A. J. J. Straathof
Bio: A. J. J. Straathof is an academic researcher from Delft University of Technology. The author has contributed to research in topics: Kinetic resolution & Enantiomeric excess. The author has an hindex of 17, co-authored 39 publications receiving 744 citations.
TL;DR: Addition of cosolvents is calculated to improve the apparent equilibrium constant of amoxicillin synthesis considerably, but probably not the synthetic yield, due to solubility restrictions of the reactants.
Abstract: The enzymatic, thermodynamically controlled synthesis of amoxicillin in aqueous solution was measured in order to study the feasibility of a `solid-to-solid' conversion. In aqueous solution, however, the synthetic yield of amoxicillin remains lower than the amoxicillin solubility. Therefore, a `solid-to-solid' synthesis of amoxicillin in aqueous solution is not feasible. Synthetic yields in enzymatic condensation reactions can often be improved by adding organic solvents in monophasic systems. Addition of cosolvents is calculated to improve the apparent equilibrium constant of amoxicillin synthesis considerably, but probably not the synthetic yield, due to solubility restrictions of the reactants.
••01 Jan 1992
TL;DR: This paper systematically treats the deviations from the model of Chen that may occur for bi-bi reactions obeying ping-pong or ternary complex kinetics for reactions with multiple substrates or products.
Abstract: The course of the kinetic resolution of a racemic compound by an enantioselective enzyme can often be described using Michaelis-Menten kinetics. This description (Chen et at., 1982, 1987) is formally not correct for reactions with multiple substrates or products. Van Tol et at. (1992) showed for the lipase-catalyzed resolution of glycidyl butanoate that the ping-pong kinetic mechanism has to be taken into account. This paper systematically treats the deviations from the model of Chen that may occur for bi-bi reactions obeying ping-pong or ternary complex kinetics. The course of the enantiomeric excess as a function of the degree of conversion was found to be dependent on two or three kinetic parameters (in contrast to the single E-value of Chen), on the thermodynamic equilibrium constant and on the ratio of initial concentrations of the reactants. This ratio can be used to some extent to manipulate the enantiomeric excess in a resolution process.
••01 Apr 2009
TL;DR: Several compounds that are formed or released during hydrolysis of lignocellulosic biomass inhibit the fermentation of the hydrolysate, and the use of a liquid extractive agent is suggested as a method for removal of these fermentation inhibitors.
Abstract: Several compounds that are formed or released during hydrolysis of lignocellulosic biomass inhibit the fermentation of the hydrolysate. The use of a liquid extractive agent is suggested as a method for removal of these fermentation inhibitors. The method can be applied before or during the fermentation. For a series of alkanes and alcohols, partition coefficients were measured at low concentrations of the inhibiting compounds furfural, hydroxymethyl furfural, vanillin, syringaldehyde, coniferyl aldehyde, acetic acid, as well as for ethanol as the fermentation product. Carbon dioxide production was measured during fermentation in the presence of each organic solvent to indicate its biocompatibility. The feasibility of extractive fermentation of hydrolysate was investigated by ethanolic glucose fermentation in synthetic medium containing several concentrations of furfural and vanillin and in the presence of decanol, oleyl alcohol and oleic acid. Volumetric ethanol productivity with 6 g/L vanillin in the medium increased twofold with 30% volume oleyl alcohol. Decanol showed interesting extractive properties for most fermentation inhibiting compounds, but it is not suitable for in situ application due to its poor biocompatibility.
TL;DR: It is shown that kinetic parameters for both enantiomers and the enantioselectivity of the enzyme may be obtained from the progress curve measurement of a racemate only.
Abstract: The present study deals with kinetic modeling of enzyme-catalyzed reactions by integral progress curve analysis, and shows how to apply this technique to kinetic resolution of enantiomers. It is shown that kinetic parameters for both enantiomers and the enantioselectivity of the enzyme may be obtained from the progress curve measurement of a racemate only.A parameter estimation procedure has been established and it is shown that the covariance matrix of the obtained parameters is a useful statistical tool in the selection and verification of the model structure. Standard deviations calculated from this matrix have shown that progress curve analysis yields parameter values with high accuracies.Potential sources of systematic errors in (multiple) progress curve analysis are addressed in this article. Amongst these, the following needed to be dealt with: (1) the true initial substrate concentrations were obtained from the final amount of product experimentally measured (mass balancing); (2) systematic errors in the initial enzyme concentration were corrected by incorporating this variable in the fitting procedure as an extra parameter per curve; and (3) enzyme inactivation is included in the model and a first-order inactivation constant is determined.Experimental verification was carried out by continuous monitoring of the hydrolysis of ethyl 2-chloropropionate by carboxylesterase NP and the alpha-chymotrypsin-catalyzed hydrolysis of benzoylalanine mathyl ester in a pH-stat system. Kinetic parameter values were obtained with high accuracies and model predictions were in good agreement with independent measurements of enantiomeric excess values or literature data. (c) 1994 John Wiley & Sons, Inc.
