Showing papers by "Delft University of Technology published in 1989"

Enzymic analysis of the crabtree effect in glucose-limited chemostat cultures of Saccharomyces cerevisiae.

Abstract: The physiology of Saccharomyces cerevisiae CBS 8066 was studied in glucose-limited chemostat cultures. Below a dilution rate of 0.30 h-1 glucose was completely respired, and biomass and CO2 were the only products formed. Above this dilution rate acetate and pyruvate appeared in the culture fluid, accompanied by disproportional increases in the rates of oxygen consumption and carbon dioxide production. This enhanced respiratory activity was accompanied by a drop in cell yield from 0.50 to 0.47 g (dry weight) g of glucose-1. At a dilution rate of 0.38 h-1 the culture reached its maximal oxidation capacity of 12 mmol of O2 g (dry weight)-1 h-1. A further increase in the dilution rate resulted in aerobic alcoholic fermentation in addition to respiration, accompanied by an additional decrease in cell yield from 0.47 to 0.16 g (dry weight) g of glucose-1. Since the high respiratory activity of the yeast at intermediary dilution rates would allow for full respiratory metabolism of glucose up to dilution rates close to mumax, we conclude that the occurrence of alcoholic fermentation is not primarily due to a limited respiratory capacity. Rather, organic acids produced by the organism may have an uncoupling effect on its respiration. As a result the respiratory activity is enhanced and reaches its maximum at a dilution rate of 0.38 h-1. An attempt was made to interpret the dilution rate-dependent formation of ethanol and acetate in glucose-limited chemostat cultures of S. cerevisiae CBS 8066 as an effect of overflow metabolism at the pyruvate level. Therefore, the activities of pyruvate decarboxylase, NAD+- and NADP+-dependent acetaldehyde dehydrogenases, acetyl coenzyme A (acetyl-CoA) synthetase, and alcohol dehydrogenase were determined in extracts of cells grown at various dilution rates. From the enzyme profiles, substrate affinities, and calculated intracellular pyruvate concentrations, the following conclusions were drawn with respect to product formation of cells growing under glucose limitation. (i) Pyruvate decarboxylase, the key enzyme of alcoholic fermentation, probably already is operative under conditions in which alcoholic fermentation is absent. The acetaldehyde produced by the enzyme is then oxidized via acetaldehyde dehydrogenases and acetyl-CoA synthetase. The acetyl-CoA thus formed is further oxidized in the mitochondria. (ii) Acetate formation results from insufficient activity of acetyl-CoA synthetase, required for the complete oxidation of acetate. Ethanol formation results from insufficient activity of acetaldehyde dehydrogenases.(ABSTRACT TRUNCATED AT 400 WORDS)

Density and uniqueness in percolation

Abstract: Two results on site percolation on thed-dimensional lattice,d≧1 arbitrary, are presented. In the first theorem, we show that for stationary underlying probability measures, each infinite cluster has a well-defined density with probability one. The second theorem states that if in addition, the probability measure satisfies the finite energy condition of Newman and Schulman, then there can be at most one infinite cluster with probability one. The simple arguments extend to a broad class of finite-dimensional models, including bond percolation and regular lattices.

The annealing algorithm

Abstract: 1 The Annealing Algorithm: A Preview.- 1.1 Combinatorial optimization.- 1.2 Moves and local minima.- 1.3 Hill climbing.- 1.4 Simulated annealing.- 1.5 Applications.- 1.6 Mathematical model.- 1.7 Discussion.- 2 Preliminaries from Matrix Theory.- 2.1 Matrices. Notation and basic properties.- 2.2 Pseudo-diagonal normal forms.- 2.3 Norms and limits of matrices.- 2.4 Quadratic forms.- 2.5 Discussion.- 3 Chains.- 3.1 Terminology.- 3.2 Linear arrangement, an example.- 3.3 The chain limit theorem.- 3.4 Reversible chains.- 3.5 Discussion.- 4 Chain Statistics.- 4.1 Density Functions.- 4.2 Expected values.- 4.3 Sampling.- 4.4 Maximum likelyhood densities.- 4.5 Aggregate functions.- 4.6 Discussion.- 5 Annealing Chains.- 5.1 Towards low scores.- 5.2 Maximal accessibility.- 5.3 The acceptance function.- 5.4 Properties of annealing chains.- 5.5 Discussion.- 6 Samples from Normal Distributions.- 6.1 Characteristic functions.- 6.2 Quadratic forms and characteristic functions.- 6.3 Sampling distributions.- 6.4 Asymptotic properties of sampling distributions.- 6.5 Discussion.- 7 Score Densities.- 7.1 The density of states.- 7.2 Weak control.- 7.3 Strong control.- 7.4 Three parameter aggregates.- 7.5 Discussion.- 8 The Control Parameter.- 8.1 Initialization.- 8.2 Decrements in the control parameter.- 8.3 A stop criterion.- 8.4 Proper convergence.- 8.5 Discussion.- 9 Finite-Time Behavior of the Annealing Algorithm.- 9.1 Rate of convergence of chains.- 9.2 Minimum number of iterations.- 9.3 Finite-time optimal schedules.- 9.4 Discussion.- 10 The Structure of the State Space.- 10.1 Chain convergence.- 10.2 The topography of the state space.- 10.3 The set of moves.- 10.4 Global convergence.- 10.5 Discussion.- 11 Implementation Aspects.- 11.1 An implementation.- 11.2 The selection function.- 11.3 Other speed-up methods.- References.

