Stability In Non-aqueous Media Research Articles

Lipase immobilized on magnetic hierarchically porous carbon materials as a versatile tool for the synthesis of bioactive quercetin derivatives

The preparation, characterization and application of a novel robust nanobiocatalyst, developed through the covalent binding of lipase B from Pseudozyma antarctica on magnetic hierarchically porous carbon materials (HPCFe), is reported. The nanobiocatalyst was characterized by combination of spectroscopic and microscopic techniques. Structural and catalytic characterization indicates that HPCFe nanostructures create a microenvironment which stabilizes the structure of the immobilized enzyme, resulting in enhanced activity and stability in non-aqueous media over other forms of the biocatalyst. The nanobiocatalyst was effectively applied for the selective deacetylation of peracetylated quercetin towards the synthesis of 3,5,7-triacetoxy-3′,4′-dihydroxyflavone, a compound with high antiproliferative activity. At the optimum bioprocess conditions, the produced amount of the bioactive quercetin derivative in a single-step process reached values up to 3.48 g L−1 which is 13 times higher than that reported to date. The immobilized enzyme retains ~100% of its catalytic activity after 10 repeated reaction cycles.

Ln polyoxocations: yttrium oxide solution speciation & solution deposited thin films.

Rare earth oxide materials, including thin film coatings, are critically important in magnetic, luminescent and microelectric devices, and few substitutes have been discovered with comparable performance. Thin film coatings from solution are almost unknown for rare earth oxides, likely due to their high activity towards hydrolysis which yields poor quality thin films. The hexamer [Ln6(O)(OH)8(H2O)12(NO3)6]2+ is a rare example of a metal-oxo cluster isolated and stabilized without additional supporting organic ligands. Herein we report a new method for both the preparation and stabilization in non-aqueous media, which makes these clusters valuable precursors for solution-processed thin films. Solution characterization (NMR, small-angle X-ray scattering and Raman spectroscopy) in wet organic solvents indicated that the clusters evolve via a fragmentation and reaggregation process. This is especially true for hexamers of the smaller Ln3+-ions: the higher charge density yields higher hydration rates. This process produced an entirely new hexadecameric cluster formulated Y16O3(OH)24(NO3)18(OSMe2)16(OCMe2)2(H2O)4. The new structure represents an intermediate hydrolysis product on the pathway from hexanuclear clusters to metal oxyhydroxide bulk solid. DMSO solvent ligands displace aqua ligands on the cluster and likely explain the additional stability observed for these clusters in organic solvents. The enhanced cluster stability in DMF and DMSO also enables solution-processing methods to create high quality thin films.

Enzyme Stability and Activity in Non-Aqueous Reaction Systems: A Mini Review

Enormous interest in biocatalysis in non-aqueous phase has recently been triggered due to the merits of good enantioselectivity, reverse thermodynamic equilibrium, and no water-dependent side reactions. It has been demonstrated that enzyme has high activity and stability in non-aqueous media, and the variation of enzyme activity is attributed to its conformational modifications. This review comprehensively addresses the stability and activity of the intact enzymes in various non-aqueous systems, such as organic solvents, ionic liquids, sub-/super-critical fluids and their combined mixtures. It has been revealed that critical factors such as Log P, functional groups and the molecular structures of the solvents define the microenvironment surrounding the enzyme molecule and affect enzyme tertiary and secondary structure, influencing enzyme catalytic properties. Therefore, it is of high importance for biocatalysis in non-aqueous media to elucidate the links between the microenvironment surrounding enzyme surface and its stability and activity. In fact, a better understanding of the correlation between different non-aqueous environments and enzyme structure, stability and activity can contribute to identifying the most suitable reaction medium for a given biotransformation.

