bluuebunny/crossref_metadata_2025_split huggingface dataset:
Controlling enzyme‐catalyzed regioselectivity in sugar ester synthesis
Authors: Rich Joseph O. , Bedell Bruce A. , Dordick Jonathan S. 
Published: December 2004

AbstractThe rational control over enzyme‐catalyzed regioselectivity has been studied using sucrose acylation by vinyl esters in organic media as a model. Subtilisins BPN' and Carlsberg preferentially acylate at the 1′‐hydroxyl of sucrose with some acylation observed at the 6‐hydroxyl. The preference for the 1′‐hydroxyl is strongly affected by the hydrophobicity of the organic solvent and the chain length of the vinyl ester. Increasingly hydrophobic solvents and longer chain lengths lower the favorable formation of the 1′‐acylation and improve 6‐acylation. Molecular modeling of sucrose in the binding pocket of subtilisin BPN' shows that the 1′‐acylation is favored in solvents that can solvate sugars (such as pyridine) as the glucose moiety is exposed to the medium, whereas 6‐acylation leaves the entire sucrose molecule buried within the enzyme's binding pocket. Thus, 1′‐acylation is sterically more favorable than 6‐acylation. Increasingly hydrophobic solvents affect regioselectivity by changing the degree of solvation of the glucose moiety in the medium and forcing the sucrose 1′‐ester completely into the binding pocket. In a related modeling, the vinyl ester chain length was shown to modulate regioselectivity by controlling the bond angles between the resulting acylenzymes and the sucrose thereby affecting the positioning of the sucrose in the binding pocket of subtilisin BPN'. This study shows that control over enzymic regioselectivity can be achieved by rational choices of substrate and solvent. © 1995 John Wiley & Sons, Inc.
Transformation capacities of chlorinated organics by mixed cultures enriched on methane, propane, toluene, or phenol
Authors: Chang Hsiao‐Lung , Alvarez‐Cohen Lisa 
Published: December 2004

AbstractThe degradation of trichloroethylene (TCE), chloroform (CF), and 1,2‐dichloroethane (1,2‐DCA) by four aerobic mixed cultures (methane, propane, toluene, and phenol oxidizers) grown under similar chemostat conditions was measured. Methane and propane oxidizers were capable of degrading both saturated and unsaturated chlorinated organics (TCE, CF, and 1,2‐DCA). Toluene and phenol oxidizers degraded TCE but were not able to degrade CF, 1,2‐DCA, or other saturated organics. None of the cultures tested were able to degrade perchloroethylene (PCE) or carbon tetrachloride (CC4). For the four cultures tested, degradation of each of the chlorinated organics resulted in cell inactivation due to product toxicity. In all cases, the toxic products were rapidly depleted, leaving no toxic residues in solution. Among the four tested cultures, the resting cells of methane oxidizers exhibited the highest transformation capacities (Tc) for TCE, CF, and 1,2‐DCA. The Tc for each chlorinated organic was observed to be inversely proportional to the chlorine carbon ratio (Cl/C). The addition of low concentrations of growth substrate or some catabolic intermediates enhanced TCE transformation capacities and degradation rates, presumably due to the regeneration of reducing energy (NADH); however, addition of higher concentrations of most amendments reduced TCE transformation capacities and degradation rates. Reducing energy limitations and amendment toxicity may significantly affect Tc measurements, causing a masking of the toxicity associated with chlorinated organic degradation. © 1995 John Wiley & Sons, Inc.
Aqueous extraction of sugarcane bagasse hemicellulose and production of xylose syrup
Authors: Saska M. , Ozer E. 
Published: December 2004

AbstractAt the optimum level of severity, the aqueous extraction of sugarcane bagasse, an abundant agricultural resdue, gave, depending on the degree of comminution, 60% to 89% yield of xylose, most of it in the form of a water soluble xylan. A process for producing xylose‐rich syrups was conceived and tested, consisting of aqueous extraction, acid hydrolysis of the concentrated aqueous extract centrifugal clarification of the hydrolysate, and recovery of the acid by continuous ion exclusion. The cost estimate indicates operating costs on the order of $0.12 to $0.15/kg xylose, in the form of xylose‐rich molasses. © 1995 John Wiley & Sons, Inc.
Studies on the lipozyme‐catalyzed synthesis of butyl laurate
Authors: Gandhi Neena N. , Sawant Sudhirprakash B. , Joshi Jyeshtharaj B. 
Published: December 2004