Brownian motion with a parabolic drift and airy functions

Abstract: Let {W(t): t ∈ ∝} be two-sided Brownian motion, originating from zero, and let V(a) be defined by V(a)=sup}t ∈ ∝: W(t)−(t−a)2 is maximal}. Then {V(a): a ∈ ℝ} is a Markovian jump process, running through the locations of maxima of two-sided Brownian motion with respect to the parabolas fa(t)=(t−a)2. We give an analytic expression for the infinitesimal generators of the processes a ∈ ℝ, in terms of Airy functions in Theorem 4.1. This makes it possible to develop asymptotics for the global behavior of a large class of isotonic estimators (i.e. estimators derived under order restrictions). An example of this is given in Groeneboom (1985), where the asymptotic distribution of the (standardized) L1-distance between a decreasing density and the Grenander maximum likelihood estimator of this density is determined. On our way to Theorem 4.1 we derive some other results. For example, we give an analytic expression for the joint density of the maximum and the location of the maximum of the process {W(t)−ct2: t ∈ ℝ}, where c is an aribrary positive constant. We also determine the Laplace transform of the integral over a Brownian excursion. These last results also have recently been derived by several other authors, using a variety of methods.

Low-voltage operational amplifier with rail-to-rail input and output ranges

Abstract: A bipolar operational amplifier (OA) with rail-to-rail input and output ranges which can operate at supply voltages down to 1 V is presented. At this supply voltage, the input offset voltage is typically 1.0 mV in an input common-mode voltage range that extends beyond both supply rails for about 300 mV, with a common-mode rejection ratio (CMRR) between 38 and 100 dB, depending on conditions. The output voltage can reach both supply rails within 100 mV, the output current is limited to +or-10 mA, the voltage gain is 100 dB, and the bandwidth is 450 kHz. The die is 2.5*5.5 mm/sup 2/. Qualities such as offset, input-bias current, and CMRR are improved when the supply voltage is increased and the dynamic level shift is autonomically turned off. The OA has been protected against unintentional reversal of the output signal when the inputs are substantially overdriven. The output stage of the circuit consists of two full complementary composite transistors, whose HF characteristics have been improved by internal Miller compensation and linearization of the transconductance. >

Coherent electron focusing with quantum point contacts in a two-dimensional electron gas

Abstract: Transverse electron focusing in a two-dimensional electron gas is investigated experimentally and theoretically for the first time. A split Schottky gate on top of a GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterostructure defines two point contacts of variable width, which are used as injector and collector of ballistic electrons. As evidenced by their quantized conductance, these are quantum point contacts with a width comparable to the Fermi wavelength. At low magnetic fields, skipping orbits at the electron-gas boundary are directly observed, thereby establishing that boundary scattering is highly specular. Large additional oscillatory structure in the focusing spectra is observed at low temperatures and for small point-contact size. This new phenomenon is interpreted in terms of interference of coherently excited magnetic edge states in a two-dimensional electron gas. A theory for this effect is given, and the relation with nonlocal resistance measurements in quantum ballistic transport is discussed. It is pointed out, and experimentally demonstrated, that four-terminal transport measurements in the electron-focusing geometry constitute a determination of either a generalized longitudinal resistance or a Hall resistance. At high magnetic fields the electron-focusing peaks are suppressed, and a transition is observed to the quantum Hall regime. The anomalous quantum Hall effect in this geometry is discussed in light of a four-terminal resistance formula.

Observation of zero-dimensional states in a one-dimensional electron interferometer.