TL;DR: The diffusion of a solute, fluorescein into lysozyme protein crystals has been studied by confocal laser scanning microscopy and the diffusion process is found to be anisotropic, and the level of anisotropy depends on the crystal morphology.
Abstract: The diffusion of a solute, fluorescein into lysozyme protein crystals has been studied by confocal laser scanning microscopy (CLSM). Confocal laser scanning microscopy makes it possible to non-invasively obtain high-resolution three-dimensional (3-D) images of spatial distribution of fluorescein in lysozyme crystals at various time steps. Confocal laser scanning microscopy gives the fluorescence intensity profiles across horizontal planes at several depths of the crystal representing the concentration profiles during diffusion into the crystal. These intensity profiles were fitted with an anisotropic model to determine the diffusivity tensor. Effective diffusion coefficients obtained range from 6.2 × 10−15 to 120 × 10−15 m2/s depending on the lysozyme crystal morphology. The diffusion process is found to be anisotropic, and the level of anisotropy depends on the crystal morphology. The packing of the protein molecules in the crystal seems to be the major factor that determines the anisotropy. © 2004 Wiley Periodicals, Inc.
TL;DR: The story of kinetic resolution from the early discoveries through fascinating historical milestones and conceptual developments is followed, and a focus on modern techniques that maximize efficiency is closed.
Abstract: The Walden memorial at the Technical University in Riga is pictured in the frontispiece to mark the recent centennial of the Walden inversion. This is a rare public monument to key events from the first era of exploration in stereocontrolled synthesis, and may be the only such monument to use the language of organic chemistry expressed at the molecular level. The reaction of racemic substrates with chiral nucleophiles is one of many methods currently known to achieve kinetic resolution, a phenomenon that ranks as the oldest and most general approach for the synthesis of highly enantioenriched substances. The first nonenzymatic kinetic resolutions as well as the original forms of the Walden inversion were studied in the 1890s. All of these investigations were conducted within the first generation following the demonstration that carbon is tetrahedral, and provided abundant evidence that the principles and importance of enantiocontrolled syntheses were understood. However, a reliable, rapid technique to quantify results and guide the optimization process was still lacking. Many decades passed before this problem was solved by the advent of HPLC and GLPC assays on chiral supports, which stimulated explosive growth in the synthesis of nonracemic substances by kinetic resolution. The Walden monument is accessible to passers-by for hands-on inspection as well as for contemplation and learning. In a similar way, kinetic resolution is experimentally accessible and can be thought-provoking at several levels. We follow the story of kinetic resolution from the early discoveries through fascinating historical milestones and conceptual developments, and close with a focus on modern techniques that maximize efficiency.
TL;DR: Production of 5-hydroxymethylfurfural and furfural from lignocellulosic biomass was studied in ionic liquid in the presence of CrCl(3) under microwave irradiation to facilitate energy-efficient and cost-effective conversion of biomass into biofuels and platform chemicals.
TL;DR: This work aims to consolidate the various findings from previous works in electrochemical, biological and metal extraction applications of ionic liquids and their use in batteries, solar panels, fuel cells, drug deliveries and biomass pretreatments.
Abstract: Ionic liquids have recently gained popularity in the scientific community owing to their special properties and characteristics. One of the reasons why ionic liquids have been termed "green solvents" is due to their negligible vapour pressure. Their use in electrochemical, biological and metal extraction applications is discussed. Wide research has been carried out for their use in batteries, solar panels, fuel cells, drug deliveries and biomass pretreatments. This work aims to consolidate the various findings from previous works in these areas.
TL;DR: Future directions are indicated including application of the ISPR approach to a wider range of product groups and the development of novel, more specific ISPR methodologies, applicable under sterile conditions in the immediate vicinity of the producing cells.
Abstract: In situ product removal (ISPR) is the fast removal of product from a producing cell thereby preventing its subsequent interference with cellular or medium components. Over the past 10 years ISPR techniques have developed substantially and its feasibility (with improvements in yield or productivity) for several processes demonstrated. Assessment of progress, however, compared to the potential benefits inherent in the ISPR approach to bioprocessing reveals that these are far from being exploited fully. Here we indicate future directions including application of the ISPR approach to a wider range of product groups and the development of novel, more specific ISPR methodologies, applicable under sterile conditions in the immediate vicinity of the producing cells. General guidelines for adaptation of an appropriate ISPR approach for a given product are also provided.