Temperature Dependence of Rb(2) Molecule Formation Rate Constant in a Magneto-Optical Trap

In this paper, we report the measurement of Rb(2) molecule formation rate constant due to a two body process in a magneto-optical trap as a function of the sample temperature. The ground state molecules are detected by two-photon ionization, through the intermediate a(3)Sigma(+)(u) -> 2(3)Pi(g) molecular band. Our results show that the Rb(2) molecules formed in the MOT could be due to a wave shape resonance, which enhances the molecule formation rate. This effect may be used to enhance the molecule production; and therefore it maybe important to future experiments involving production and trapping of cold ground state molecules.

Thiothionyl bromide (S=SBr2): The elusive isomer of dibromodisulfane (BrSSBr)

CCSD(T) with a series of correlation consistent basis up to quadruple-zeta is used to investigate the structures, vibrational spectra, relative stability, heats of formation, and barrier to isomerization of S=SBr2 and BrSSBr. It represents the most accurate and detailed characterization of these molecules to date. We show that the frequency mode at 302 cm(-1), detected in various studies and assigned to impurities by some authors, and to the anti-symmetric SBr stretch in BrSSBr by others, thus in fact corresponds to the anti-symmetric SBr stretch in the elusive S=SBr2 species; it thus corroborates and complements an earlier partial IR spectra study attributable to S=SBr2. Including corrections for relativistic and core-valence correlation effects, we also predict 26.33 (12.74) kcal/mol for Delta H-f (298.15 K) of S=SBr2 (BrSSBr). For the S=SBr2 -> BrSSBr reaction, our best estimates for the Gibbs free energy and the enthalpy of the reaction at 298.15 K are -13.71 and -13.44 kcal/mol, respectively. For a value of Delta G(#) equal to 23.52 kcal/mol, we estimate a TST rate constant, at 298.15 K, of 3.57 x 10(-5) s(-1). (c) 2007 Elsevier B.V. All rights reserved.

Excited-State Dynamics in fac-[Re(CO)(3)(Me(4)phen)(L)](+)

Excited-state dynamics in fac-[Re(CO)(3)(Me(4)phen)(cis-L)](+) (Me(4)phen = 3,4,7,8-tetramethyl-1,10-phenanthroline, L = 4-styrylpyridine (stpy) or 1,2-bis(4-pyridyl)ethylene (bpe)) were investigated by steady-state and time-resolved techniques. A complex equilibrium among three closely lying excited states, 3IL(cis-L), (3)MLCT(Re -> me4phen), and (3)IL(Me4phen), has been established. Under UV irradiation, cis-to-trans isomerization of coordinated cis-L is observed with a quantum yield of 0.15 in acetonitrile solutions. This photoreaction competes with radiative decay from (3)MLCT(Re -> Me4phen) and (3)IL(Me4phen) excited states, leading to a decrease in the emission quantum yield relative to the nonisomerizable complex fac-[Re(CO)(3)(Me(4)phen)(bpa)](+) (bpa = 1,2-bis(4-pyridyl)ethane). From temperature-dependent time-resolved emission measurements in solution and in poly(methyl methacrylate) (PMMA) films, energy barriers (Delta E(a)) for interconversion between (3)MLCT(Re -> me4Phen) and (3)IL(Me4phen) emitting states were determined. For L = cis-stpy, Delta E(a) = 11 (920 cm(-1)) and 15 kJ mol(-1) (1254 cm(-1)) in 5:4 propionitrile/butyronitrile and PMMA, respectively. For L = cis-bpe, Delta E(a) = 13 kJ mol(-1) (1087 cm(-1)) in 5:4 propionitrile/butyronitrile. These energy barriers are sufficient to decrease the rate constant for internal conversion from higher-lying (3)IL(me4phen) state to (3)MLCT(Re -> Me4phen), k(i) congruent to 10(6) s(-1). The decrease in rate allows for the observation of intraligand phosphorescence, even in fluid medium at room temperature. Our results provide additional insight into the role of energy gap and excited-state dynamics on the photochemical and photophysical properties of Re(I) polypyridyl complexes.

Electrochemical Reduction Using Glassy Carbon Electrode in Aqueous Medium of a Potential Anti-Chagas Drug: NFOH

Nitrofurazone (NF) presents activity against Chagas' disease, yet it has a high toxicity. Its analog, hydroxymethylnitrofurazone (NFOH), is more potent against Trypanosoma cruzi and much less toxic than the parent drug, NF. The electrochemical reduction of NFOH in an aqueous medium using a glassy carbon electrode (GCE) is presented. By cyclic voltammetry in anacidic medium, one irreversible reduction peak related to hydroxylamine derivative formation was registered, being linearly pH dependent. However, from pH > 7, a reversible reduction peak at a more positive potential appears and corresponds to the formation of a nitro radical anion. The radical-anion kinetic stability was evaluated by Ip(a)/Ip(c) the current ratio of the R-NO(2)/R-NO(2)-redox couple. The nitro radical anion decays with a second-order rate constant (k(2)) of 6.07, 2.06, and 1.44(X 10(3)) L mol(-1) s(-1) corresponding to pH 8.29, 9.29, and 10.2, respectively, with a corresponding half-time life (t(1/2)) of 0.33, 0.97, and 1.4 s for each pH value. By polishing the GCE surface with diamond powder and comparing with the GCE surface polished with alumina, it is shown that the presence of alumina affects the lifetime of the nitro radical anion. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3130082] All rights reserved.