Abstract: We have studied the electron transport in a one-dimensional electron interferometer. It consists of a disk-shaped two-dimensional electron gas, to which quantum point contacts are attached. Discrete zero-dimensional states are formed due to constructive interference of electron waves traveling along the circumference of the disk in one-dimensional magnetic edge channels. The conductance shows pronounced Aharonov-Bohm–type oscillations, with maxima occurring whenever the energy of a zero-dimensional state coincides with the Fermi energy. Good agreement with theory is found, taking energy averaging into account.

Aerobic Denitrification in Various Heterotrophic Nitrifiers

Abstract: Various heterotrophic nitrifiers have been tested and found to also be aerobic denitrifiers. The simultaneous use of two electron acceptors (oxygen and nitrate) permits these organisms to grow more rapidly than on either single electron acceptor, but generally results in a lower yield than is obtained on oxygen, alone. One strain, formerly known as “Pseudomonas denitrificans”, was grown in the chemostat and shown to achieve nitrification rates of up to 44 nmol NH3 min−1 mg protein−1 and denitrification rates up to 69 nmol NOinf3sup−1min−1 mg protein−1.

A general-purpose version of the fast decoupled load flow

Abstract: A new version of the fast decoupled load flow, in which a more broad range of power systems can be solved, is presented. The key lies in the different way in which the resistances are ignored and in a different iteration scheme. In the standard algorithm the resistances are ignored while building the B' load flow matrix: it is shown that it is preferable that the resistances are ignored in the B" matrix instead of the B' matrix. For normal test systems there is hardly any difference in the number of iterations. However, the new algorithm iterates faster if one or more problematic R/X ratios are present. An iteration scheme with strict successive P and Q iterations prevents cycling convergence behavior which can be found in some low voltage systems. The advantages of the new version are demonstrated with runs on IEEE test systems, with both uniformly and nonuniformly scaled reactances. R-scaling up to 3 is always possible, and sometimes values up to 5 can be used. X-scaling of at least 0.1 is possible without losing convergence and with iteration counts which are significantly lower than with the standard scheme. >

A nonlinear laplace operator as edge detector in noisy images

Abstract: An edge detection scheme is developed robust enough to perform well over a wide range of signal-to-noise ratios. It is based upon the detection of zero crossings in the output image of a nonlinear Laplace filter. Specific characterizations of the nonlinear Laplacian are its adaptive orientation to the direction of the gradient and its inherent masks which permit the development of approximately circular (isotropic) filters. We have investigated the relation between the locally optimal filter parameters, smoothing size, and filter size, and the SNR of the image to be processed. A quantitative evaluation shows that our edge detector performs at least as well—and in most cases much better—than edge detectors. At very low signal-to-noise ratios, our edge detector is superior to all others tested.

Piezoresistance in polysilicon and its applications to strain gauges

Abstract: A full investigation into the piezoresistive effect in polycrystalline silicon is described. A theoretical and experimental approach is applied and a new theoretical model developed. Transport across the grain boundary is based on thermionic emission/diffusion theory. The change in conductivity due to stress, in bulk material, is due to a shift in the sub-bands relative to each other and the subsequent transfer of carriers. An additional effect in p -type material, not found in n -type material, is a change of the shape of the bands. This is due to the lifting of the degeneracy at k = 0. The barrier piezoresistive coefficients are calculated here by considering the contribution to thermionic emission/diffusion from each band minima and calculating the relative change when the system is strained. These coefficients are used in conjunction with the polysilicon model to estimate the effects of physical parameters of the film on the strain sensitivity. The temperature coefficients of resistivity and gauge factor are also modelled and calculated. Previous models assumed the barrier to be insensitive to strain but found poor agreement for lower doping concentration. By considering both thermionic emission and diffusion as conducting mechanisms and their sensitivity to strain, a model valid over a wide range of doping concentrations is achieved. Comparison between theory and experiment shows reasonable agreement for both n and p -type material, longitudinal and transverse strain measurements. The difference in magnitude between longitudinal and transverse gauge factor depends on the texture and is found to be explained by the anisotropy of piezoresistance in silicon. A peak longitudinal gauge factor of ∼43 for p -type material was achieved experimentally. Theoretical estimates of gauge factor and temperature coefficients are used to predict sensor characteristics. An optimum fabrication process is suggested.

