Stereoselectivity and competing reactions as studied by lipase-catalyzed esterifications in aqueous lecithin-based gelatin gels

 The enzyme catalyzed conversion of R/S-(±)-2-octanol with hexanoic acid to R/S-(±)-2-octyl hexanoate has been studied in different microenvironments and in the presence of the competing substrate ethanol. The reactions were performed in various gels made from aqueous gelatin solutions and liposome dispersions or isotropic liquid solutions, with or without oil and ethanol. The lipase Candida sp. (SP 525) was dissolved in the dispersions or solutions stabilized by the naturally occurring zwitterionic surfactant soybean lecithin. The sectioned porous gel was immersed in hexane containing 0.33 mol dm^-3 of racemic 2-octanol and hexanoic acid. Since ethanol acts both as a substrate and as a part of the gel it is of fundamental interest to know the phase behaviour of the used systems. Partial phase diagrams for the systems ethanol–water–soybean lecithin and ethanol/water (7:3)–oil–soybean lecithin were determined at 298.2 K. The oil was either castor oil or hexadecane. The conversion of R-2-octyl hexanoate was about 0.45 when no or small amounts of ethanol was present, but decreased considerably with high amounts of ethanol present and ethyl hexanoate became the main product. Hydrolysis of R-2-octyl hexanoate was favoured in the latter systems and hexanoic acid formed was immediately esterified to ethyl hexanoate. The conversion of R-2-octyl hexanoate and ethyl hexanoate depends only on the ethanol content present in the systems and is thus independent of the environment of the enzyme. However, the chiral esters synthesized from racemic 2-octanol and hexanoic acid showed high optical purities regardless of the ethanol content. Received: 1 July 1996 Accepted: 30 August 1996     

Entrapment of enzyme in water-restricted microenvironment for enzyme-mediated catalysis under microemulsion-based organogels

Nonaqueous enzymology has emerged as a major area of biotechnology research and development. Enzymes in organic solvents offer great potential for the biocatalysis of a wide range of chemical processes that cannot occur in water. One of the most commonly used methods for carrying out enzymatic conversions in organic solvents is enzymes solubilized in water-in-oil (w/o) microemulsions or water containing reverse micelles. In reverse micelles, enzyme molecules are solubilized in discrete hydrated micelles formed by surfactants within a continuous phase, i.e., nonpolar organic solvent. Under appropriate conditions, these solutions are homogeneous, thermodynamically stable, and optically transparent. However, there are very few examples of preparative-scale enzyme-catalyzed synthesis in water-in-oil microemulsion. One reason is that despite the advantages offered by microemulsion media, product isolation and enzyme reuse from such singlephase liquid medium is more complex than in competing methodologies in which the catalyst is present as a separate solid phase. Therefore, the approach simplifying product isolation, and enzyme reuse from microemulsion-based media, has been the use of a gelled microemulsion system, especially gelatin silica nanocomposite.     

少量离子液体对有机相R.miehei脂肪酶催化合成辛酸戊酯的影响

研究了疏水性离子液体[BMIM][PF6]对脂肪酶R. miehei有机相催化合成辛酸戊酯反应的影响,并结合荧光发射光谱分析R. miehei构象变化与酶活性表达的关联。结果表明,该酯化反应在饱和烷烃中的初速度比在芳烃中的大,且当溶剂logP为1.5～3.5时,反应初速度随logP的增加而降低。在不同有机溶剂中,加入占辛酸质量1%的[BMIM][PF6]均导致反应初速度降低15%左右,降低幅度几乎不与溶剂种类有关。表明[BMIM][PF6]对该反应的影响与所在溶剂系统无关,而只与离子液体的加入有关。[BMIM][PF6]加入量为辛酸质量1%～3%时,以壬烷为溶剂时的反应初速度随[BMIM][PF6]加入量的增加而降低,进一步表明离子液体的加入量是影响反应初速度的重要因素。荧光光谱分析表明,酶活性的表现与酶分子的构象变化有一定的相关性。     

Monte Carlo study of polyelectrolyte gels

The discontinuous transition between dense and dilute phases in polyelectrolyte gels is observed in Bond-Fluctuation Method Monte Carlo simulations of gels. The transition is driven by the competition between local attractive interactions of a poor-quality solvent and global repulsive interactions from counter-ion pressure. A procedure is introduced that prevents local attractive interactions from destroying ergodicity. Under good solvent conditions, lengths and volumes of gels are found to follow self-avoiding random walk scaling. © 1996 John Wiley & Sons, Inc.     

