Showing papers by "Instituto Superior Técnico published in 1983"

Desulfovibrio Gigas Hydrogenase: Redox Properties of the Nickel and Iron‐Sulfur Centers

Abstract: Below 30 K, oxidized Desulfovibrio gigas hydrogenase presents an intense electron paramagnetic resonance (EPR) signal centered at g= 2.02, typical of an iron-sulfur center. In addition a rhombic EPR signal, attributed to Ni(III) species, is also observed [LeGall, J., Ljungdahl, P., Moura, I., Peck, H. D., Jr, Xavier, A. V., Moura, J. J. G., Teixeira, M., Huynh, B. H., and DerVartanian, D. V. (1982) Biochem. Biophys. Res. Commun. 106, 610–616; and Cammack, R., Patil, D., Aguirre, R., and Hatchikian, E. C., (1982) FEBS Lett. 142, 289–292]. At higher temperatures (77 K) the iron-sulfur EPR signal is broader and all the EPR features of the rhombic nickel signal can easily be observed. We have now obtained additional information concerning the redox properties of these EPR active centers, using an EPR redox titration method in the presence of dye mediators at pH = 8.5. The mid-point potential was determined to be - 70 mV for the Fe, S cluster and - 220 mV for the Ni center. Intermediate oxidation states were obtained upon partial reduction with either dithionite or hydrogen. Although upon dithionite reduction the centers are reduced in the order of decreasing mid-point reduction potentials, under a hydrogen atmosphere the nickel center reduces preferentially. This suggests a catalytic involvement of the nickel redox center in the binding of hydrogen. Preliminary Mossbauer studies on Desulfovibrio gigas hydrogenase reveal the presence of a paramagnetic 3 Fe center and two 4 Fe centers. The 3 Fe center is responsible for the g= 2.02 EPR signal but the two 4 Fe centers have been so far undetectable by EPR.

Nonstationary spectral modeling of voiced speech

Abstract: The main purpose of this paper is to present a novel model for voiced speech. The classical model, which is being used in many applications, assumes local stationarity, and consequently imposes a simple and well known line structure to the short-time spectrum of voiced speech. The model derived in this paper allows for local non-stationarities not only in terms of pitch perturbations, but in terms of vocal tract variations as well. The resulting structure of the short-time spectrum becomes more complex, but can still be interpreted in terms of generalized lines. The proposed model supports new forms of spectral prediction, which can be put to advantage in speech coding applications. Experimental results are presented supporting the validity of both the model itself and the prediction relationships. Finally, a new class of speech coders, denoted harmonic coders, based on the presented model, is proposed, and a specific implementation is presented.

Thermal conductivity of toluene in the temperature range 35–90°C at pressures up to 600 MPa

Abstract: New, absolute measurements of the thermal conductivity of liquid toluene are reported. The measurements extend over the temperature range 35–90°C and the pressure range 0.8–600 MPa. A new analytic evaluation of the contribution of radiation in an absorbing emitting fluid to the measurement process is presented. This analysis indicates that the thermal conductivity determined in a transient hot-wire instrument is the radiation-free value. As a consequence it is possible to assert that the overall uncertainty in the experimental data is one of ± 0.3%. A comparison of the data with the results of independent measurements by the same technique shows that the various sets of data are consistent within their mutual uncertainty.

Thermal conductivity of five hydrocarbons along the saturation line

Abstract: The paper presents the results of new, absolute measurements of the thermal conductivity of five alkanes as a function of temperature at their saturation vapor pressure. The alkanes studied include the four normal alkanes n-octane, n-nonane, n-undecane, and n-tetradecane as well as the branched alkane, iso-octane. The results, which extend over the temperature range 282–373 K, have an estimated uncertainty limit of 1.5% deriving mainly from a correction for the effects of radiation absorption in the measurement process. The results are employed to generate effective core volumes for the fluids studied which may be employed to predict the density dependence of the thermal conductivity by means of an existing correlation.

A Binary Decision Model for the Stock Control of Very Slow Moving Items

Abstract: In this note a model for very slow moving items is developed in order to decide whether it is economic to have 0 or 1 item in stock. The model is applied to a real situation and substantial savings are obtained.

On waves in non-isothermal, compressible, ionized and viscous atmospheres

Abstract: A review is given of the properties of waves in atmospheres, with particular emphasis on (Section 1) the variation of amplitude and phase with altitude for propagating waves (Figures 1 to 4) and the waveforms of standing modes (Figure 5). The cases dealt with concern waves under the combined influences of gravity and compressibility, and examine the effects of. (Section 2) temperature gradients in a non-isothermal atmospheric model; (Section 3) external magnetic field, either vertical or horizontal; (Section 4) dissipation by viscosity and electrical resistance. The results are relevant to (Section 5) the assessment of atmospheric wave growth and shock formation, and to the calculation of heating functions describing the deposition of wave energy.

