Showing papers by "Instituto Superior Técnico published in 1993"

Steady-state drainage of an aqueous foam.

Abstract: The process of wetting of a monodisperse foam by the continuous addition of liquid from the top has been investigated for a cylindrical column. The velocity of the interface between dry and wet foam was found to vary as the square root of the rate of addition of liquid. Evidence of a structural transformation was found around gas fraction \ensuremath{\Phi}=0.87. It was interpreted as arising from the collapse of the four-sided faces of the (Kelvin) polyhedra which constitute the foam cells, and this was directly observed when large bubbles were used. There is a second transition at around \ensuremath{\Phi}=0.6, with the onset of bubble motion.

An introduction to the angular Fourier transform

Abstract: The author introduces the angular Fourier transform (AFT), a generalization of the classical Fourier transform. The AFT can be interpreted as a rotation on the time-frequency plane. An AFT with an angle of alpha = pi /2 corresponds to the classical Fourier transform, and an AFT with alpha =0 corresponds to the identity operator. The angles of successively performed AFTs simply add up, as do the angles of successive rotations. A number of properties of the AFT are given. Most important among these are the AFT's relationships with time-frequency representations such as the Wigner distribution, the ambiguity function, the short-time Fourier transform, and the spectrogram. These relationships have a very simple and natural form, which further enhances the AFT's interpretation as a rotation operator. An example of the application of the AFT to the study of swept-frequency filters is given. >

Synthesis of fatty acid esters by a recombinant cutinase in reversed micelles

Abstract: Fusarium solani pisi recombinant cutinase solubilized in AOT/isooctane reversed micelles could effectively catalyze the esterification of fatty acids with aliphatic alcohols. Some relevant parameters for the enzyme activity such as pH, Wo (molar ratio water/surfactant) and temperature were investigated. The cutinase showed selectivity for short chain fatty acids and amphiphilic alcohols.

An ultrafiltration membrane bioreactor for the lipolysis of olive oil in reversed micellar media

Abstract: The enzymatic hydrolysis of olive oil using Chromobacterium viscosum lipase B encapsulated in reversed micelles of dioctyl sodium sulfosuccinate (AOT) in isooctane was investigated in an ultrafiltration ceramic membrane reactor of tubular type, operating in a batch mode. Water concentration was found to be a critical parameter in the enzyme kinetics and hydrolysis yield of the reaction. The size of micelles, recirculation rate, and substrate concentration were found to be the major factors affecting the separation process. A correlation that enables the prediction of final conversion degrees in this bioreactor from the initial reaction conditions was established.

Vibrational spectra and structure of fluoroindate glasses

Abstract: A series of fluoroindate glasses, both binary (x InF3·[1 − x]MF2, M = Sr, Ba) and multicomponent (InF3-ZnF2-SrF2-BaF2-CdF2), have been prepared and their infrared absorption and reflection spectra were measured. Polarized Raman spectra were also recorded for all the different glass compositions. Starting with the binary compositions, some structural conclusions were derived for these fluoroindate glasses, whose structure is still virtually unknown. The In atoms appear to be always in octahedral coordination, as inferred from both the infrared and Raman spectra, the latter being dominated by a highly polarized high frequency mode at ∼ 509 cm−1, in a fashion similar to fluorozirconate glasses. Other features in the vibrational spectra provided additional insight into the structural environments of the glass cations, namely, Zn.

Pseudodifferential Operators on Compact Manifolds with Lipschitz Boundary

Abstract: Pseudodifferential operators with non-smooth symbols on a manifold M with Lipschitz boundary are considered. Theorems about order reduction and localization of such operators in Bessel potential H and Holder-Zygmund Z(IRn) spaces are proved. A pseudodifferential operator A with locally sectorial matrix symbol is proved to be Fredholm in the space H (M) and Ind A = 0 where s depends on A. Application to a boundary value problem for an elastic body with crack is discussed in conclusion.

Cytochrome c6 from Monoraphidium braunii A cytochrome with an unusual heme axial coordination

Abstract: A soluble monoheme c-type cytochrome (cytochrome c6) has been isolated from the green alga Monoraphidium braunii. It has a molecular mass of 9.3 kDa, an isoelectric point of 3.6 and a reduction potential of 358 mV at pH 7. The determined amino acid sequence allows its classification as a class-I c-type cytochrome. The ferric and ferrous cytochrome forms and their pH equilibria have been studied using 1H-NMR, ultraviolet/visible, EPR and Mossbauer spectroscopies. The pH equilibria are complex, several pKa values and pH-dependent forms being observed. The amino acid sequence, the reduction-potential value and the visible and NMR spectroscopies data in the pH range 4-9 indicate that the heme iron has a methionine-histidine axial coordination. However, the EPR and Mossbauer data obtained for the ferricytochrome show that in this pH range two distinct forms are present: form I, gz = 3.27, gy = 2.05 and gx = 1.05; form II, gz = 2.95, gy = 2.29 and gx = 1.43. While form I has crystal-field parameters typical of a methionine-histidine coordination, those associated with form II would suggest a histidine-histidine axial ligation. This possibility was extensively analyzed by spectroscopic methods and by chemical modification of a histidine residue. It was concluded that form II actually corresponds to an unusual type of methionine-histidine axial coordination. Straightforward examples of this type of coordination have recently been found in other c-type hemeproteins [Teixeira, M., Campos, A. P., Aguiar, A. P., Costa, H. S., Santos, H., Turner, D. L. & Xavier, A. V. (1993) FEBS Lett. 317, 233-236], corroborating our proposal. Since both forms, with very distinct crystal-field parameters, are shown to have the same reduction potential, it may be concluded that the axial and rhombic distortions of the heme-iron ligand field cannot be directly correlated with the heme-reduction potential. The pH-dependence studies have also shown that the form I and form II are interconvertible, with pKa approximately 5. To establish a possible physiological significance for this process, in particular for the interaction of the cytochrome with the membrane-bound electron-transfer complexes b6f and photosystem I, the effect of surfactants on the spectroscopic characteristics of cytochrome c6 has been studied.