AbstractThe effects of temperature, speed of agitation, enzyme concentration, etc., on butyl laurate synthessis using Mucor miehei lipase (Lipozyme™) have been studied. Although the soluble enzyme was quite thermcstable in aqeous solution, it deactivated rapidly at and above 40°C in the presence of butanol. This enzyme immobilized on an anion‐exchange resin (Lipozyme™) showed enhanced stability (as compared to the soluble form) to denaturation by butanol under the same conditions. The denaturation of M. miehei lipase was found to be a function of the butanol concentration in the aqueous phase, and rapid denaturation takes place at the concentration corresponding to its saturation at that temperature. © 1995 John Wiley & Sons, Inc.
Assessment of a disc stack centrifuge for use in mammalian cell separation
Authors: Kempken R. , Preissmann A. , Berthold W. 
Published: December 2004

AbstractA prototype disc stack centrifuge was tested for the separation of mammalian cell cultures from 80‐ and 2000‐L fermentations. The clarification capacity for mammalian cells was excellent, but some smaller particles remained in the supernatant and reduced its usefulness for downstream processing. In order to identify the source of such particle formation, several parameters were assessed and minimum particle size for separation was calculated. An analysis of particle distribution was performed. Temperature and pressure effects inside the centrifuge bowl were measured. Some modifications of mechanical engineering can be suggested for the improvement of the use of standard disc stack centrifuges for mammalian cells. © 1995 John Wiley & Sons, Inc.
A cometabilic kinetics model incorporating enzyme inhbition, inactivation, and recovery: I. Model development, analysis, and testing
Authors: Ely Roger L, , Williamson Kenneth J. , Guenther Ronald B. , Hyman Michael R. , Arp Daniel J. 
Published: December 2004

AbstractCometabolic biodegradation prcesses are important for bioremediation of hazardous waste sites. However, these proceeses are not well understood and have not been modeled thoroughly. Traditional Michaelis–Menten kinetics models often are used, but toxic effects and bacterial responses to toxicity may cause changes in enzyme levels, rendering such models inappropriate. In this article, a conceptual and mathematical model of cometabolic enzyme kinetics i described. Model derivation is based on enzyme/growth‐substrate/nongrowth‐substrate interaction and incorporates enzyme inhibition (caused by the presence of a cometabolic compound), inactivation (resulting from toxicity of a cometabolic product), and recovery (associated with bacterial synthesis of new enbzyme in response to inactivation). The mathematical model consists of a system of two, nonlinear ordinary differential equations that can be solved implicitly using numerical methods, providing estimates of model parameters. Model analysis shows that growth substraate adn nongrowth substrate oxidation rates are related by a dimensionless constant. Reliability of tehy model solution prcedure is verifiedl by abnalyzing data ses, containing random error, from simulated experimentss with trichhloroethyylene (TCE) degradation by ammonia‐oxidizing bacterialunder various conditions. Estimation of the recovery rate contant is deterimined to be sensitive to intial TCE concentration. Model assumptions are evaluated in a companion article using data from TCE degradation experiments with amoniaoxidizing bacteria. © 1995 John Wiley & Sons, Inc.
Selective separation of tryptic β‐casein peptides through ultrafiltration membranes: Influence of ionic interactions
Authors: Nau F. , Kerhervé F. L. , Leonil J. , Daufin G. 
Published: December 2004

AbstractPeptide separation by selective membrane filtration has numerous potential applications such as production of peptides with biological activities or spectific enrichment in compounds acting as flavoring agents or as growth factors required by the fermentation industry. The retention of peptides arising from tryptic hdroysis of β‐casein using an M5 Carbosep membrane (molecular wieght cutoll = 10,000 D) has been studied. The peptides with known sequences were characterized by their molecular weight, isoelectric point, and hydrophobicity. Our experiments highlighted that their transmission involves mechanisms other than size exclusion as developed elsewhere. The effect of ionic interactions between peptides and membrance has been investgated by vrying pH, ionic strength of bulk, and electric potential of filtering material. The charge of both peptides and membrane plays an important role in the transmission, particularly with small size and high or lkow isoelectric point. Then, peptides with the same sign as the membrane have lower transmission than expected from the size xclusion model, whereas peptides with opposite sign have higher trnsmission than expected, and even higher than 1 with some of them. © 1995 John Wiley & Sons, Inc.
A kinetic model for product formation of microbial and mammalian cells
Authors: Zeng An‐Ping 
Published: December 2004