Non-commutative Banach Function Spaces

Abstract: In this paper we survey some aspects of the theory of non-commutative Banach function spaces, that is, spaces of measurable operators associated with a semi- finite von Neumann algebra. These spaces are also known as non-commutative symmetric spaces. The theory of such spaces emerged as a common generalization of the theory of classical (“commutative”) rearrangement invariant Banach function spaces (in the sense of W.A.J. Luxemburg and A.C. Zaanen) and of the theory of symmetrically normed ideals of bounded linear operators in Hilbert space (in the sense of I.C. Gohberg and M.G. Krein). These two cases may be considered as the two extremes of the theory: in the first case the underlying von Neumann algebra is the commutative algebra L ∞ on some measure space (with integration as trace); in the second case the underlying von Neumann algebra is B (ℌ), the algebra of all bounded linear operators on a Hilbert space ℌ (with standard trace). Important special cases of these non-commutative spaces are the non-commutative L p-spaces, which correspond in the commutative case with the usual L p-spaces on a measure space, and in the setting of symmetrically normed operator ideals they correspond to the Schatten p-classes \( \mathfrak{S}_p \) .

Boriding of Fe and Fe–C, Fe–Cr, and Fe–Ni alloys; Boride-layer growth kinetics

Abstract: Specimens of pure Fe and of Fe-0.8 mass % C, Fe-0.5 mass % Cr, Fe-4.0 mass % Cr, Fe-4.0 mass% Ni, and Fe-10.0 mass% Ni alloys were borided in boriding powder. A boron-compound layer developed consisting of a surface-adjacent “FeB” sublayer on top of an “Fe2B” sublayer. Layer-growth kinetics were analyzed by measuring the extent of penetration of the “FeB” and “Fe2B” sublayers as a function of boriding time and temperature in the range 1025–1275 K. Layer growth is dominated by B diffusion through “FeB/Fe2B”. This diffusion process is of strongly anisotropic nature. Consequently, ragged interfaces occur between the substrate and the boride layers. The depths of the tips of the most deeply penetrated “FeB” and “Fe2B” needles have been taken as measures for diffusion in the easy [001] diffusion directions. Assuming unidirectional B diffusion and parabolic growth, a simple model of layer growth has been given. It accounts for the specific volume difference between “FeB” and “Fe2B”. In contrast with earlier work, the model provides values for the kinetic parameters for growth of each of the phases in the boron-compound layer.

Charging effects and quantum coherence in regular Josephson junction arrays

Abstract: Two-dimensional arrays of very-small-capacitance Josephson junctions have been studied. At low temperatures the arrays show a transition from superconducting to insulating behavior when the ratio of charging energy to Josephson-coupling energy exceeds the value 1. Insulating behavior coincides withthe occurrence of a charging gap inside the BCS gap, with an S-shaped I-V characteristic. This so far unobserved S shape is predicted to arise from macroscopic quantum coherent effects including Bloch oscillations.

An efficient uniform cost algorithm applied to distance transforms

Abstract: The uniform-cost algorithm is a special case of the A*-algorithm for finding the shortest paths in graphs. In the uniform-cost algorithm, nodes are expanded in order of increasing cost. An efficient version of this algorithm is developed for integer cost values. Nodes are sorted by storing them at predefined places (bucket sort), keeping the overhead low. The algorithm is applied to general distance transformation. A constrained distance transform is an operation which calculates at each pixel of an image the distance to the nearest pixel of a reference set, distance being defined as minimum path length. The uniform-cost algorithm, in the constrained case, proves to be the best solution for distance transformation. It is fast, the processing time is independent of the complexity of the image, and memory requirements are moderate. >

Cell and tissue cultures of Catharanthus roseus (L.) G. Don: a literature survey

Abstract: The literature concerning the formation of secondary metabolites in cell and tissue cultures ofCatharanthus roseus has been reviewed. Several aspects involved in the formation of secondary metabolites are discussed; e.g. regulation of secondary metabolism, environmental factors influencing secondary metabolism, biosynthesis and enzymology of the products, analysis of product formation, immobilization of cultured cells and stability of cell lines. Some economical aspects of production processes are discussed.

Anomalous integer quantum Hall effect in the ballistic regime with quantum point contacts.

Abstract: The Hall conductance of a wide two-dimensional electron gas has been measured in a geometry in which two quantum point contacts form controllable current and voltage probes, separated by less than the transport mean free path. Adjustable barriers in the point contacts allow selective population and detection of Landau levels. The quantization of the Hall conductance is determined by the number of quantum channels in the point contacts and is independent of the number of occupied bulk Landau levels. A theoretical description based on the Landauer-B\"uttiker formalism is given.