The Thermal Conductivity of Toluene and Water

Abstract: A new instrument is presented to measure the thermal conductivity of polar and electrically conducting liquids based on the transient coated hot-wire method. The performance of the apparatus has been assessed with toluene and water, which are recognized as standard reference materials for nonpolar and polar fluids, respectively. New results are reported fort the thermal conductivity of these liquids between 298 and 370 K and at pressures slightly above the saturation. The results show that the instrument is capable of an accuracy better than ±0.5%, while the precision and reproducibility are better than ±0.3%.

The surface tension of cork from contact angle measurements

Abstract: The surface tension of cork is evaluated from contact angle measurements in cork-water-air and cork-methylene iodide-air systems, using the geometric and the harmonic mean methods. The contact angles were studied as a function of time and, in each case, the extrapolated value for t = 0 was used in the calculations to obtain γs. In spite of the chemical and physical complexity of cork, it was possible to obtain an average value for its surface tension γs = 32 ± 3 dyne cm -1 and polarity γ P S /γ S = 0.25. These values were compared to others obtained for several natural products such as wood or cellulose derivatives.

Polymerization of ethylene using a high-activity Ziegler–Natta catalyst. I. Kinetic studies

Abstract: Factors affecting the particular shape of kinetic rate–time profiles in the polymerization of ethylene with a MgCl2-supported TiCl4 catalyst activated by Al(C2H5)3 have been investigated Examination of the dependence of the polymerization rate on the concentration of Al(C2H5)3 resulted in a Langmuir–Hinshelwood rate law Analysis of the polymerization rate as a function of the polymerization temperature gave about 46 kJ mol−1 for the overall activation energy Examination of the rapid decay of the polymerization rate with time showed that this decay is represented better by a first-order decay law than by a second-order one © 1993 John Wiley & Sons, Inc

Subunit composition, crystallization and preliminary crystallographic studies of the Desulfovibrio gigas aldehyde oxidoreductase containing molybdenum and [2Fe-2S] centers

Abstract: The Desulfovibrio gigas aldehyde oxidoreductase contains molybdenum bound to a pterin cofactor and [2Fe-2S] centers. The enzyme was characterized by SDS/PAGE, gel-filtration and analytical ultracentrifugation experiments. It was crystallized at 4 degrees C, pH 7.2, using isopropanol and MgCl2 as precipitants. The crystals diffract beyond 0.3-nm (3.0-A) resolution and belong to space group P6(1)22 or its enantiomorph, with cell dimensions a = b = 14.45 nm and c = 16.32 nm. There is one subunit/asymmetric unit which gives a packing density of 2.5 x 10(-3) nm3/Da (2.5 A3/Da), consistent with the experimental crystal density, rho = 1.14 g/cm3. One dimer (approximately 2 x 100 kDa) is located on a crystallographic twofold axis.

Fluorescence of acridine and acridine 9-carboxylic acid in anionic micelles

Abstract: Using steady state and transient state fluorescence techniques, a comparative study of fluorescence parameters of acridine and acridine 9-carboxylic acid in water and sodium dodecyl sulphate (SDS) micelles, at controlled pH, is presented. When fluorescence probes are micellized, the fluorescence quantum yields are low, φf=0.02-0.06, and the fluorescence decays are complex. The radiative constants kf are (1-2)x107 s− in both media, but the non-radiative constants increase 20 times in acridine and twentyfold to thirtyfold in acridine 9-carboxylic acid. The data point to a lowest excited state π-π*, with increased mixing of n-π* in acridine 9-carboxylic acid accounting for a significant increase in the non-radiative constants knr (2X 108 s−1) in H2O and SDS in spite of the efficiency of hydrogen bonding.

Modeling lipolysis in a reversed micellar system: part I. conventional batch reactor.

Abstract: Triglyceride hydrolysis using Chromobacterium viscosum lipase B in a reversed micellar media was investigated in a batch-type reactor. The dynamic evolution of the product concentration was analyzed using several mechanistic models, both from the literature and developed in this work. A kinetic model with nonlinear product inhibition was found to be the most adequate for the description of batch hydrolysis data over an extensive range of time and substrate concentration. The obtained rate equation described the time course of not only the reactions performed in this work, at different water contents (W0 = 7, 24) and pH values, but also the experimental results obtained in the literature with a Candida rugosa lipase. © 1993 John Wiley & Sons, Inc.

Hydroxyl radical induced damage to the purine bases of DNA : in vitro studies

Abstract: Il est bien connu que le radical hydroxyle (OH o ) joue un role tres important dans la degradation des tissus vivants sous l'action des rayonnements ionisants. L'une des causes principales de ce phenomene est le changement chimique induit sur les bases de l'ADN par ce radical. Ce travail rend compte de la reaction du radical OH o avec les bases puriques de l'ADN. Des resultats relatifs a l'adenine et a la guanine sont presentes, qui concernent l'etude cinetique (par radiolyse pulsee avec detection optique et conductimetrique) des radicaux primaires formes, de leurs reactions de transformation ainsi que l'analyse par HPLC des produits stables correspondants