A quasiclassical trajectory study of the OH plus SO reaction: The role of rotational energy

A full dimensional quasiclassical trajectory study of the OH+SO reaction is presented with the aim of investigating the role of the reactants rotational energy in the reactivity. Different energetic combinations with one and both reactants rotationally excited are studied. A passive method is used to correct zero-point-energy leakage in the classical calculations. The reactive cross sections, for each combination, are calculated and fitted to a capturelike model combined with a factor accounting for recrossing effects. Reactivity decreases as rotational energy is increased in any of both reactants. This fact provides a theoretical support for the experimental dependence of the rate constant on temperature.

Effect of terbium(III) on the binding of aromatic guests with sodium taurocholate aggregates

The effect of binding Tb(3+) to sodium taurocholate aggregates containing polyaromatic hydrocarbon guests was examined using pyrene and 1-ethylnaphthalene as guests that bind to the primary aggregate, and 1-naphthyl-1-ethanol as a secondary aggregate guest. Time-resolved fluorescence quenching studies were used to study the binding site properties, while laser flash photolysis quenching studies provided information on the dynamics of the guest-aggregate system. Both the primary and secondary aggregate binding sites became more compact in the presence of bound Tb(3+), while only the primary aggregate became more accessible to anionic molecules. The binding dynamics for the guest-primary aggregate system became faster when Tb(3+) was bound to the aggregate. In contrast, for the guest-secondary aggregate the presence of Tb(3+) resulted in a small decrease in the dissociation rate constant. The influence of bound Tb(3+) on the primary and secondary bile salt aggregates is significantly different, which affects how these aggregates can be used as supramolecular host systems to modify guest reactivity.

Modeling Vinyl Acetate and n-Butyl Acrylate Emulsion Copolymerization Process

This work presents a mathematical model for the vinyl acetate and n-butyl acrylate emulsion copolymerization process in batch reactors. The model is able to explain the effects of simultaneous changes in emulsifier concentration, initiator concentration, monomer-to-water ratio, and monomer feed composition on monomer conversion, copolymer composition and, to lesser extent, average particle size evolution histories. The main features of the system, such as the increase in the rate of polymerization as temperature, emulsifier, and initiator concentrations increase are correctly represented by the model. The model accounts for the basic features of the process and may be useful for practical applications, despite its simplicity and a reduced number of adjustable parameters.

Photoinduced Degradation of the Herbicide Clomazone Model Reactions for Natural and Technical Systems

The photodegradation of the herbicide clomazone in the presence of S(2)O(8)(2-) or of humic substances of different origin was investigated. A value of (9.4 +/- 0.4) x 10(8) m(-1) s(-1) was measured for the bimolecular rate constant for the reaction of sulfate radicals with clomazone in flash-photolysis experiments. Steady state photolysis of peroxydisulfate, leading to the formation of the sulfate radicals, in the presence of clomazone was shown to be an efficient photodegradation method of the herbicide. This is a relevant result regarding the in situ chemical oxidation procedures involving peroxydisulfate as the oxidant. The main reaction products are 2-chlorobenzylalcohol and 2-chlorobenzaldehyde. The degradation kinetics of clomazone was also studied under steady state conditions induced by photolysis of Aldrich humic acid or a vermicompost extract (VCE). The results indicate that singlet oxygen is the main species responsible for clomazone degradation. The quantum yield of O(2)(a(1)Delta(g)) generation (lambda = 400 nm) for the VCE in D(2)O, Phi(Delta) = (1.3 +/- 0.1) x 10(-3), was determined by measuring the O(2)(a(1)Delta(g)) phosphorescence at 1270 nm. The value of the overall quenching constant of O(2)(a(1)Delta(g)) by clomazone was found to be (5.7 +/- 0.3) x 10(7) m(-1) s(-1) in D(2)O. The bimolecular rate constant for the reaction of clomazone with singlet oxygen was k(r) = (5.4 +/- 0.1) x 10(7) m(-1) s(-1), which means that the quenching process is mainly reactive.

