Keungarp Ryu

Immobilization of laccase on carbon nanomaterials

Laccase from Trametes versicolor was readily immobilized on carbon nanomaterials including multiwalled carbon nanotubes (MWNTs), carboxylated multiwalled carbon nanotubes (MWNT-COOHs), and graphene oxides (GOs), by physical adsorption without using coupling agents. The immobilized amount of laccase strongly depends on the pH of the aqueous buffers of the immobilization mixture. As the pH of the aqueous buffer for immobilization increases, the immobilized amount of laccase decreases. The activity of the immobilized laccase on the three carbon nanomaterials exhibits a bell-shaped dependence on the pH of the immobilization solution with maximum activity at pH 6 or 7. When the immobilization solution becomes acidic or basic, the activity of the immobilized laccase declines significantly. The amount and the activity of immobilized laccase were maximum for graphene oxides as substrate material for immobilization.

Removal of dibenzothiophene and its oxidized product in anhydrous water-immiscible organic solvents by immobilized cytochrome c

In various anhydrous water-immiscible organic solvents, dibenzothiophene (DBT) was removed by more than 80% by cumene hydroperoxide (α,α-dimethylbenzyl hydroperoxide) catalyzed by the immobilized cytochrome c on Celite. DBT-sulfone, the oxidation product of DBT, strongly adsorbed on Celite in highly hydrophobic organic solvents such as hexadecane, decane and n-octane. The adsorbed DBT-sulfone was desorbed from Celite by a simple washing of the Celite with a less hydrophobic solvent such as octanol.

Quantitative and predictive correlations for peroxidase catalysis in organic media

Quantitative and predictive relationships have been developed for horseradish peroxidase catalysis in both aqueous and organic media. These relationships take into account the physicochemical characteristics of both substrate (e.g., hydrophobicity and electronic characteristics) and solvent (e.g., hydrophobicity and polarity). The results suggest that solvent effects on hydrophobic interactions within proteins are secondary to effects on electrostatic forces.

Adsorption of a xylanase purified from Pulpzyme HC onto alkali-lignin and crystalline cellulose

Adsorption of a xylanase purified from a commercial xylanase, Pulpzyme HC, onto two model components of kraft pulp, crystalline cellulose (Avicel) and alkali-lignin (Indulin AT), was studied at 40°C. A considerable amount of the purified xylanase was adsorbed onto alkali-lignin in alkaline solutions. The adsorption of the purified xylanase onto crystalline cellulose was not significant and could be described by the Langmuir-type adsorption isotherm. The adsorption of the purified xylanase onto alkali-lignin was assumed to be caused by physical or van der Waals interaction based on the result that NaCl did not change the adsorption isotherm.

Enzymatic catalysis on coal-related compounds in organic media: kinetics and potential commercial applications

Abstract Enzymatic catalysis in nonaqueous media has been considered for use in the modification of coal and related compounds including lignin and phenolic resins. This paper reviews the use of horseradish peroxidase and several other oxidative enzymes in organic media, and selected applications of these enzymes in such environments. These applications include phenolic polymerizations, lignin depolymerization, and S-oxidation of dibenzothiophene. The latter application is particularly relevant to coal desulfurization. Specifically, horseradish peroxidase, laccase, and chloroperoxidase were capable of oxidizing dibenzothiophene in 25% dimethylformamide. Yields of over 40% were obtained. Such an enzymatic reaction, used in conjunction with further microbial steps can be used to remove sulfur from coal.

Oxidation of dibenzothiophene catalyzed by immobilized hemoproteins in water-immiscible organic solvents

In various water-immiscible organic solvents, hemoglobin, myoglobin and cytochrome c deposited on Celite catalyzed the oxidation of dibenzothiophene when peroxides having nonpolar substituents, such as t-butyl hydroperoxide and cumene hydroperoxide (α, α-dimethylbenzyl hydroperoxide), were used as oxidants. In hexadecane, oxidation of dibenzothiophene by immobilized cytochrome c, with a Km value of 0.15 mM for dibenzothiophene, was inhibited by cumene hydroperoxide above 0.5 mM.

