showed that the reaction is substrate, acid concentration and temperature dependence with maximum yield of 20.24 g/L for a liquid/substrate ratio of 5 L/g. Morphological characterization shows that the hydrolysis impacted greatly on the structure of the substrate. Optimization study showed that mild acid concentration of 0.3 mol/dm3, as the temperature approaches 120 °C and reaction time of 60 min produced 18.4 g/L of glucose. The results obtained positioned M. esculenta peel as a source of glucose for alcohol production in bio-fuel production.

Kinetics of Vinyl Acetate Biodegradation by Pseudomonas fluorescens PCM 2123

Abstract The microbial degradation of vinyl acetate (VA) by Pseudomonas fluorescens PCM 2123 strain was studied in both batch and continuous modes. The purpose of the experiments was to determine the kinetic model of the cell growth and biodegradation rate of vinyl acetate (VA), which was the sole carbon and energy source for tested microorganisms. The experiments, carried out in a batch reactor for several initial concentrations of growth substrate in the liquid phase ranging from 18.6 to 373 gsubstrate·m−3 (gs·m−3) made it possible to choose the kinetic model and to estimate its constants. The Haldane inhibitory model with the values of constants: μm = 0.1202 h−1, KS = 17.195 gs·m−3, Ki = 166.88 gs·m−3 predicted the experimental data with the best accuracy. To set the parameters of maintenance metabolism it was necessary to carry out a series of continuous cultures at different dilution rates (0.05 to 0.072 h−1) and concentrations of VA in the liquid supplied to the chemostat ranging from 30.9 to 123.6 gs·m−3. The obtained data-base enabled to determine the coefficient for maintenance metabolism (me = 0.0251 gsubstrate gcell dry weight −1·h−1 (gs·gcdw −1·h−1)) as well as the maximal and observed values of yield coefficients, Yxs M = 0.463 gcdw·gs −1 and (Yxs ) obs = 0.411 gcdw·gs −1, respectively. The developed kinetics was verified by comparison of the computed and obtained in batch experiments profiles of changes in biomass and growth substrate concentrations.

Application of ion-exchange resin as solid acid for buffer-free production of γ-aminobutyric acid using Enterococcus faecium cells

In this study, ion-exchange resin was applied as solid acid catalyst for the γ-aminobutyric acid (GABA) production using a strain of Enterococcus faecium (GZ2) by whole-cell bioconversion. It was observed that the accumulated GABA resulted in the inhibition of the bioconversion activity. To improve the productivity, a cation-exchange resin-based in situ product removal (ISPR) approach was developed. Three cation-exchange resins were tested to select a suitable adsorbent used in the bioconversion. The strong acidic cation-exchange resin IRP-69 exhibited the highest GABA production and remained helpful in reducing the free GABA concentration in the reaction solution. Compared to the bioconversion in sodium acetate buffer, the GABA production, with the addition of resin, was significantly improved by 2.12-fold. Furthermore, a fed-batch bioconversion strategy with the feeding of L-monosodium glutamate (L-MSG) was tested. The maximum production of GABA, 406.1 mM, was achieved after 12 h reaction with an initial substrate concentration of 200 mM, and a triple feeding of 100 mM L-MSG afterwards at 4 h, 6 h and 8 h. The result shows that the operation of bioconversion reaction in a feeding substrate batch mode is an effective approach to improve the efficiency and productivity of GABA.

Production of (S)-1,2-propanediol from l-rhamnose using the moderately thermophilic Clostridium strain AK1

Clostridium strain AK1 was investigated for its capacity of producing 1,2-propanediol from l-rhamnose but not l-fucose. The maximum yields of 1,2-propanediol from rhamnose was 0.81 mol 1,2-PD/mol l-rhamnose. The influence of different initial substrate concentrations as well as the effect of temperature and pH on 1,2-PD production was investigated.

Progress Curve Analysis Within BioCatNet: Comparing Kinetic Models for Enzyme-Catalyzed Self-Ligation.

The estimation of kinetic parameters provides valuable insights into the function of biocatalysts and is indispensable in optimizing process conditions. Frequently, kinetic analysis relies on the Michaelis-Menten model derived from initial reaction rates at different initial substrate concentrations. However, by analysis of complete progress curves, more complex kinetic models can be identified. This case study compares two previously published experiments on benzaldehyde lyase-catalyzed self-ligation for the substrates benzaldehyde and 3,5-dimethoxybenzaldehyde to investigate 1) the effect of using different kinetic model equations on the kinetic parameter values, and 2) the effect of using models with and without enzyme inactivation on the kinetic parameter values. These analyses first highlight possible pitfalls in the interpretation of kinetic parameter estimates and second suggest a consistent strategy for data management and validation of kinetic models: First, Michaelis-Menten parameters need to be interpreted with care, complete progress curves are necessary to describe the reaction dynamics, and all experimental conditions have to be taken into consideration when interpreting parameter estimates. Second, complete progress curves should be stored together with the respective reaction conditions, to consistently annotate experimental data and avoid misinterpretation of kinetic parameters. Such a data management strategy is provided by the BioCatNet database system.