Active Vibration Control of Simple Beam

Abstract: An experiment on active vibration control of structures is studied. A tendon control mechanism (“King-post Truss”) is used to reduce the vibration of a simple beam, subjected to a sinusoidal driving force. Experimental results are compared with theoretical ones. Conclusions about the efficiency of the control scheme are drawn and practical difficulties are pointed out.

The effects of oxide impurities on the optical properties of fluoride glasses

Abstract: The effects of oxide impurities on the vibrational spectra of ZrF 4 -based ternary glasses have been investigated. Samples have been prepared in the ZrF 4 BaF 2 ThF 4 and ZrF 4 BaF 2 LaF 3 systems, to which selected amounts of ZrO 2 , BaO and La 2 O 3 were added, up to a O:F atomic ratio ∼1.2%. A pronounced effect was observed on the IR absorption edge of bulk samples. Polarized Raman spectra have also been recorded. It appears that the oxygen atoms tend to occupy bridging positions in the zirconium fluoride chain-like glass skeleton.

Current Research Topics in Low-Pressure Glow Discharges in Rare Gases and in Pure Nitrogen

Abstract: The present work reports some results of the research in progress concerning the positive column of glow discharges in pure rare gases and in pure nitrogen, at pressures typically below 1 torr and discusses some of the limitations of currently available models for predicting the discharge characteristics and for explaining the kinetics of some important excited atomic and molecular species, such as the metastable ones, in the positive column. The paper is divided into two main sections: the first devoted to the modeling of the positive column in Argon and the second, to the kinetics of the metastable state N2(A3∑+ U) in the positive column of a pure nitrogen glow discharge.

A spatial smoothing formulation for location systems

Abstract: Two extensions of the classic passive location problem are considered. The first examines the ranging ability of passive receivers. The second presents alternative ways of describing the geometric content in positioning problems. The implications of the different approaches on the structure and performance of the location receiver are discussed. Because range determination by passive means is missing, the classical formulation of passive location may be viewed as a local geometry demodulation problem. With the explicit consideration of range, passive location becomes a global problem. At stake is what may be gained by processing the small but valuable amount of information carried by the wavefront curvature of the signals. Relevant questions relate to the design of passive receivers that aptly demodulate the range and the remaining quantities defining the geometry. There are passive applications where models exhibiting a high degree of (geometric) regularity are viable from a practical point of view. These occur, for example, when one can assume that the array sensors are collinear and that the moving target follows a deterministic linear path. In these models, the geometry is completely determined by a finite set of (unknown) parameters (e.g., range, bearing, speed, etc.). Accordingly, it is said that the regular models use an integral or ensemble approach for the description of the geometry. In many other problems, the geometry is more adequately described by statistical processes. Examples arise when the source follows a disturbed path, or when, due to towing, the array shape deforms, acquiring a not-completely-known shape. The paper models these constraints via a set of stochastic differential equations. The resulting representation is termed a differential description. It is emphasized that the differential approach is not only applied to the time content (relative dynamics), but also to the spatial dimension (array shape). The technique dualizes the space and time aspects of the problem. It provides a more flexible framework than the previous one. More general motions and array shapes than the traditional collinear ones can be considered by the analysis, e.g., irregular line arrays or arrays where the sensors are located at positions with a certain degree of randomness. Each approach fits a different design framework. The ensemble description is associated with the maximum likelihood technique. The differential representation uses recursive estimation methods (as provided by the Kalman-Bucy filtering theory). The paper discusses the main aspects of the structure of the resulting receivers and the associated measures of error performance. A second advantage of the differential model is immediately apparent The recursiveness of the differential receiver reduces its computational load. The speed-up obtained is fully appreciated in tracking applications, where the observations are sequentially updated. Finally, it is interesting to note that the time/space duality provided by the differential approach exhibits a remarkable distinction: the location recursive receiver behaves in time as a filter, while it behaves in space as a smoother.

Boundary element analysis of rotationally symmetric systems under general boundary conditions

Abstract: Engineering systems often exhibit cyclic symmetry (or rotational periodicity) of geometrical and physical properties. This paper deals with the use of this feature in the context of the direct boundary element method, and shows how to decompose an elastostatic and steady state b.v. problem with general (not cyclically symmetric) boundary conditions into a number of much smaller problems over a single segment. There is no need to discretize the interfaces between adjacent segments. The decomposition is achieved by rotational transformations of physical into cyclic components

Phase Demodulation: A Nonlinear Filtering Approach

Abstract: The paper studies nonlinear filtering techniques. It discusses their application to several classes of phase modulation (PM) problems: cyclic and absolute PM problems. It considers the long time or tracking behavior of the PM demodulators, and their short time or acquisition ability. After motivating examples, the mathematical model is presented. The continuous and the discrete optimal nonlinear filtering solutions are then analysed.The PM problems are described. A brief exposition of the PLL theory follows, with emphasis on a operator type solution of the associated Kolmogoroff equations. Finally, the implementation of the nonlinear demodulator and their performance in the context of absolute PM tracking and acquisition is pursued.

