969 research outputs found
Oxidation of azaheterocycles by free and immobilized xanthine oxidase and xanthine dehydrogenase
The objective of the study described in this thesis was to show that enzymes, especially immobilized enzymes, can be advantageously used in synthetic organic chemistry.In Chapter 1 enzymes are introduced and the advantages discussed when these highly active and specific biocatalysts are immobilized, i.e. , attached to a solid support. Criteria for general acceptation of a specific enzyme as a routine catalyst in the organic-chemistry laboratory are stated. The reasons for the choice of xanthine oxidase as a model enzyme and some relevant properties are given.The solid support used for the immobilization determines to a large extent the ultimate characteristics of the immobilized enzyme. In Chapter 2 properties of an ideal support and guidelines for the evaluation of a specific support arediscussed. Various supports are treated in some detail.The mode of immobilization is also very important, especially with respect to the eventual specific activity of the immobilized enzyme. In Chapter 3 the various methods of immobilization are discussed. Just as with the supports (Chapter 2), special attention is given to the ones investigated in this study.In Chapter 4 various properties of immobilized milk xantine oxidase are described, as well as the stabilization by coimmobilization with protein, superoxide dismutase and catalase. It is concluded that conversion of substrate on a preparativescale can be performed conveniently, but that for efficient application a more stable enzyme preparation is desired.Chicken-liver xanthine dehydrogenase is an enzyme very similar to milk xanthine oxidase. In Chapter 5 the reasons are given why it was expected that the liver enzyme would be more stable and practical for synthetic application. The immobilization and the properties of the free and immobilized enzyme are described. Analogous to the milk enzyme, the operational stability of xanthine dehydrogenase is much lower than the storage stability.In order to determine the substrate limits of xanthine oxidase more systematical ly, so that more-reliable predictions would be possible, series of substrates were synthesized for this purpose. In Chapter 6 the synthesis of 7-(p-X-phenyl) pteridin-4-ones and the oxidation by free and immobilized xanthine oxidase are presented. As X becomes more electron withdrawing, the rate of oxidation decreases. It is therefore concluded that the electron donation from substrate to enzyme must be the rate-limiting step.Convenient oxidation of the above substrates and easy product isolation was possible on a small preparative scale. The limited stability of xanthine oxidase demanded however a relatively large amount of enzyme. In Chapter 7 an easy and mild immobilization procedure, using gelatin as support and glutaraldehyde as cross-linking agent, is presented. Whole milk can be used as starting enzyme solution and no isolation is required. The resulting preparation of immobilized xanthine oxidase is very cheap, highly active, relatively stable and suitable for application in organic synthesis in a continuous manner.In Chapter 8 the improvement of the immobilization method with gelatin is described and the extension to the immobilization of whole cells, i.e. , Arthrobacter cells containing xanthine-oxidase activity. These cells were chosen because of their high specific activity, their substrate activation instead of inhibition and their expected higher stability. Immobilized Arthrobacter xanthineoxidase is indeed highly active, more stable and has a different substrate specificity than milk xanthine oxidase. Therefore, a wider spectrum of substrates can be conveniently and efficiently oxidized.In the final Chapter 9 additional information, mostly gathered at a later stage than the time of publication, is given and discussed.The end conclusion is: Xanthine oxidase, either from whole milk or Arthrobacter, when immobilized in glutaraldehyde-crosslinked gelatin, can be conveniently used for the oxidation of a wide range of substrates in a continuous fashion.<p/
Accommodating the difference in students’ prior knowledge of cell growth kinetics
This paper describes the development and benefits of an adaptive digital module on cell growth to tackle the problem of educating a heterogeneous group of students at the beginning of an undergraduate course on process engineering. Aim of the digital module is to provide students with the minimal level of knowledge on cell growth kinetics they need to comprehend the content knowledge of the subsequent lectures and pass the exam. The module was organised to offer the subject matter in a differentiated manner, so that students could follow different learning paths. Two student groups were investigated, one consisting of students who had received their prior education abroad and one of students that had not. Exam scores, questionnaires, and logged user data of the two student groups were analysed to discover whether the digital module had the intended effect. The results indicate that students did indeed follow different learning paths. Also, the differences in exam scores between the two student groups that was present before the introduction of the digital module was found to have decreased afterwards. In general, students appreciated the use of the material regardless of their prior education. We therefore conclude that the use of adaptive digital learning material is a possible way to solve the problem of differences in prior education of students entering a course
The influence of student characteristics on the use of adaptive e-learning material
Adaptive e-learning materials can help teachers to educate heterogeneous student groups. This study provides empirical data about the way academic students differ in their learning when using adaptive elearning materials. Ninety-four students participated in the study. We determined characteristics in a heterogeneous student group by collecting demographic data and measuring motivation and prior knowledge. We also measured the learning paths students followed and learning strategies they used when working with adaptive e-learning material in a molecular biology course. We then combined these data to study if and how student characteristics relate to the learning paths and strategies they used. We observed that students did follow different learning paths. Gender did not have an effect, but (mainly Dutch) BSc students differed from (international) MSc students in the intrinsic motivation they had and the learning paths and strategies they followed when using the adaptive e-learning materia
Development and Evaluation of an Adaptive Digital Module on Enzyme Kinetics
An adaptive module on basic enzyme kinetics was developed for first- and second-year university students. The module offers more assignments to students who have less knowledge of the theory than to more advanced students. The aim of the research was to investigate what influence students’ backgrounds have on their use and appreciation of this module. Both freshmen and second-year students showed a large variation in the number of assignments they needed to perform in order to finish the module, indicating that the module’s adaptive feature was exploited by all the students. Findings indicated that the prior knowledge was of influence of students’ motivation and perception of difficulty of the modul
The properties of single WO stars
The enigmatic oxygen sequence Wolf-Rayet (WO) stars represent a very late
stage in massive star evolution, although their exact nature is still under
debate. The spectra of most of the WO stars have never been analysed through
detailed modelling with a non-local thermodynamic equilibrium expanding
atmosphere code. Here we present preliminary results of the first homogeneous
analysis of the (apparently) single WOs.Comment: 2 pages, 1 figure, Proceedings IAU Symposium No. 307, 2014, 'New
windows on massive stars: asteroseismology, interferometry, and
spectropolarimetry
Low-metallicity massive single stars with rotation. II. Predicting spectra and spectral classes of chemically-homogeneously evolving stars
Context. Metal-poor massive stars are supposed to be progenitors of certain
supernovae, gamma-ray bursts and compact object mergers, potentially
contributing to the early epochs of the Universe with their strong ionizing
radiation. However, they remain mainly theoretical as individual spectroscopic
observations of such objects have rarely been carried out below the metallicity
of the SMC.