Co-fermentation of cellobiose and xylose by mixed culture of recombinant Saccharomyces cerevisiae and kinetic modeling.

Efficient conversion of cellulosic sugars in cellulosic hydrolysates is important for economically viable production of biofuels from lignocellulosic biomass, but the goal remains a critical challenge. The present study reports a new approach for simultaneous fermentation of cellobiose and xylose by using the co-culture consisting of recombinant Saccharomyces cerevisiae specialist strains. The co-culture system can provide competitive advantage of modularity compared to the single culture system and can be tuned to deal with fluctuations in feedstock composition to achieve robust and cost-effective biofuel production. This study characterized fermentation kinetics of the recombinant cellobiose-consuming S. cerevisiae strain EJ2, xylose-consuming S. cerevisiae strain SR8, and their co-culture. The motivation for kinetic modeling was to provide guidance and prediction of using the co-culture system for simultaneous fermentation of mixed sugars with adjustable biomass of each specialist strain under different substrate concentrations. The kinetic model for the co-culture system was developed based on the pure culture models and incorporated the effects of product inhibition, initial substrate concentration and inoculum size. The model simulations were validated by results from independent fermentation experiments under different substrate conditions, and good agreement was found between model predictions and experimental data from batch fermentation of cellobiose, xylose and their mixtures. Additionally, with the guidance of model prediction, simultaneous co-fermentation of 60 g/L cellobiose and 20 g/L xylose was achieved with the initial cell densities of 0.45 g dry cell weight /L for EJ2 and 0.9 g dry cell weight /L SR8. The results demonstrated that the kinetic modeling could be used to guide the design and optimization of yeast co-culture conditions for achieving simultaneous fermentation of cellobiose and xylose with improved ethanol productivity, which is critically important for robust and efficient renewable biofuel production from lignocellulosic biomass.

Why is the hydrolytic activity of acetylcholinesterase pH dependent? Kinetic study of acetylcholine and acetylthiocholine hydrolysis catalyzed by acetylcholinesterase from electric eel.

Abstract The dependence of the activity of acetylcholinesterase from electric eel at a pH value range of 4.8–9.8 (phosphate buffer), regarding acetylcholine and acetylthiocholine hydrolysis, was determined at 25 °C, ionic strength of 0.11 M, and initial substrate concentration of 4 mM. At a pH range of 4.8–9.8, the dependences A(pH) form a sigmoid increasing curve with the maximum catalytic activity at a pH range 8–9.5. For acetylcholine hydrolysis, the kinetic reason for such an increase in A consists mainly of an increase in the rate constant k 2 (Michaelis-Menten) model with increasing pH of the reaction mixture. For acetylthiocholine hydrolysis, the kinetic explication of the determined dependence A(pH) is more complicated.

Kinetic modeling of cellobiose by a β-glucosidase from Aspergillus fumigatus

The final step in lignocellulose enzymatic saccharification is the cellobiose conversion to glucose by β-glucosidases (BG). In this work, a valid kinetic model to describe cellobiose degradation for an industrial mixture of BG enzymes present in Aspergillus fumigatus is selected. Firstly, the enzyme mixture was characterised in terms of protein content and enzymatic activity on p-NPG (1326UmLpreparation−1), determining the molecular weight of the only BG activity band observed in zymograms by SDS-PAGE and MALDI-TOF: 94kDa. Subsequently, to select the correct kinetic model for the enzymatic hydrolysis of cellobiose, a combined strategy was performed: Firstly, non-linear regressions were applied to initial hydrolysis rate data for different enzyme concentrations and initial substrate and product concentrations, observing inhibition by cellobiose and glucose. Secondly, the optimal kinetic model was discriminated by a coupled non-linear regression-DOE numerical integration approach, by fitting several possible kinetic models involving different product inhibition mechanisms to progress curve data from runs at various initial substrate concentrations and temperatures. The best kinetic model involves non-competitive substrate inhibition and product competitive inhibition with two binding sites for glucose.