AbstractGrowth of microbial and mammalian cells can be classified into substrate‐limited and substrate‐sufficient growth according to the relative availability of the substrate (carbon and energy source) and other nutrients. It has been observed for a number of microbial and mammalian cells that the consumption rate of substrate and energy (ATP) is generally higher under substratesufficient conditions than under substrate limitation. Accordingly, the product formation under substrate excess often exhibits different patterns from those under substrate limitation. The extent of increase or decrease in product formation may depend not only on the nature of limitation and cell growth rate but also on the residual substrate concentration in a relatively wide range. The product formation kinetic models existing in literature cannot describe these effects. In this study, the Luedeking–Piret kinetic is extended to include a term describing the effect of residual substrate concentration. The extended model has a similar structure to the kinetic model for substrate and energy consumption rate recently proposed by Zeng and Deckwer. The applicability of the extended model is demonstrated with three microbial cultures for the production of primary metabolites and three hybridoma cell cultures for the production of ammonia and lactic acid over a wide range of substrate concentration. The model describes the product formation in all these cultures satisfactorily. Using this model, the range of residual substrate concentration, in which the product formation is affected, can be quantitatively assessed. © 1995 John Wiley & Sons, Inc.
Competition between nitrate and nitrite reduction in denitrification by Pseudomonas fluorescens
Authors: Almeida J. S. , Reis M. A. M. , Carrondo M. J. T. 
Published: December 2004

AbstractA pure culture of Pseudomonas fluorescens was used as a model system to study the kinetics of denitrification. An exponentially growing culture was harvested and resuspended in an anoxic acetate solution buffered with K/Na phosphate at pH values of 6.6, 7.0, 7.4, and 7.8. The temperature was kept at 28°C in all assays. Nitrate pulses of approximately 0.2 mg N/L caused nitrite to accumulate due to a faster rate of nitrate reduction over nitrite reduction. The rate of nitrate reduction was observed to depend on its concentration as predicted by the Michaelis–Menten equation. At nonlimiting nitrate concentrations, nitrite reduction was described by the same equation. Otherwise, nitrite reduction also depended on nitrate concentration. Consequently, nitrate and nitrite reductions compete with each other for the oxidation of common electron donors. A kinetic model for nitrate competitive inhibition of nitrite reduction is proposed. The model was used to interpret the nitrate and nitrite profiles observed at the four pH values: the optimum pH value was 7.0 in both cases; the affinity for nitrite was also not affected by the medium pH in the range of values 6.6 to 7.4 (K = 0.04 mg N/L); the affinity for nitrite was also not affected by the medium pH in the range of values 6.6 to 7.4 (K = 0.06 mg N/L), but it decreased sharply for the pH value of 7.8. Although the ratio between the two maximum reduction rates (V/V) is constant, nitrite accumulation depends on the medium pH value. Therefore, the regulation mechanism that shifts the electron flow between the two terminal reductases is readily reversible and does not change their relative maximum reduction rates. © 1995 John Wiley & Sons, Inc.
Growth kinetics of glucose‐limited petunia hybrida cells in chemostat cultures: Determination of experimental values for growth and maintenance parameters
Authors: de Gucht Luuk P. E. , van der Plas Linus H. W. 
Published: December 2004

AbstractWith glucose‐limited continuous cultures of Petunia hybrida six steady states were obtained at specific growth rates varying from 0.0035 to 0.012 h−1 (corresponding with culture residence times varying from 285 to 85 h). The macromolecular and the elemental biomass composition which were determined in four steady states showed no major differences over the range of growth rates examined. During all six steady states specific subtrate and oxygen consumption as well as biomass and extracellular product formation rates were monitored. Moreover the specific activities of the mitochondrial cytochrome and alternative pathway were determined and used to estimate specific adenosine triphosphate (ATP) production rates. Data thus obtained were used in the determination of maintenance and true growth yield parameters. For the maintenance on glucose and ATP values of 0.0070 C‐mol/C‐mol/h and 0.034 mol/C‐mol/h were obtained, respectively. True yields of biomass on glucose and ATP were 0.50 C‐mol/C‐mol and 0.28 C‐mol/mol, respectively. © 1995 John Wiley & Sons, Inc.
Two‐stage batch process for the production of ajmalicine by Catharanthus roseus: The link between growth and production stage
Authors: Schlatmann J. E. , Moreno P. R. H. , Sellés M. , Vinke J. L. , ten Hoopen H. J. G. , Verpoorte R. , Heijnen J. J. 
Published: December 2004