Aims. This work aims at exploring what our state-of-the-art theories of
stellar evolution combined with those of stellar atmospheres predict about a
certain type of metal-poor (0.02 Z) hot massive stars, the chemically
homogeneously evolving ones, called TWUIN stars.
Methods. Synthetic spectra corresponding to a broad range in masses (20-130
M) and covering several evolutionary phases from the zero-age
main-sequence up to the core helium-burning stage were computed.
Results. We find that TWUIN stars show almost no emission lines during most
of their {core hydrogen-burning} lifetimes. Most metal lines are completely
absent, including nitrogen. During their core helium-burning stage, lines
switch to emission and even some metal lines (oxygen and carbon, but still
almost no nitrogen) show up. Mass loss and clumping play a significant role in
line-formation in later evolutionary phases, particularly during core
helium-burning. Most of our spectra are classified as an early O type giant or
supergiant, and we find Wolf-Rayet stars of type WO in the core helium-burning
phase.
Conclusions. An extremely hot, early O type star observed in a
low-metallicity galaxy could be the outcome of chemically homogeneous evolution
and therefore the progenitor of a long-duration gamma-ray burst or a type
Ic supernova. TWUIN stars may play an important role in reionizing the Universe
due to their being hot without showing prominent emission lines during the
majority of their lifetimes.Comment: Accepted by Astronomy and Astrophysics. In Pres
Time-Integrated Position Error Accounts for Sensorimotor Behavior in Time-Constrained Tasks
Several studies have shown that human motor behavior can be successfully described using optimal control theory, which describes behavior by optimizing the trade-off between the subject's effort and performance. This approach predicts that subjects reach the goal exactly at the final time. However, another strategy might be that subjects try to reach the target position well before the final time to avoid the risk of missing the target. To test this, we have investigated whether minimizing the control effort and maximizing the performance is sufficient to describe human motor behavior in time-constrained motor tasks. In addition to the standard model, we postulate a new model which includes an additional cost criterion which penalizes deviations between the position of the effector and the target throughout the trial, forcing arrival on target before the final time. To investigate which model gives the best fit to the data and to see whether that model is generic, we tested both models in two different tasks where subjects used a joystick to steer a ball on a screen to hit a target (first task) or one of two targets (second task) before a final time. Noise of different amplitudes was superimposed on the ball position to investigate the ability of the models to predict motor behavior for different levels of uncertainty. The results show that a cost function representing only a trade-off between effort and accuracy at the end time is insufficient to describe the observed behavior. The new model correctly predicts that subjects steer the ball to the target position well before the final time is reached, which is in agreement with the observed behavior. This result is consistent for all noise amplitudes and for both tasks
The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries
The initial distribution of spin rates of massive stars is a fingerprint of
their elusive formation process. It also sets a key initial condition for
stellar evolution and is thus an important ingredient in stellar population
synthesis. So far, most studies have focused on single stars. Most O stars are
however found in multiple systems. By establishing the spin-rate distribution
of a sizeable sample of O-type spectroscopic binaries and by comparing the
distributions of binary sub-populations with one another as well as with that
of presumed single stars in the same region, we aim to constrain the initial
spin distribution of O stars in binaries, and to identify signatures of the
physical mechanisms that affect the evolution of the massive stars spin rates.
We use ground-based optical spectroscopy obtained in the framework of the
VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial
rotational velocities (\vrot) for components of 114 spectroscopic binaries in
30 Doradus. The \vrot\ values are derived from the full-width at half-maximum
(FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we
derive based on previous line analysis methods applied to single O-type stars
in the VFTS sample. The overall \vrot\ distribution of the primary stars
resembles that of single O-type stars in the VFTS, featuring a low-velocity
peak (at \vrot < 200 kms) and a shoulder at intermediate velocities (200 <
\vrot < 300 kms). The distributions of binaries and single stars however
differ in two ways. First, the main peak at \vrot \sim100 kms is broader and
slightly shifted toward higher spin rates in the binary distribution compared
to that of the presumed-single stars. Second, the \vrot distribution of
primaries lacks a significant population of stars spinning faster than 300 kms
while such a population is clearly present in the single star sample.Comment: 16 pages, 16 figures, paper accepted in Astronomy & Astrophysic
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