Characterization of lecithin-based microemulsions used as media for a cholesterol oxidase-catalyzed reaction

Microemulsions stabilized by soybean lecithin and ethanol have been characterized with respect to phase behavior, distribution of the ethanol cosolvent, conductivities, viscosities, and volume fractions of the different phases in Winsor III systems. The conductivities and viscosities of the surfactant-rich phase in the Winsor III system indicate that this phase exhibits a bicontinuous structure. The reaction yield at 298.2K for the enzymatic conversion of cholesterol to cholestenone by cholesterol oxidase performed in a Winsor III system containing water, lecithin, hexadecane, and ethanol is low.     

Fabrication of palladium nanoparticles as effective catalysts by using supramolecular gels

Two-component supramolecular gels were made through self-assembly of tetrazolyl derivatives and Pd(OAc)₂. The robust gels indicated high storage modulus(>10,000 Pa) and loss modulus, which were studied by rheological measurements. The formed Pd nanoparticles(~9 nm) obtained during the formation of the gel showed effective catalytic hydrogenation of nitrobenzene and could be recovered and reused without loss of activity.     

Process development for production of medium chain triglycerides using immobilized lipase in a solvent-free system

The synthesis of tricaprylin, tricaprin, trilaurin, and trimyristin in a solvent-free system was conducted by mixing a commercial immobilized lipase with the organic reagents (glycerol and fatty acid) in a 20-mL batch reactor with constant stirring. The effects of temperature, fatty acid/glycerol molar ratio, and enzyme concentration on the reaction conversion were determined. The reactions were carried out for 26 h and the nonpolar phase was analyzed by gas chromatography. Appreciable levels of medium chain triglycerides were achieved, except for tricaprylin. The higher selectivity values for the production of triglycerides were attained under the following conditions: a fatty acid/glycerol molar ratio of 5; enzyme concentration of 5 or 9% (w/w); and temperatures of 70°C (tricaprin), 80°C (trilaurin), and 90°C (trimyristin). After completion of the esterification reaction under these conditions, the recovery of the triglyceride and fatty acids, and the reusability of the enzyme were studied. The unreacted fatty acid and the produced triglyceride were satisfactorily recovered. The commercial immobilized lipase was used in 10 consecutive batch reactions at 80°C, with 100% selectivity in the trilaurin and trimyristin synthesis. The possibility of enzyme reuse and the recovery of residual fatty acid are relevant results that contribute to increasing the viability of the process.     

Studies of phase transformations in the gelatine-water system using near-IR spectroscopy

Near-IR spectroscopy studies of gel formation mechanism are reported for gelatin-water systems. Relative absorbance in 1.4-μm and 1.9-μm bands was shown to be sensitive to the helix to coil phase transition in the pre-melting region. The formation mechanism of a metastable “coil” phase is in agreement with DSC and TGA data obtained in parallel experiments on the 10% gelatin gel. Received: 12 April 2000 Accepted: 4 October 2000     

Electrochromatography in packed tubes using 1.5 to 50 μm silica gels and ODS bonded silica gels

Summary Electrochromatography (that is HPLC where the eluent is driven along the column by electro-osmosis using fields of up to 100 kV m⁻¹) promises plate efficiencies for HPLC which are comparable to those attained in capillary gas chromatography, but this requires that narrow-bore columns can be successfully packed with submicron particles. This paper demonstrates that we have now moved a considerable distance towards this goal. We show (1) that, following theory, there is no evidence of any reduction in electroosmotic velocity in columns packed with particles down to 1.5 μm diameter, (2) that reduced plate heights as low as unity are attainable for unretained solutes using both slurrypacked and drawn-packed columns 30 to 200 μm bore and up to 1 m long when packed with conventional 3 and 5 μm silica gels or with 1.5 μm impermeable silica spheres, (3) that columns driven electrically show higher plate efficiencies than identical columns driven by pressure, and (4) that 100,000 plate HPLC separations can be achieved in relatively short times of 30 minutes using in situ derivatised drawn packed capillaries containing 3 and 5 μm ODS-silica gels.     