Glucose Transport in Crabtree-positive and Crabtree-negative Yeasts

Abstract: SUMMARY: The kinetic parameters of glucose transport in four Crabtree-positive and four Crabtree-negative yeasts were determined. The organisms were grown in aerobic glucose-limited chemostats at a dilution rate of 0·1 h-1. The results show a clear correlation between the presence of high-affinity glucose transport systems and the absence of aerobic fermentation upon addition of excess glucose to steady-state cultures. The presence of these H+-symport systems could be established by determination of intracellular accumulation of 6-deoxy-[3H]glucose and alkalinization of buffered cell suspensions upon addition of glucose. In contrast, the yeasts that did show aerobic alcoholic fermentation during these glucose pulse experiments had low-affinity facilitated-diffusion carriers only. In the yeasts examined the capacity of the glucose transport carriers was higher than the actual glucose consumption rates during the glucose pulse experiments. The relationship between the rate of sugar consumption and the rate of alcoholic fermentation was studied in detail with Saccharomyces cerevisiae. When S. cerevisiae was pulsed with low amounts of glucose or mannose, in order to obtain submaximal sugar consumption rates, fermentation was already occurring at sugar consumption rates just above those which were maintained in the glucose-limited steady-state culture. The results are interpreted in relation with the Crabtree effect. In Crabtree-positive yeasts, an increase in the external glucose concentration may lead to unrestricted glucose uptake by facilitated diffusion and hence, to aerobic fermentation. In contrast, Crabtree-negative yeasts may restrict the entry of glucose by their regulated H+-symport systems and thus prevent the occurrence of overflow metabolism.

Early incorporation of polysulphides in sedimentary organic matter

Abstract: THE increase in sulphur content of organic matter with depth in Recent sediments has been attributed to incorporation of either H2S (ref. 1) or polysulphides2,3 or to a combination of both4,5. Indeed, the widespread occurrence of organic sulphur compounds6–12 with carbon skeletons that bear an unambiguous link with natural precursors indicates that organic matter may act as a sink for inorganic sulphur species with an efficacy second only to reactive iron minerals13. Laboratory and field observations14–18 indicate that all compounds identified so far are consistent with incorporation of H2S; molecular evidence for incorporation of polysulphides has previously been lacking. Here we report the identification of homologous series of cyclic disulphides with a linear carbon skeleton and of a cyclic di- and trisulphide with a C20 isoprenoid carbon skeleton in sediments of Quaternary to Pliocene age. Although incorporation of H2S can still explain the presence of cyclic disulphides, the cyclic trisulphide implies incorporation of inorganic polysulphides in sedimentary organic matter at the earliest stages of diagenesis.

Excess nitrogen in the ferrite matrix of nitrided binary iron-based alloys

Abstract: On nitriding ferritic Fe-M (M = Ti, V, Cr or Mo) alloys, three types of absorbed nitrogen atoms can be distinguished: (i) nitrogen incorporated in the stoichiometric nitride; (ii) nitrogen adsorbed at the nitride/matrix interface, and (iii) nitrogen dissolved in octahedral interstices of the ferrite lattice. As compared with pure ferrite, enhanced nitrogen lattice solubility is observed for the ferritic matrix of Fe-M alloys. This excess nitrogen can be ascribed to an average lattice dilation induced by the volume misfit between nitrides and matrix. A model is developed for a quantitative description of the amount of excess nitrogen in ferrite. The treatment originates from an application of a theory due to Eshelby, developed originally for the case of elastic distortions by point imperfections, in conjunction with the thermodynamics of stressed solids. The model is applied to published data on excess nitrogen in various Fe-M alloys where coherent nitrides develop. The dependence of the amount of...

Kinetics of growth and glucose transport in glucose-limited chemostat cultures of Saccharomyces cerevisiae CBS 8066.

Abstract: The glucose transport capacity of Saccharomyces cerevisiae CBS 8066 was studied in aerobic glucose-limited chemostat cultures Two different transport systems were encountered with affinity constants of 1 and 20 mM, respectively The capacity of these carriers (Vmax) was dependent on the dilution rate and the residual glucose concentration in the culture From the residual glucose concentration in the fermenter and the kinetic constants of glucose transport, their in situ contribution to glucose consumption was determined The sum of these calculated in situ transport rates correlated well with the observed rate of glucose consumption of the culture The growth kinetics of S cerevisiae CBS 8066 in glucose-limited cultures were rather perculiar At low dilution rates, at which glucose was completely respired, the glucose concentration in the fermenter was constant at 110 μM, independent of the glucose concentration in the reservoir At high dilution rates, characterized by the occurrence of both respiration and alcoholic fermentation, the residual substrate concentration followed Monod kinetics In this case, however, the overall affinity constant was dependent on the reservoir glucose concentration