Substrate-induced coagulation (SIC) of nano-disperse titania in non-aqueous media: The dispersibility and stability of titania in N-methyl-2-pyrrolidinone

Dispersions of 1 wt% titandioxide (titania or TiO 2) were investigated with respect to their stability in non-aqueous media. The objective of this work was to find conditions for the substrate-induced coagulation (SIC) process in N-methyl-2-pyrrolidinone (NMP). The SIC process is a dip-coating process that enhances adsorption of fine dispersed particles on a pre-conditioned surface. The wetting behavior of titania and NMP was investigated by the powder contact angle method. The absorption process of the polar solvent NMP on the acid oxide TiO 2and Aerosol OT (AOT) (bis-2-ethylhexyl sodium sulfosuccinate) was also investigated and a polar or hydrophilic interaction was found. The stability of titania in NMP dispersions and the influence of the solvent AOT and the electrolyte LiCl was investigated. By studying the electrophoretic mobilities of titania particles in NMP and the influence on solutes by the electrophoretic method phase analysis light scattering (PALS) and the electroacoustic method this paper explores suitable conditions for non-aqueous substrate-induced coagulation of titania.

Microbial lipases: At the interface of aqueous and non-aqueous media

In recent times, biotechnological applications of microbial lipases in synthesis of many organic molecules have rapidly increased in non-aqueous media. Microbial lipases are the 'working horses' in biocatalysis and have been extensively studied when their exceptionally high stability in non-aqueous media has been discovered. Stability of lipases in organic solvents makes them commercially feasibile in the enzymatic esterification reactions. Their stability is affected by temperature, reaction medium, water concentration and by the biocatalyst's preparation. An optimization process for ester synthesis from pilot scale to industrial scale in the reaction medium is discussed. The water released during the esterification process can be controlled over a wide range and has a profound effect on the activity of the lipases. Approaches to lipase catalysis like protein engineering, directed evolution and metagenome approach were studied. This review reports the recent development in the field ofnon-aqueous microbial lipase catalysis and factors controlling the esterification/transesterification processes in organic media.

Enzyme-coated micro-crystals: a 1-step method for high activity biocatalyst preparation

A rapid, inexpensive method for producing water-soluble enzyme-coated micro-crystals which exhibit dramatically enhanced catalytic activity and stability in non-aqueous media and can be re-dissolved easily in aqueous solution is described.

Probing Polymeric Stabilization in Nonaqueous Media by Direct Measurements

The steric repulsion induced by adsorbed layers of the commercial dispersant Hypermer KD3 has been probed by direct measurements in decalin. The forces are long range (commencing at 30-40 nm) and repulsive, and the distance dependence can be modeled with a simple scaling theory expression valid for polymer brushes. We obtain layer thicknesses of similar to 9-15 nm for the compressed layers, depending on KD3 concentration, whereas the undisturbed layers have a thickness of similar to 23-24 nm, independent of polymer concentration. Comparison of the measured interaction lengths with previous layer thickness estimates based on rheological studies shows that the polymer layers are compressed in dense suspensions.

How do additives affect enzyme activity and stability in nonaqueous media?

The catalytic activities of lyophilized powders of alpha-chymotrypsin and Candida antarctica lipase were found to increase 4- to 8-fold with increasing amounts of either buffer salts or potassium chloride in the enzyme preparation. Increasing amounts of sorbitol in the chymotrypsin preparation produced a modest increase in activity. The additives are basically thought to serve as immobilization matrices, the sorbitol being inferior because of its poor mechanical properties. Besides their role as supports, the buffer species were indispensable for the transesterification activity of chymotrypsin because they prevented perturbations of the pH during the course of the reaction. Hence, increasing amounts of buffer species yielded a 100-fold increase in transesterification activity. Effects of pH changes were not as predominant in the peptide synthesis and the lipase-catalyzed reactions. Immobilization of the protease on celite resulted in a remarkable improvement of transesterification activity as compared to the suspended protease, even in the absence of buffer species. Immobilization of the lipase caused a small improvement of activity. The activity of the immobilized enzymes was further enhanced 3-4 times by including increasing amounts of buffer salts in the preparation.The inclusion of increasing amounts of sodium phosphate or sorbitol to chymotrypsin rendered the catalyst more labile against thermal inactivation. The denaturation temperature decreased with 7 degrees C at the highest content of sodium phosphate, as compared to the temperature obtained for the denaturation of the pure protein. The apparent enthalpy of denaturation increased with increasing contents of the additives. The enhancement of hydration level and flexibility of the macromolecule upon addition of the compounds partly provides the explanation for the observed results.