Enzymatic Synthesis of 6'-Sialyllactose, a Dominant Sialylated Human Milk Oligosaccharide, by a Novel exo-α-Sialidase from Bacteroides fragilis NCTC9343.

Gut bacteria provide a rich source of glycosidases that can recognize and/or hydrolyze glycans for nutrition. Interestingly, some glycosidases have also been found to catalyze transglycosylation reactions in vitro and thus can be used for oligosaccharide synthesis. In this work, six putative and one known exo-α-sialidase genes-three from Bacteroides fragilis NCTC9343, three from Clostridium perfringens ATCC 13124, and one known from Bifidobacterium bifidum JCM1254-were subjected to gene cloning and heterogeneous expression in Escherichia coli The recombinant enzymes were purified, characterized for substrate specificity, and screened for transglycosylation activity. A sialidase, named BfGH33C, from B. fragilis NCTC9343 was found to possess excellent transglycosylation activity for the synthesis of sialylated human milk oligosaccharide. The native BfGH33C was a homodimer with a molecular weight of 113.6 kDa. The Km and kcat values for 4-methylumbelliferyl N-acetyl-α-d-neuraminic acid and sialic acid dimer were determined to be 0.06 mM and 283.2 s-1, and 0.75 mM and 329.6 s-1, respectively. The enzyme was able to transfer sialyl from sialic acid dimer or oligomer to lactose with high efficiency and strict α2-6 regioselectivity. The influences of the initial substrate concentration, pH, temperature, and reaction time on transglycosylation were investigated in detail. Using 40 mM sialic acid dimer (or 40 mg/ml oligomer) and 1 M lactose (pH 6.5) at 50°C for 10 min, BfGH33C could specifically produce 6'-sialyllactose, a dominant sialylated human milk oligosaccharide, at a maximal conversion ratio above 20%. It provides a promising alternative to the current chemical and enzymatic methods for obtaining sialylated oligosaccharides.IMPORTANCE Sialylated human milk oligosaccharides are significantly beneficial to the neonate, as they play important roles in supporting resistance to pathogens, gut maturation, immune function, and brain and cognitive development. Therefore, access to the sialylated oligosaccharides has attracted increasing attention both for the study of saccharide functions and for the development of infant formulas that could mimic the nutritional value of human milk. Nevertheless, nine-carbon sialic acids are rather complicated for the traditional chemical modifications, which require multiple protection and deprotection steps to achieve a specific glycosidic bond. Here, the exo-α-sialidase BfGH33C synthesized 6'-sialyllactose in a simple step with high transglycosylation activity and strict regioselectivity. Additionally, it could utilize oligosialic acid, which was newly prepared in an easy, economical way to reduce the substrate cost, as a glycosyl donor. All the studies laid a foundation for the practical use of BfGH33C in large-scale synthesis of sialylated oligosaccharides in the future.

Photocatalytic activity and mechanism of bisphenol a removal over TiO2−x/rGO nanocomposite driven by visible light

In the present study, Ti3+ and oxygen vacancies (Ti3+/Ov) self-doped TiO2 and coupled with a reduced graphene oxide (rGO) nanocomposite (TiO2−x/rGO) was successfully synthesized via a facile hydrothermal-calcination method for the efficient degradation of bisphenol A (BPA) under visible light. The results of XPS, ERP and TEM analyses revealed that the presence of Ti3+/Ov in the lattice of TiO2−x/rGO leads to the narrowed band gap and enhanced visible light harvesting. The chemical bonds (TiOC) between TiO2−x and rGO act as the channel for electron transfer, consequently resulting in the efficient charge separation, which was investigated and confirmed by such methods as PL spectra and time-resolved PL spectra. In addition, the coupling of TiO2−x and rGO could strongly suppress the aggregation of TiO2−x particles and therefore improve the adsorption of organic pollutants. For the synergistic effect of the three preeminent features mentioned above, TiO2−x/rGO exhibited a 6.16-, 2.92- and 2.55-fold faster reaction rate for BPA degradation than that of pristine TiO2, TiO2/rGO and TiO2−x, respectively. Moreover, the effects of the initial substrate concentration, initial solution pH, catalyst dosage and inorganic anions on BPA removal were also investigated in depth. EPR measurements indicated that ·O2− as the major oxidizing species, is responsible for the degradation of BPA. Next, the feasible pathway of BPA degradation by TiO2−x/rGO was proposed based on the analysis of intermediate products. Finally, the mechanism of the enhanced photocatalytic performance by TiO2−x/rGO under visible light was discussed. Based on these results, the TiO2−x/rGO nanocomposite could be an efficient and promising photocatalyst for the degradation of organic pollutants in water.