Showing papers by "Instituto Superior Técnico published in 1984"

NMR studies of electron transfer mechanisms in a protein with interacting redox centres: Desulfovibrio gigas cytochrome c3.

Abstract: The proton NMR spectra of the tetrahaem cytochrome c3 from Desulfovibrio gigas were examined while varying the pH and the redox potential The analysis of the NMR reoxidation pattern was based on a model for the electron distribution between the four haems that takes into account haem-haem redox interactions The intramolecular electron exchange is fast on the NMR time scale (larger than 105 s−1) The NMR data concerning the pH dependence of the chemical shift of haem methyl resonances in different oxidation steps and resonance intensities are not compatible with a non-interacting model and can be explained assuming a redox interaction between the haems A complete analysis at pH*= 72 and 96, shows that the haem-haem interacting potentials cover a range from -50mV to +60 mV The midpoint redox potentials of some of the haems, as well as some of their interacting potentials, are pH-dependent The physiological relevance of the modulation of the haem midpoint redox potentials by both the pH and the redox potential of the solution is discussed

Variable-frequency synthesis: An improved harmonic coding scheme

Abstract: The Harmonic Coding concept has already shown its potential for efficiently coding speech. Previous implementations have usec a frame rate of one every 16 ms. This was mainly due to the fact that, with longer frames, even a nonstationary spectral model (of low order) cannot reproduce the zones of fast-varying pitch with the desirable quality. However, the high framing rate is a limitation, since it implies that fewer bits will be available for encoding each frame. A solution for this problem has been devised: the signal is synthesized in the time domain, as a superimposition of "harmonics" whose instantaneous frequency varies continuously along an interpolation curve, within each frame. In this way, fast pitch variations can be tracked with no difficulty. Experimental results are presented, confirming these facts. The integration of this synthesis scheme in a speech coder is discussed.

On the Theory of the Wells Turbine

Abstract: Presentation d'une etude theorique du comportement thermodynamique de la turbine de Wells, turbine a ecoulement axial adaptee a l'extraction d'energie a partir d'un ecoulement d'air alternatif. Developpement d'une analyse bidimensionnelle, et obtention d'expressions pour la forme d'ailette rendant maximal le rendement de la turbine. Analyse tridimensionnelle montrant que des distorsions radiales importantes du profil de vitesse axial peuvent se produire en fonction de la forme de l'ailette

On a Concept of Derivative of Complex Order with Applications to Special Functions

Abstract: On introduit un concept de derivee Dν d'ordre complexe ν, d'une fonction f(z), generalisant a la fois les integrales de Cauchy et de Weyl, qui sont les cas particuliers de, respectivement, ν=+n/−n, un entier positif/negatif, correspondant a la derivation/integration ordinaire d'ordre n. On demontre des theoremes d'existence, d'analyticite et d'integrabilite pour la derivee d'ordre complexe general ν

Purification, characterization and redox properties of hydrogenase from Methanosarcina barkeri (DSM 800).

Abstract: A soluble hydrogenase from the methanogenic bacterium, Methanosarcina barkeri (DSM 800) has been purified to apparent electrophoretic homogeneity, with an overall 550-fold purification, a 45% yield and a final specific activity of 270 mumol H2 evolved min-1 (mg protein)-1. The hydrogenase has a high molecular mass of approximately equal to 800 kDa and subunits with molecular masses of approximately equal to 60 kDa. The enzyme is stable to heating at 65 degrees C and to exposure to air at 4 degrees C in the oxidized state for periods up to a week. The overall stability of this enzyme is compared with other hydrogenase isolated from strict anaerobic sulfate-reducing bacteria. Ms. barkeri hydrogenase shows an absorption spectrum typical of a non-heme iron protein with maxima at 275 nm, 380 nm and 405 nm. A flavin component, identified as FMN or riboflavin was extracted under acidic conditions and quantified to approximately one flavin molecule per subunit. In addition to this component, 8-10 iron atoms and 0.6-0.8 nickel atom were also detected per subunit. The electron paramagnetic resonance (EPR) spectrum of the native enzyme shows a rhombic signal with g values at 2.24, 2.20 and approximately equal to 2.0. probably due to nickel which is optimally measured at 40 K but still detectable at 77 K. In the reduced state, using dithionite or molecular hydrogen as reductants, at least two types of g = 1.94 EPR signals, due to iron-sulfur centers, could be detected and differentiated on the basis of power and temperature dependence. Center I has g values at 2.04, 1.90 and 1.86, while center II has g values at 2.08, 1.93 and 1.85. When the hydrogenase is reduced by hydrogen or dithionite the rhombic EPR species disappears and is replaced by other EPR-active species with g values at 2.33, 2.23, 2.12, 2.09, 2.04 and 2.00. These complex signals may represent different nickel species and are only observable at temperatures higher than 20 K. In the native preparation, at high temperatures (T greater than 35 K) or in partially reduced samples, a free radical due to the flavin moiety is observed. The EPR spectrum of reduced hydrogenase in 80% Me2SO presents an axial type of spectrum only detectable below 30 K.

