Kinetics of photoinduced birefringence in the guest-host system of poly(methyl methacrylate) doped with azobenzene-containing crown ethers

The kinetics of the buildup and decay of photoinduced birefringence was examined in a series of host-guest systems: azobenzene-containing crown ethers, differing in the size of the crowns, dissolved in a poly (methyl methacrylate) matrix. In all samples, the kinetics of the buildup of the birefringence was reasonably described by a sum of two exponential functions, the time constants being inversely proportional to the intensity of the pumping light and the magnitudes of the signals at the saturation level depending on the pumping light intensity and sample thickness. The dark decays were best described by the stretched exponential function, with the characteristic parameters (time constant and stretch coefficient) being practically independent of the type of crown ether. The time constants of the signal decay were orders of magnitude shorter than the respective constants of the dark isomerization of the azo crown ethers, thus indicating that the process controlling the decay was a relaxation of the polymer matrix and/or a rearrangement of the flexible parts of the crowns. (C) 2007 Wiley Periodicals, Inc.

Nucleation and growth of mullite whiskers from lanthanum-doped aluminosilicate melts

Nucleation and growth of mullite whiskers in the La2O3-Al2O3-SiO2 system were investigated in the 1500degrees-1700degreesC temperature range. A differential thermal analysis (DTA) showed that the mullitization temperature decreases from 1350degreesC to 1240degreesC as a result of lanthania doping. In the temperature range of 1250-1500degreesC, most of the mullite grains have an Al2O3/SiO2 = 1.5 composition throughout the ceramic body; however, from 1400degreesC upward, the number of anisotropic grains with the Al2O3/SiO2 = 1.3 composition begins to increase. The concentration of alumina in the composition of the grain-boundary phase decreases as firing temperatures increase. At temperatures > 1500degreesC, alumina grains and whiskers grow on the internal and external surfaces of the ceramic body with the characteristic Al2O3/SiO2 = 1.3 composition. Removal of the mullite whisker layer by acid attack revealed an alumina-rich, rosace-like patterned microstructure correlated with the process of whisker nucleation and growth. In the early stages, whisker growth rates were found to be near 60 mum/h. Experimental evidence pointed to nucleation inside the thin glass layer on the external surface.

The role of hydration on the mechanism of allosteric regulation: In situ measurements of the oxygen-linked kinetics of water binding to hemoglobin

We report here the first direct measurements of changes in protein hydration triggered by a functional binding. This task is achieved by weighing hemoglobin (Hb) and myoglobin films exposed to an atmosphere of 98%, relative humidity during oxygenation. The binding of the first oxygen molecules to Hb tetramer triggers a change in protein conformation, which increases binding affinity to the remaining empty sites giving rise to the appearance of cooperative phenomena. Although crystallographic data have evidenced that this structural change increases the protein water-accessible surface area, isobaric osmotic stress experiments in aqueous cosolutions have shown that water binding is linked to Hb oxygenation. Now we show that the differential hydration between fully oxygenated and fully deoxygenated states of these proteins, determined by weighing protein films with a quartz crystal microbalance, agree with the ones determined by osmotic stress in aqueous cosolutions, from the linkage between protein oxygen affinity and water activity. The agreements prove that the changes in water activity brought about by adding osmolytes to the buffer solution shift biochemical equilibrium in proportion to the number of water molecules associated with the reaction. The concomitant kinetics of oxygen and of water binding to Hb have been also determined. The data show that the binding of water molecules to the extra protein surface exposed on the transition from the low-affinity T to the high-affinity R conformations of hemoglobin is the rate-limiting step of Hb cooperative reaction. This evidences that water binding is a crucial step on the allosteric mechanism regulating cooperative interactions, and suggests the possibility that environmental water activity might be engaged in the kinetic control of some important reactions in vivo.

Kinetics of zinc uptake and anatomy of roots and leaves of coffee trees as affected by zinc nutrition

Zinc (Zn) uptake kinetics and root and leaf anatomy were studied in coffee trees grown in nutrient solutions with or without Zn. Leaves and roots were sampled and cuts were made in the medium part of the leaves and in root tips and observed under an optical microscope. Plants grown without Zn showed an increase in root and in root stele diameter. There was also an increase in epidermis thickness and in the cross-sectional area of the cortex and stele due to Zn deficiency, but the diameter of xylem vessels was decreased. An increase in root cortex and stele diameter provided for an increased surface for nutrient uptake. Accordingly, C(min) was decreased from 13.8 to 3.4 mu mol L(-1) and V(max) increased from 0.50 to 2.1 mu mol cm(-2) h(-1) .