Partial purification and characterization of an extracellular protease fromXenorhabdus nematophilus, a symbiotic bacterium isolated from an entomopathogenic nematode,Steinernema glaseri

Entomopathogenic nematodes are used for insect control. Herein, an extracellular protease was partially purified from a culture supernatant ofXenorhabdus nematophilus, a symbiotic bacterium of an entomopathogenic nematode,Steinernema glaseri, using precipitation with 80% v/v isopropyl alcohol followed by gel permeation chromatography with a packed Sephacryl S-300 HR media. The partially purified protease exhibited maximal activity at pH 7 in the presence of 1 mM CaCl2. The protease was identified as a metallo-protease based on the inhibition of its activity by the metal chelating agent, EDTA.

Peroxidase-catalyzed polymerization ofp-cresol in supercritical CO2

The feasibility of the catalysis of horseradish peroxidase (HRP) in supercriticalCO2 was studied for the polymerization ofp-cresol in the presence of H2O2. The reactions were performed at 40‡C and 74.8 aim (1 100 psia) above the critical conditions ofCO2. In the initial tests with 2 mM ofP-crcsol and 1 mM of H2O2, more oligomers ofp-cresol were produced as more HRP was added. This indicates that HRP is active in supercritical CO2. HRP was not completely inhibited by H2O2 at concentrations up to 20 mM. Increasing the initial concentrations ofp-cresol and H2O2 to 20 mM, respectively, resulted in the formation of precipitates which were undissolved either in water or in dimcthylforamide (DMF). The effects of adding water and/or methanol as cosolvents on the reactivity of HRP were studied subsequently. When more than 13.3 mL of water per liter of reaction volume was added, the formation of precipitates was not observed. The reactivity of HRP was sustained when up to 11.8 mL of methanol per liter of reaction volume was added. In most cases conversion ofP-cresol was less than 50% for 5 hours of reaction time.

Kinetic characterization of a fungal peroxidase from Coprinus cinereus in aqueous and organic media

Catalytic and structural characteristics of a fungal peroxidase from Coprinus cinereus (CiP) expressed by Aspergillus niger were investigated in aqueous media and aqueous/dioxane solvents, and compared to the well-known peroxidase from horseradish (HRP). CiP is more thermostable than HRP; the half lives for the irreversible inactivation of CiP and HRP at 85°C were 40 min and 21 min, respectively. The observed oxidation potential of CiP is similar to that of HRP. The kinetics for the oxidation of substituted phenols catalyzed by both peroxidases were measured to construct linear free energy relationships between the catalytic efficiency, Vmax/Km, of both enzymes and the electronic and hydrophobic properties of the phenolic substrates as represented by the Hammett constant, s` and Hansch constant, π respectively. CiP and HRP are similar in response to substrate hydrophobicity in different concentrations of dioxane; a finding not unexpected as substrate desolation dominates hydrophobic interactions in organi...

Thermal stability and degradation kinetics of polyphenols and polyphenylenediamines enzymatically synthesized by horseradish peroxidase

Various substituted phenols and phenylenediamines were enzymatically polymerized by horseradish peroxidase in 80% (v/v) organic solvents-aqueous buffer (100 mM sodium acetate, pH 5) mixtures with H2O2 as the oxidant. The thermal stability of the polymers was investigated by thermogravimetric analysis (TGA) and represented by the char yield (wt% of the initial polymer mass) after being heated at 800 °C. Poly(p-phenylphenol) had the highest thermal stability among the synthesized polymers with a char yield of 47 wt%. The polymers containing amino groups such as poly(p-aminophenol) and polyphenylenediamines were also shown to possess high thermal stabilities. The activation energies for the thermal degradation of the polymers determined by derivative thermogravimetric analysis (DTG) using Horowitz-Metzger’s pseudo-first-order kinetics were in the range between 23-65 kJ/mol and comparable to those of the chemically synthesized polymers. Dynamic structural changes of the enzymatically synthesized polymers upon heating were studied by differential scanning calorimetry (DSC). The DSC curves of poly(p-phenylphenol) showed a broad exothermic peaks between 150-250 °C, indicating that the polymer undergoes complex structural transitions in the temperature range. On the other hand, the DSC curves of the poly(p-aminophenol) and the poly(p-phenylenediamine) which contain amino groups showed strong sharp endothermic peaks near 150 °C, implying that these polymers possess homogeneous oriented structures which undergo a concerted structural disintegration upon heating.