Electrostatic stabilization in non-aqueous media

This paper discusses electrostatic stabilization of dispersions in non-aqueous media. It begins with the theory of repulsion, with particular attention to the roles of the dielectric constant and ionic strength of the liquid medium. Results for flat plates and spheres are compared. Methods are reviewed to measure the dielectric and electric parameters in non-aqueous media, in conjunction with procedures to determine the mechanical properties of electrostatically stabilized, concentrated suspensions. From theoretical considerations it appears that the extent of the electrostatic stabilization in non-aqueous media is extremely sensitive to the dielectric constant ϵ of the liquid, affecting stability in particular through the degree of dissociation of the stabilizing electrolyte. It is essential that, besides the presence of charge on the particles, there also are certain levels of ions in the solution to ensure a sufficient force of repulsion. It is expedient to distinguish three regimes of ϵ: (a) ϵ ⩾ 11, the (semi-)polar range, where systems can be charge-stabilized more or less as in aqueous systems, (b) the low-polar regime (5 ⩽ ϵ ⩽ 11), where electrostatic stabilization is possible provided some dissociated electrolyte is present and (c) the apolar range (ϵ⩽ 5), where screening is exclusively determined by the polarization of the solvent, and where electrostatic stabilization may be more problematic. Concentrated dispersions of solids in liquid nonionic carriers with dodecyl-benzene sulphonic acid (HDBS) as the stabilizer arc good models for the ‘low polar’ category, as detailed experimental data illustrate. In such media HDBS creates a ζ-potential, and enhances the dielectric constant and the ionic strength of the continuous phase. Especially when attraction between suspended salt particles in liquid nonionics is weak, electrostatic stabilization is easily achieved. This is particularly noticed in the rheology of the concentrated suspensions of some salts.

Effect of aminosilane adsorption on rheology of silica powders in nonaqueous media

An aminosilane,N-(3-acryloxy-2-hydroxyl propyl)-3-amino-propyltriethoxysilane (AHAS), was used to disperse and stabilize Stober silica suspensions via steric stabilization in nonaqueous media. The effect of variables, such as the calcination temperature of the silica, the water content of the solvents, the physicochemical properties of solvent media, the particle size distributions of the silica, and the proportions of mixed solvent were studied. Viscosity measurements were conducted to assess the dispersion behaviour of the silica suspensions. Fourier transform-infrared analysis was used to deduce the mechanism of AHAS adsorption on the silica surface. The effect of the water content in the silica suspension was found to be highly dependent upon the degree of silica surface hydration before AHAS addition and also upon the kind of solvent medium used. It was observed that electrostatic contribution to the silica stabilization was appreciably greater than anticipated and was found to be a function of the relative acid-base nature of the organic solvent. Analyses of rheological data and infrared spectra were used to explain the behaviour of the silica suspensions.

Enthalpic stabilization in nonaqueous media

It is shown that polyacrylonitrile latexes stabilized by polyisobutylene in 2-methylbutane (isopentane) flocculate on heating to the θ-point for the stabilizing chains in free solution. This demonstrates unequivocally that the latexes are enthalpically stabilized, at least just below the critical flocculation temperature; the latexes represent the first quantitative documentation of enthalpic stabilization in nonaqueous media. The occurrence of enthalpic stabilization in nonaqueous media is attributed to the difference in free volume between the polymeric stabilizing chains and the dispersion medium.