Titanium(IV)-EDTA complex - Kinetics of thermal decomposition by non-isothermal procedures

This work aims the evaluation of the kinetic triplets corresponding to the two successive steps of thermal decomposition of Ti(IV)-ethylenediaminetetraacetate complex. Applying the isoconversional Wall-Flynn-Ozawa method on the DSC curves, average activation energy: E=172.4 +/- 9.7 and 205.3 +/- 12.8 kJ mol(-1), and pre-exponential factor: logA = 16.38 +/- 0.84 and 18.96 +/- 1.21 min(-1) at 95% confidence interval could be obtained, regarding the partial formation of anhydride and subsequent thermal decomposition of uncoordinated carboxylate groups, respectively.From E and logA values, Dollimore and Malek methods could be applied suggesting PT (Prout-Tompkins) and R3 (contracting volume) as the kinetic model to the partial formation of anhydride and thermal decomposition of the carboxylate groups, respectively.

Kinetics of B cell response during Yersinia enterocolitica infection in resistant and susceptible strains of mice

In this study we analyze the B-cell response in murine yersiniosis. To this end, we determined whether polyclonal activation of B-lymphocytes occurs during infection of susceptible (BALB/c) and resistant (C57BL/6) mice with Y. enterocolitica 0:8 and compared the immunoglobulin (Ig) isotypes produced in response to the infection by the two strains. The number of splenic cells secreting nonspecific and specific immunoglobulins was determined by ELISPOT. The presence of anti-Yersinia antibodies in serum was detected by ELISA. In both strains, the number of specific Ig-secreting cells was relatively low. Polyclonal B-cell activation was observed in both strains of mice, and the greatest activation was observed in the BALB/c mice, mainly for lgG(1)- and IgG(3)- secreting cells. The C57BL/6 mice showed a predominance of IgG(2a)-secreting cells. The peak production of anti-Yersinia IgG antibodies in the sera of BALB/c mice was seen on the 28th day after infection. The greatest increase in IgM occurred on the 14th day. A progressive increase of specific IgG antibodies was observed in C57BL/6 mice up to the 28th day after infection while IgM increased on the 21st day after infection. The production of specific IgA antibodies was not detected in either BALB/c or C57BL/6 mice. We conclude that polyclonal. activation of B lymphocytes occurs in both the Yersinia resistant and Yersinia-susceptible mice and that the more intense activation of B lymphocytes observed in the susceptible BALB/c mice does not enhance their resistance to Y. enterocolitica infection.

Air-drying of fresh and osmotically pre-treated pineapple slices: Fixed air temperature versus fixed slice temperature drying kinetics

In this investigation, the air drying characteristics of fresh and osmotically pre-treated pineapple slices in a tray dryer were studied under different operating conditions. The air velocity varied from 1.5 to 2.5 m/s and the air temperature from 40 to 70 degreesC. The analytical solution of the second Fick's law for an infinite slab was used to calculate effective diffusion coefficients and their temperature dependence could be well represented by an Arrhenius-type equation. Comparison of the results showed that the diffusion coefficients were lower for the pre-treated fruit. By means of automatic control, it was possible to obtain drying curves under conditions of constant product temperature, which showed to be an alternative to reduce the drying time of pineapple slices.

Diffusion-controlled growth of aggregates in layer-by-layer films of poly(o-methoxyaniline)

The adsorption process in layer-by-layer (LBL) films of poly(o-methoxyaniline) alternated with poly(vinyl sulfonic acid) is explained using the Avrami equation. This equation was used due to its mathematical simplicity and adequate description of experimental data in real polymer systems. The Avrami parameters are a convenient means to represent empirical data of crystallization, and if microscopic knowledge is available these parameters can also be associated with adsorption mechanisms. The growth of spherulites in the LBL films was studied as a function of time using atomic force microscopy and the data were used to estimate the number and radii of aggregates, from which the Avrami parameters were determined. We find that the adsorption mechanism may correspond to a tri dimensional, diffusion-controlled growth, with increasing nucleation rate, consistent with results from kinetics of adsorption.

Effect of processing conditions on the nucleation and growth of indium-tin-oxide nanowires made by pulsed laser ablation

Indium-tin oxide nanowires were deposited by excimer laser ablation onto catalyst-free oxidized silicon substrates at a low temperature of 500 degrees C in a nitrogen atmosphere. The nanowires have branches with spheres at the tips, indicating a vapor-liquid-solid (VLS) growth. The deposition time and pressure have a strong influence on the areal density and length of the nanowires. At the earlier stages of growth, lower pressures promote a larger number of nucleation centers. With the increase in deposition time, both the number and length of the wires increase up to an areal density of about 70 wires/mu m(2). After this point all the material arriving at the substrate is used for lengthening the existing wires and their branches. The nanowires present the single-crystalline cubic bixbyite structure of indium oxide, oriented in the [100] direction. These structures have potential applications in electrical and optical nanoscale devices.

Controlling the growth of zirconia needles precursor from a liquid crystal template

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Capacitance Spectroscopy: A Versatile Approach To Resolving the Redox Density of States and Kinetics in Redox-Active Self-Assembled Monolayers

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)