Phase behaviour of pseudo-binary systems of pressurized ((propane + l,l-lactide)) at different ethanol to l,l-lactide mole ratios

This work reports phase equilibrium data of pressurized (propane + l,l-lactide) system at different ethanol to monomer mole ratios (9:1; 7:1; 5:1). Phase equilibrium experiments were accomplished in a high-pressure variable-volume view cell employing the static synthetic method. (Vapour + liquid) equilibrium data for the pseudo-binary systems were determined within the temperature range from (323 to 353) K and pressures up to 3.3 MPa. For the systems investigated, (vapour + liquid) equilibrium (VLE) was visually recorded. It was observed that an increase in temperature or in propane concentration led to a pronounced rise in pressure transition values. On the other hand, an increase in the ethanol to l,l-lactide mole ratio led to a reduction in pressure transitions, whereas a reduction in ethanol concentration complicates the achievement of one-phase homogeneous system. Thus, rapid complete miscibility of the system can be controlled by the amount of ethanol added as a co-solvent. The experimental results were modelled using the Peng–Robinson (PR) equation of state with the Wong–Sandler (PR–WS) mixing rule, providing a good representation of the experimental phase transition points.     

Reaction kinetics of the esterification of lauric acid in iso-octane using an immobilized biocatalyst

The kinetics of the esterification of lauric acid with geraniol catalyzed by a commercially immobilized lipase preparation fromMucor miehei, Lipozyme, was studied in well-stirred flasks under conditions of no external mass transfer limitations. It was shown that the reaction is inhibited by lauric acid and the reaction mechanism can be described as a Ping-Pong Bi-Bi with Dead-End inhibition caused by lauric acid.     

Self-organisation in the assembly of gels from mixtures of different dendritic peptide building blocks

This paper investigates dendritic peptides capable of assembling into nanostructured gels, and explores the effect on self-assembly of mixing different molecular building blocks. Thermal measurements, small angle X-ray scattering (SAXS) and circular dichroism (CD) spectroscopy are used to probe these materials on macroscopic, nanoscopic and molecular length scales. The results from these investigations demonstrate that in this case, systems with different "size" and "chirality" factors can self-organise, whilst systems with different "shape" factors cannot. The "size" and "chirality" factors are directly connected with the molecular information programmed into the dendritic peptides, whilst the shape factor depends on the group linking these peptides together--this is consistent with molecular recognition hydrogen bond pathways between the peptidic building blocks controlling the ability of these systems to self-recognise. These results demonstrate that mixtures of relatively complex peptides, with only subtle differences on the molecular scale, can self-organise into nanoscale structures, an important step in the spontaneous assembly of ordered systems from complex mixtures.     

Identification of the phase of a fluid using partial derivatives of pressure, volume, and temperature without reference to saturation properties: Applications in phase equilibria calculations

The phase of a fluid (liquid, vapor) at a given state is normally identified by comparing the properties of the fluid at the given state with saturation properties. In this paper we present a thermodynamic method for determining the phase of a fluid from the partial derivatives of pressure, volume and temperature without reference to saturated properties. The method has immediate application in a variety of phase equilibria calculations, particularly for liquid-liquid or vapor-liquid-liquid equilibria calculations in process simulators.     

High performance liquid chromatography of raw sugars and polyols using bonded silica gels

Summary Bonded silica columns have been evaluated for their ability to separate carbohydrates and polyols. Mobile phases consisting of dichloromethane/methanol produced the best separations in comparison with the acetonitrile/water mixtures commonly used with amino columns. Of all the bonded phases tested, LiChrospher Diol silica provided the best separations, and selectivities were not very different from those obtained on the most popular system using an amino bonded phase and acetonitrile/water as eluent. In addition, diol columns with a dichloromethane/methanol eluent offer excellent stability with no Schiffs base formation of reducing sugars. Using an evaporative light scattering detector, low limit detection is obtainable (20 ng of glucose from a column) and gradient elution is quite feasible.     

Modelling of phase equilibria for associating mixtures using an equation of state

In the present work, the group contribution with association equation of state (GCA-EoS) is extended to represent phase equilibria in mixtures containing acids, esters, and ketones, with water, alcohols, and any number of inert components. Association effects are represented by a group-contribution approach. Self- and cross-association between the associating groups present in these mixtures are considered. The GCA-EoS model is compared to the group-contribution method MHV2, which does not take into account explicitly association effects. The results obtained with the GCA-EoS model are, in general, more accurate when compared to the ones achieved by the MHV2 equation with less number of parameters. Model predictions are presented for binary self- and cross-associating mixtures.     