Anisotropy of thermopower in N-methyl-N-ethylmorpholinium bistetracyanoquinodimethane, MEM ( TCNQ ) 2 , in the region of the high-temperature phase transitions

Abstract: Measurements of the thermoelectric power along the three crystallographic axes of N-methyl-N-ethylmorpholinium bistetracyanoquinodimethane, ${\mathrm{MEM}(\mathrm{TCNQ})}_{2}$, in the temperature range 280-360 K are reported. The first-order metal-insulator phase transition at 338 K is observed as a discontinuous change between the temperature-independent values ${S}_{a}=\ensuremath{-}50$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$, ${S}_{b}=\ensuremath{-}35$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$, and ${S}_{c}=\ensuremath{-}65$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$ in the metallic phase ($Tg338$ K) and the values ${S}_{a}=\ensuremath{-}7$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$, ${S}_{b}=\ensuremath{-}48$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$, and ${S}_{c}=+35$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$. In the semiconducting phase, ${S}_{b}$ is almost temperature independent, but both ${S}_{a}$ and ${S}_{c}$ increase upon cooling. At 314 K, a strong anomaly is made evident as a drastic change in the slope of ${S}_{a}$ and ${S}_{c}$ versus temperature. The electrical conductivity accurately measured along the three crystallographic axes, also shows an anomalous decrease for $Tl314$ K. The thermopower and conductivity anomalies at 314 K are consistent with a second-order phase transition which originated in a change of cation motion, and is associated mainly with a decrease in the mobility of the charge carriers, the role of a possible increase in the gap being secondary. For $Tl314$ K there is a predominant decrease of the electron mobility mainly along the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ axis but also along the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ axis, while, along the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$ axis, both the electron and hole mobilities decrease, remaining equal to each other.

Heat Transfer Applications

Abstract: Before dealing with the application of the Boundary Element Method (BEM) to heat transfer problems we shall summarize the basic theory of heat conduction. It is known from experience that heat flows from the hotter parts of a body to the cooler. The mechanisms responsible for this phenomena are conduction, radiation and convection. Since we are concerned in this paper exclusively with solid bodies, convection only intervenes through boundary conditions in solid-fluid interfaces. Radiation is usually negligible, except eventually when setting up boundary conditions.

NMR and electron-paramagnetic-resonance studies of a dihaem cytochrome from Pseudomonas stutzeri (ATCC 11607) (cytochrome c peroxidase)

Abstract: A dihaem cytochrome (Mr 37 400) with cytochrome c peroxidase activity was purified from Pseudomonas stutzeri (ATCC 11 607). The haem redox potentials are far apart: one of the haems is completely ascorbate-reducible and the other is only reduced by dithionite. The coordination, spin states and redox properties of the covalently bound haems were probed by visible, NMR and electron paramagnetic resonance (EPR) spectroscopies in three oxidation states. In the oxidized state, the low-temperature EPR spectrum of the native enzyme is a complex superimposition of three components: (I) a low-spin haem indicating a histidinyl-methionyl coordination; (II) a low-spin haem indicating a histidinyl-histidinyl coordination; and (III) a minor high-spin haem component. At room temperature, NMR and optical studies indicate the presence of high-spin and low-spin haems, suggesting that for one of the haems a high-spin to low-spin transition is observed when temperature is decreased. In the half-reduced state, the component I (high redox potential) of the EPR spectrum disappears and induces a change in the g-values and linewidth of component II; the high-spin component II is no longer detected at low temperature. Visible and NMR studies reveal the presence of a high-spin ferric and a low-spin (methionyl-coordinated) ferrous state. The NMR data fully support the haem-haem interaction probed by EPR. In the reduced state, the NMR spectrum indicates that the low-potential haem is high-spin ferrous.

Isolation and selection of phenol - degrading microorganisms from an industrial effluent

Abstract: Among 12 bacterial and fungal isolates from a phenol-containing effluent, an Acinetobacter sp. and a Fusarium flocciferum strain could be adapted to growth on up to lg/1 of phenol.