AbstractThe link between the growth stage and the production stage in a two‐stage batch process was investigated using (filtered) inocula from different periods of the stationary phase of the growth cycle. In the production stage, ajmalicine production by Catharanthus roseus in a 3‐L stirred tank reactor was induced with a high glucose concentration (80 g/L). Ajmalicine production in cultures started with cells from the late stationary phase was five times higher than in cultures started with cells from the early stationary phase. After transfer to the production stage, cells from the early stationary phase showed a transient increase in respiration and enzyme induction, followed by culture browning. In contrast, cells in the late stationary phase showed a typical induction pattern: constant respiration, and permanent enzyme induction. A striking similarity between the geraniol‐10‐hydroxylase (G10H) activity and the ajmalicine accumulation profile could be observed in all cultures, suggesting that G 10H regulated ajmalicine production in this investigation. The intracellular nitrate concentration was significantly higher in the inoculum showing a high ajmalicine production than in the inoculum with a low production. Consequently, nitrate may act as a marker for the start of the production stage: as soon as the nitrate is depleted in the growth medium secondary metabolism can be induced. © 1995 John Wiley & Sons, Inc.
In situ microscopy for on‐line characterization of cell‐populations in bioreactors, including cell‐concentration measurements by depth from focus
Authors: Suhr H. , Wehnert G. , Schneider K. , Bittner C. , Scholz T. , Geissler P. , Jähne B. , Scheper T. 
Published: December 2004

AbstractA new technique is presented which allows the use of a front‐end sensor head for in situ and on‐line characterization of cell concentration and cell size during fermentation. An epifluorescence microscope is mounted in a port of a bioreactor viewing directly into the agitated broth. Still images from cells are generated using pulsed illumination. They are directly visualized on a monitor and used for automatic image analysis. The cell concentration and morphological information are determined by counting and evaluating the cell images with respect to their depth from focus characteristic. An in situ microscope was successfully tested during yeast fermentations and yielded results which correlated well with results from a hemocytometer. © 1995 John Wiley & Sons, Inc.
Biological sulphide oxidation in a fed‐batch reactor
Authors: Janssen A. J. H. , Sleyster R. , van der Kaa C. , Jochemsen A. , Bontsema J. , Lettinga G. 
Published: December 2004

AbstractThis study shows that, in a sulphide‐oxidizing bioreactor with a mixed culture of Thiobacilli, the formation of sulphur and sulphate as end‐products from the oxidation of sulphide can be controiledinstantaneously and reversibiy by the amount of oxygen supplied. It was found that at sulphide loading rates of up to 2.33 mmol7/L · h, both products can be formed already at oxygen concentrations below 0.1 mg/L. Because the microorganisms tend to form sulphate rather than forming sulphur, the oxygen concentration is not appropriate to optimize the sulphur production. Within less than 2 h, the system can be switched reversibly from sulphur to sulphate formation by adjusting the oxygen flow. This is below the minimum doubling time (2.85 h) of, e.g., Thiobacillus neapolitanus and Thiobacillus 0,18 which indicates that one metabolic type of organism can probably perform both reactions. Under highly oxygen‐limited circumstances, that is, at an oxygen/sulphide consumption ratio below 0.7 mol · h−1 mol · h−1 thiosulphate is abundantly formed. Because the chemical sulphide oxidation results mainly in the formation of thiosulphate, it is concluded that, under these circumstances, the biological oxidation capacity of the system is lower than the chemical oxidation capacity. The oxidation rate of the chemical sulphide oxidation can be described by a first‐order process (k =−0.87 h−1).© 1995 John Wiley & Sons, Inc
The effective diffusion coefficient and the distribution constant for small molecules in calcium‐alginate gel beads
Authors: Øyaas Jorun , Storrø Ivar , Svendsen Hallvard , Levine David W. 
Published: December 2004