Syntheses of enantiopure Si-centrochiral silicon-based muscarinic antagonists using an enantioselective enzymatic esterification as the key step

The muscarinic antagonists (R)-cyclopentyl(hydroxymethyl)phenyl[2-(piperidin-1-yl)ethyl]silane[(R)-1] and(R)-1-{2-[cyclopentyl(hydroxymethyl)phenylsilyl]ethyl}-1-methylpiperidinium iodide [(R)-2] were synthesized using anenantioselective enzymatic transformation as the key step. Apapain-catalyzed (E.C. 3.4.22.2) esterification ofrac-cyclopentyl(hydroxymethyl)phenyl(vinyl)silane(rac-3) with 5-phenylpentanoic acid afforded(R)-cyclopentyl(phenyl)[(5-phenylpentanoyloxy)methyl]vinylsilane[(R)-4] which upon chemical hydrolysis gave enantiomericallyenriched (R)-3 (68% ee). Repetition of thisesterification/hydrolysis sequence, starting from enantiomericallyenriched (R)-3, finally gave the enantiopure silane (R)-3(98% ee) which served as the starting material for thesubsequent chemical synthesis of (R)-1 and (R)-2{(R)-3 (R)-cyclopentyl(phenyl)[(trimethylsilyloxy)methyl]vinylsilane[(R)-5] (R)-1 (R)-2}.     

Effects of plasticizers on novel electromechanical actuations with different poly(vinyl chloride) gels

Four different plasticizers were applied to make different poly(vinyl chloride) (PVC) gels, poly(vinyl chloride)‐bis(2‐ethylhexyl)phthalate (PVC‐DOP), poly(vinyl chloride)‐di‐n‐butylphthalate, poly(vinyl chloride)‐bis(2‐ethylhexyl)adipate, and poly(vinyl chloride)‐tris(2‐ethylhexyl)trimellitate. In our previous work, we reported that PVC‐DOP gel exhibits novel and reversible deformations of creeping and jointlike bending induced by direct current electric fields. In this article, we scrutinize the effects of plasticizers on electromechanical actuations, that is, reversible creeping and bending actuation with four of the different aforementioned gels. We measured the relative creeping distance, creeping area, creeping velocity, current observed, and bending angle as a function of applied electric fields for different PVC gels and found significant differences among them. To explain these variations, we compared the utility of plasticizers on the basis of the properties of different PVC gels, such as plasticizer‐retention ability, bending modulus, elongation at break, and the dielectric constant. The mentioned properties of the PVC gels played vital roles on their electromechanical actuations. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2119–2127, 2003     

Green synthesis of banana flavor using different catalysts: a comparative study of different methods

ABSTRACT

Esterification of isoamyl alcohol with acetic acid was studied using different ion-exchange resins, namely Amberlyst 15 dry, Amberlyst 16 wet, Amberlite 120-IR. Esterification was carried out using different esterification methods that are quite new (ohmic, ultrasonic probe, and ultrasonic bath) and the results were compared with microwave-assisted esterification (MAE). The highest isoamyl acetate yield (99%) was obtained by MAE, using a mixture of acetic acid and isoamyl alcohol (mole ratio of 1:2) after 2?h of reaction time. In this process, 2% Amberlyst 15 dry was used. MAE had the least specific energy consumption (0.42?kWh/g isoamyl acetate) and specific CO₂ emission (34?g/g isoamyl acetate). According to the images obtained by scanning electron microscopy, lower amounts of Amberlyst 15 dry beads were destroyed by MAE method compared to other esterification methods. In conclusion, MAE proved to be an economic and environmentally-friendly method for esterification of different flavoring compounds.     

高效液相色谱法同时测定阿达帕林凝胶剂中的主药阿达帕林和两种防腐剂

建立了同时测定阿达帕林凝胶剂中主药阿达帕林和防腐剂(苯氧乙醇、对羟基苯甲酸甲酯)含量的反相高效液相色谱法(HPLC)。采用Tigerkin C18柱(150 mm×4.6 mm,5 μm),以pH 3.0的0.02 mol/L醋酸盐缓冲液-四氢呋喃-乙腈为流动相进行梯度洗脱,在270 nm波长条件下检测。线性范围:苯氧乙醇,10～100 mg/L(r=0.9999);对羟基苯甲酸甲酯,4～40 mg/L(r=0.9999);阿达帕林,4～40 mg/L(r=0.9999)。3种组分的平均回收率为98.0%～98.6%。该方法简便、可靠,可用于阿达帕林凝胶剂中阿达帕林、苯氧乙醇和对羟基苯甲酸甲酯的同时测定。