Time Dependent Potential Problems

Abstract: The use of singularities to represent instantaneous sources or sinks of heat for solving time dependent heat conduction problems is described in Carslaw and Jaeger, 1969, ch.X, where Kelvin is credited with having made systematic use of this method to obtain analytical solutions. The integral representation to be derived below in Section 2 appears in Boley and Weiner, 1960, but without any numerical treatment.

Automated High-Temperature PVT Apparatus With Data for Propane.

Abstract: An apparatus is described which can be used for PVT and compressibility measurements on 5upercritical fluids from near room temperature to 600 °C and pressures to 35 MPa. Two separate experimental techniques are employed to obtain PVT data over a broad range of the state surface. Burnett expansions are performed to generate compressibility factor (or equivalently density) data along a well-behaved supercritical isotherm. A series of isochoric measurements is then made to extend the temperature range. Densities assigned to the isochores are determined from their intersection with the previously measured Burnett isotherm or gravimetri~ cally. A computer is used for experimental control and for data logging. Isochoric measurements lasting several days can be performed routinely and without operator attention. The apparatus has been tested on propane to a temperature of 325 ·C. The density data, estimated accurate to ±O.l percent, are in excellent agreement with other existing data.

An X-ray diffraction study of the structure of barium fluorozirconate and fluorohafnate glasses

Abstract: Two binary glasses have been prepared in the ZrF 4 BaF 2 and HfF 4 BaF 2 systems, containing 64 mol.% ZrF 4 and 66 mol.% HfF 4 , respectively. The two plate-like glass samples were studied by X-ray diffraction, and the resulting data were subjected to a Fourier analysis. The pair correlation functions thus obtained yield a ZrF (or HfF) nearest neighbor coordination number of ∼ 7.6 for both glasses. The second peak in the correlation functions has been assigned to a combination of FF and BaF interactions, while the third peak is probably due in part to ZrZr (Hf-Hf) interactions. The diffraction data have been compared with the predictions of structural models for barium fluorozirconate and fluorohafnate glasses.

Interconversion from 3Fe into 4Fe clusters in the presence of Desulfovibrio gigas cell extracts.

Abstract: Desulfovibrio gigas ferredoxin II (FdII) contains a single 3Fe cluster [Huynh, B. H., Moura, J. J. G., Moura, I., Kent, T. A., LeGall, J., Xavier, A. V., and Munck, E. (1980) J. Biol. Chem. 255, 3242–3244]. In the oxidized state the protein exhibits an intense electron paramagnetic resonance (EPR) signal at g= 2.02. Upon one-electron reduction the center becomes EPR silent. In the presence of D. gigas crude cell extracts, devoid of acidic electron carriers and supplemented with pyruvate and FdII, an EPR signal typical of reduced [4Fe–4S] centers is obtained. The appearance of this signal correlates with the beginning of stimulation of the phosphoroclastic reaction as judged by the production of H2. These results, supported by the occurrence of easy chemical conversion of the 3Fe cluster of D. gigas ferredoxin into 4Fe structures [Moura, J. J. G., Moura, I., Kent, T. A., Lipscomb, J. D., Huynh, B. H., LeGall, J., Xavier, A. V., and Munch, E. (1982) J. Biol. Chem. 257, 6259–6267], suggest that cluster conversion takes place in conditions close to the situation in vivo. This cluster interconversion is discussed in the context of some of the relevant metabolic pathways of Desulfovibrio spp.

A study on short-time phase and multipulse LPC

Abstract: Multipulse LPC, as is often designated the model proposed by Atal and Remae, has been directed towards 9.6 kb/s speech coding. However, at such bit rate, the speech quality is not yet generally acceptable. This paper has a double purpose - one is to investigate the role of short-time phase in multipulse LPC. The other is to look for different modelling structures to be used with this method. This study was prompted by direct observation of the structure of the excitation, where patterns of pulses may be found which are associated with phase-correcting mechanisms of the LPC impulse response. Consequently, a new multipulse technique was developed, based on a special ARMA model formed by cascading an all-pole with an all-pass network. This new model will be referred to as the MAPAP (Multipulse All-Pole All-Pass) method. Another one was tried in which the synthetic speech is formed by combination of several MAPAP signals. We therefore denoted it "multichannel multipulse" method. The potential advantages of both single and multichannel models seem rather promising.

Nuclear-magnetic-resonance studies of Desulfuromonas acetoxidans cytochrome c551.5 (c7).