The use of the inverted linear model of enzyme decay to plan a strategy for enzymatic batch reactions and to assess the industrial applicability of immobilized enzyme preparations.

Abstract: This article is concerned with the development of a model to plan a strategy for an enzymatic batch process where enzyme is subjected to deactivation described by the inverted linear decay model. The particular system studied is the enzymatic hydrolysis of penicillin to 6-amino penicillanic acid (6 APA), but the model can be utilized with other batch systems as long as the decay of the immobilized enzyme (IME) preparation is described by the inverted linear decay model. The model developed is eminently practical and simple and several example of its application are shown. Experimental data obtained in a small pilot plant batch recirculated reactor on the average are well fitted by this model. For IME systems whose decay is best described by the first-order decay model, it is not possible to use the same approach.

Geometric Aspects in Array Processing

Abstract: The theory of array processing is usually directed to nonmoving, particular array shapes, like the linear, rectangular, disc, crossed arrays. These are geometric figures whose description is incorporated into the problem in an expedite way. In practice, due to causes beyond the control of the designer, the actual array departs from these simple geometries. Colinearity is difficult of maintaining in very long arrays. Uncertainty may arise concerning the exact location of the array sensors. If the array is a towed flexible aggregate, its steady state shape may significantly depart from a closed type description. The array may move, through irregular paths, eg. random accelerating. These motions are best modeled as stochastic dynamics.

On Waves in Non-Isothermal, Compressible, Ionized and Viscous Atmospheres *

Abstract: A review is given of the properties of waves in atmospheres, with particular emphasis on (Section 1) the variation of amplitude and phase with altitude for propagating waves (Figures 1 to 4) and the waveforms of standing modes (Figure 5). The cases dealt with concern waves under the combined influences of gravity and compressibility, and examine the effects of: (Section 2) temperature gradients in a non-isothermal atmospheric model; (Section 3) external magnetic field, either vertical or horizontal; (Section 4) dissipation by viscosity and electrical resistance. The results are relevant to (Section 5) the assessment of atmospheric wave growth and shock formation, and to the calculation of heating functions describing the deposition of wave energy.

Obtention of Protoplasts of Sylibum Marianum and its Use in the Coagulation of Milk

Abstract: The production of cheese involves two stages:1) the enzymatic hydrolysis of K casein in which its micelle is desestabilized and 2) its subsequent aggregation. In most cases, this operation is carried out by microbial proteases, being used also, animal and plant enzymes.

Transport processes in porous charged membranes

Abstract: A porous membrane interposed between homogeneous electrolytic aqueous solutions of different concentrations, if subjected to an external electric field, very often gives rise to a volume flow between the two homogeneous phases. This phenomenon, conventionally called electro-osmosis, has been related to the formation of an electrical double layer at the solid matrix-electrolyte interface. In our laboratories, this effect has been shown to be of great importance for materials characterized by a high surface area. Our objective is to relate the electro-osmotic flow rates to the geometry of the porous matrix, the solid-liquid interface, and the external conditions (membraneadjacent solution concentrations, electrical potential difference, and pressure difference). The porous structure is modeled as a network of cylindrical pores crossing the membrane from one face to the other. Its characterization is then made by measured or estimated parameters such as porosity, tortuosity, and surface area.' For the situation in which a membrane separates two solutions of unequal concentrations (without an external electric field), the continuity equation for the solute is solved assuming two semi-infinite compartments adjacent to the membrane and considering the membrane as a pseudohomogeneous phase. The mass fluxes at the two interfaces, membrane-electrolyte, are continuous. The time-average solute fluxes are calculated as a function of both the initial concentration ratio of the two electrolytic solutions and the geometrical characteristics of the membrane. When an external electrical field is introduced, the Nernst-Planck-Poisson equation, coupled with the steady-state Navier-Stokes equation for creeping flow, is solved for a pore.' The Debye-Hiickel aproximation is used for the ion-distribution calculation on the electrical double layer. The electro-osmotic flow rates are calculated and correlated to the characteristic parameters of our system. The mass fluxes with and without electrical field are compared. The experimental cell is made of a Plexiglass parallelepiped with two rectangular plane-parallel graphite electrodes mounted on opposite lateral faces. The membrane is fixed on a rectangular frame between the electrodes. The electrodes and the membrane have plane-parallel concentric faces with the same area. The electrodes are connected to a power supply source. The electrolyte is an aqueous potassium chloride solution. Experimental data were obtained with cellophane membranes. Different concentration ratios of the membrane-adjacent compartments were investigated.