AbstractThe effective diffusion coefficient, De, and the distribution constant, Ki, for selected mono‐ and disaccharides and organic acids were determined in homogeneous calcium‐alginate gel with and without entrapped bacteria. Results were obtained from transient concentration changes in well‐stirred solutions of limited volume, in which the gel beads were suspended. The effective diffusioncoefficients and the distribution constants were estimated by fitting mathematical model predictions to the experimental data using a nonlinear model fitting program (MODFIT). Both single solute diffusion and multiple solute diffusion were performed. A small positive effect was obtained onthe values of De for the system of multiple solute diffusion; however, the values of Ki were not significantly influenced. For the nine solutes tested, De for 2% Ca‐alginate gel beads was found to be approximately 85% of the diffusivity measured in water. The effects on De and Ki, for lactose and lactic acid were determined for variations of alginate concentration, pH, temperature, and biomass content in the beads. De decreased linearly for both lactose and lactic acid with increasing cell concentration in the Ca‐alginate gel. Ki, was constant for both lactose and lactic acid with increasing cell concentration. De was significantly lower at pH 4.5 than at pH 5.5 and 6.5 for both lactose and lactic acid. Furthermore, De seemed to decrease with increased alginate concentration in the range of 1% to 4%. The diffusion rate increased with increasing temperature, and the activation energy for the diffusion process for both lactose and lactic acid was constant in the temperature range tested. © 1995 John Wiley & Sons Inc.
Nonlinear estimation of specific growth rate for aerobic fermentation processes
Authors: Cazzador Lorenzo , Lubenova Vellislava 
Published: December 2004

AbstractThe specific growth rate of the biomass, a very important parameter of almost every fermentation process, cannot be measured directly or estimated from related variables, as the concentrations of biomass, substrates, or products, due to the lack of reliable and cheap sensors. In this article a stable adaptive estimator of the specific growth rate is designed for those aerobic processes where the measurement of the oxygen uptake rate is available on‐line. This particular approach can be applied also for other reaction rates if the model of the process satisfies some very general assumptions, which make the dynamics of the measured reaction rate a nonlinear function only of two unknown parameters, the specific growth rate and its time derivative. With respect to a previous similar approach, the new estimator has one additional parameter and a different nonlinear structure. From the analysis of the dynamics of the estimation error, a tuning criterion is derived, by which the two different algorithms can be compared under similar conditions. Simulation results show a good performance of both estimators for various kind of processes and disturbances. © 1995 John Wiley & Sons, Inc.
Hydraulic resistance and fouling of microfilters by Candida utilis in fermentation broth
Authors: Liew M. K. H. , Fane A. G. , Rogers P. L. 
Published: December 2004

AbstractThe hydraulic resistance and membrane fouling effects of Candida utilis in fermentation broth were investigated using Millipore PVDF 0.22‐μm membranes (GVWP and GVHP) in a stirred‐cell system at 50 kPa and 700 rpm. With the various components of broth, spent medium, which contains colloidal particles and macromolecules having sizes (0.32 to 2.67 μm) comparable with the membrane pores (actual range 0.26 to 0.63 μm), was found to be the major contributing factor to the membrane fouling by broth through pore plugging. This led the spent medium to exhibit the highest hydraulic resistance (Rsm of 5.8E + 12 m−1) and percentage flux loss (81.0%) when compared with either intact cells alone in buffer or to whole broth. Intact cells appeared to physically block and protect the pores without significant adhesion, because of the relatively hydrophilic nature of their cell walls (hydrophobicity of 5.9% at hour 36), resulting in the lowest hydraulic resistance (Rsbc of 7.5E + 11m−1) and percentage flux loss (19.3%).However, the hydraulic resistance and percentage flux loss of broth increased as cells aged. This was attributed to the increase in particle loading (intact cells by 15.37%, released cell contents and cell fragments) and in the hydrophobicity of cell walls. Autoclaved broth, lysed broth and aged broth, which contained a larger portion of colloidal particles and released cell contents caused a more pronounced fouling effect. This was revealed by the absence of flux recovery after depressurization with continuous stirring, even when a hydrophilic membrane was used. Furthermore, the hydrophobicity of C. utilis was found to increase with yeast extract present in medium, and use of hydrophobic membranes helped enhance the fouling effect. Overall, the degree of irreversible membrane fouling could be revealed by the value of Rsm/Rt′ and the hydraulic resistance, which resulted from concentration polarrzation, could be revealed by the value of Rc/Rt′ where Rt = Rm + Rsm + Rc′ and Rm is the clean membrane resistance. © 1995 John Wiley & Sons, Inc.