Abstract: 1H nuclear magnetic resonance (NMR) spectroscopy has been used to examine cytochrome c551.5 (c7) from the sulfur reducer, Desulfuromonas acetoxidans. This protein contains three hemes. Two stable oxidation states (the fully oxidized and the fully reduced) as well as intermediate oxidation states were studied. The axial ligands of the iron were found to be neutral histidines. The redox properties of cytochrome c7 were examined and good quantitative agreement found between the NMR results and previously reported redox potential measurements. The properties of cytochrome c7 are discussed together with those of the homologous tetraheme cytochromes c3 isolate from sulfate-reducing bacteria.

NMR studies of a dihaem cytochrome from Pseudomonas perfectomarinus (ATCC 14405)

Abstract: Pseudomonas perfectomarinus (ATCC 14405) dihaem cytochrome c552 was studied by 300-MHz proton magnetic resonance. Some of the haem resonances were assigned in the fully reduced and fully oxidized states. No evidence was found for methionine haem axial coordination. The oxidation-reduction equilibrium was studied in detail. Due to the large difference in mid-point redox potential between the two haems (+174mV, for haem II and -180mV for haem I) an intermediate oxidation state could be obtained containing reduced haem I and oxidized haem II. In this way the total paramagnetic shift at different oxidation levels could be decomposed in the intrinsic and extrinsic contributions. It was found that the two haems interact. The rate of electron exchange is slow on the NMR time scale. The redox equilibria are discussed for four possible redox species in solution.

Comparison of Immobilization Methods for Plant Cells and Protoplastsa

Abstract: Plant cells have been immobilized by entrapment methods using polysaccharides from seaweeds and synthetic matrices for the biosynthesis and biotransformation of valuable plant cell products.' Protoplasts, which are obtained by the removal of the plant cell walls, have been mechanically stabilized by immobilization in calcium alginate gels.' In an attempt to develop inexpensive and simple immobilization techniques for these sensitive cells, several experiments with alternative methods were carried out with Silybum marianum cells and protoplasts. The flowers and leaves of Silybum marianum are used traditionally in Portugal for the small-scale production of Serpa cheeses. The coagulant properties of their petals or leaves are due to their high rennin

Non-adiabatic processes in alkali metal-alkyl halide molecule collisions

Abstract: The mechanism for vibronic excitation and ion pair production in alkali metal-alkyl halide collisions is investigated. A classical trajectory surface hopping model is developed to account for the fourteen exit channels for which data is available. It is shown that the ground ionic intermediate accounts for the observed A state excitation in CH3I as well as the production of excited alkali atoms. It is further shown that the other, more highly excited, processes can be understood on the basis of an electron harpoon to form one of a sequence of strongly ionic doorway states. Each doorway state connecting to a manifold of exit channels. The nature of these doorway states and the role of vacancy production in their excitation is discussed.

A vibrational spectroscopy study of the structure of binary thorium fluorohafnate glasses

Abstract: Binary glasses have been prepared in the HfF4ThF4 system, containing ∼ 32–43 mol.% HfF4. Glass transition and crystallization temperatures have been determined by Differential Thermal Analysis. Infrared absorption and polarized Raman spectra have been measured at room temperature. The results have been compared with those previously obtained for HfF4BaF2 glasses and those of crystalline HfF4 and ThF4. A vibrational mode assignment has been made. While HfF4BaF2 glasses had a predominantly one-dimensional structure, the absence of a modifier in the HfF4ThF4 system led to a three-dimensional network and a decrease in the glass-forming ability. The most probable coordination numbers for Hf and Th atoms are also discussed.

Numerical Integration and other Computational Techniques

Abstract: In this lecture we review the problems related to the computation of the several types of integrals present in the BEM context. Special emphasis is given to numerical integration techniques. The main reasons for this approach lie in its generality and in the fact that this is the only approach possible with higher order isoparametric elements.

Coherent structures in the near field of round jets

Abstract: The present paper reports measurements of axial and radial velocities obtained with laser-Doppler anemometry in the transitional region of a round jet The predominant frequencies of the coherent structures detected in the non-artificially excited jet, correspond to Strouhal numbers, around 033 and 055 and an interpretation of these observations is made The use of external excitation can modify some characteristics of the jets disturbing the balance between the different flow structures, namely those corresponding to the two values of St The jet diameter was 300 mm and the velocities ranged from near zero up to 15 m/s, corresponding to Reynolds number of 287 × 104