Impedimetric immunosensor for electronegative low density lipoprotein (LDL(-)) based on monoclonal antibody adsorbed on (polyvinyl formal)-gold nanoparticles matrix

Monoclonal antibodies (MAb) have been commonly applied to measure LDL in vivo and to characterize modifications of the lipids and apoprotein of the LDL particles. The electronegative low density lipoprotein (LDL(-)) has an apolipoprotein B-100 modified at oxidized events in vivo. In this work, a novel LDL-electrochemical biosensor was developed by adsorption of anti-LDL(-) MAb on an (polyvinyl formal)-gold nanoparticles (PVF-AuNPs)-modified gold electrode. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to characterize the recognition of LDL-. The interaction between MAb-LDL(-) leads to a blockage in the electron transfer of the [Fe(CN)(6)](4-)/K(4)[Fe(CN)(6)](3-) redox couple, which may could result in high change in the electron transfer resistance (R(CT)) and decrease in the amperometric responses in CV analysis. The compact antibody-antigen complex introduces the insulating layer on the assembled surface, which increases the diameter of the semicircle, resulting in a high R(CT), and the charge transferring rate constant k(0) decreases from 18.2 x 10(-6) m/s to 4.6 x 10(-6) m/s. Our results suggest that the interaction between MAb and lipoprotein can be quantitatively assessed by the modified electrode. The PVF-AuNPs-MAb system exhibited a sensitive response to LDL(-), which could be used as a biosensor to quantify plasmatic levels of LDL(-). (C) 2011 Elsevier B.V. All rights reserved.

Brazilian sugar cane bagasse ashes from the cogeneration industry as active pozzolans for cement manufacture

For proper management of wastes and their possible recycling as raw materials, complete characterization of the materials is necessary to evaluate the main scientific aspects and potential applications. The current paper presents a detailed scientific study of different Brazilian sugar cane bagasse ashes from the cogeneration industry as alternative cementing materials (active addition) for cement manufacture. The results show that the ashes from the industrial process (filter and bottom ones) present different chemical and mineralogical compositions and pozzolanic properties as well. As a consequence of its nature, the kinetic rate constant (K) states that the pozzolanic activity is null for the bottom ash and very low for the filter ash with respect to a sugar cane bagasse ash obtained in the laboratory under controlled burning conditions (reference). The scarce pozzolanic activity showed by ashes could be related to a possible contamination of bagasse wastes (with soils) before their use as alternative combustibles. For this reason, an optimization process for these wastes is advisable, if the ashes are to be used as pozzolans. (C) 2011 Elsevier Ltd. All rights reserved.

Pozzolanic behavior of bamboo leaf ash: Characterization and determination of the kinetic parameters

The paper presents a characterization and study of the pozzolanic behavior between calcium hydroxide (CH) and bamboo leaf ash (BLAsh), which was obtained by calcining bamboo leaves at 600 degrees C for 2 h in a laboratory electric furnace. To evaluate the pozzolanic behavior the conductometric method was used, which is based on the measurement of the electrical conductivity in a BLAsh/CH solution with the reaction time. Later, the kinetic parameters are quantified by applying a kinetic-diffusive model. The kinetic parameters that characterize the process (in particular, the reaction rate constant and free energy of activation) were determined with relative accuracy in the fitting process of the model. The pozzolanic activity is quantitatively evaluated according to the values obtained of the kinetic parameters. Other experimental techniques, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM), were also employed. The results show that this kind of ash is formed by silica with a completely amorphous nature and a high pozzolanic activity. The correlation between the values of free energy of activation (Delta G(#)) and the reaction rate constants (K) are in correspondence with the theoretical studies about the rate processes reported in the literature. (C) 2010 Elsevier Ltd. All rights reserved.

Effects of calcining conditions on the microstructure of sugar cane waste ashes (SCWA): Influence in the pozzolanic activation

in this paper a study of calcining conditions on the microstructural features of sugar cane waste ash (SCWA) is carried out. For this purpose, some microparticles (< 90 mu m) of sugar cane straw ash and sugar cane bagasse ash of samples calcined at 800 degrees C and 1000 are studied by combining the bright field and the dark field images with the electron diffraction patterns in the transmission electron microscopy (TEM). It is appreciated that the morphology and texture of these microparticles change when silicon or calcium are present. Furthermore, it is observed that iron oxide (magnetite Fe(3)O(4)) is located in the calcium-rich particles. The microstructural information is correlated with the results of a kinetic-diffusive model that allows the computing of the kinetic parameters of the pozzolanic reaction (mainly the reaction rate constant). The results show that the sugar cane wastes ash calcined at 800 and 1000 degrees C have properties indicative of high pozzolanic activity. The X-ray diffraction patterns, the TEM images and the pozzolanic activity tests show the influence of different factors on the activation of these ashes. (c) 2008 Elsevier Ltd. All rights reserved.

Study of the pozzolanic reaction kinetics in sugar cane bagasse-clay ash/calcium hydroxide system: kinetic parameters and pozzolanic activity

This paper presents a study of the pozzolanic reaction kinetics between calcium hydroxide and a mixture of sugar cane bagasse with 20 and 30% of clay, burned at 800 and 1000 degrees C (SCBCA) by electrical conductivity measurements. A kinetic-diffusive model produced in previous studies by some of the authors was used. The model was fitted to the experimental data, which allowed the computation of the kinetic parameters of the pozzolanic reaction (reaction rate constant and free energy of activation) that rigorously characterised the pozzolanic activity of the materials. The results show that SCBCA demonstrated reactivity and good pozzolanic qualities in the range 800-1000 degrees C.

Thiol and Sulfenic Acid Oxidation of AhpE, the One-Cysteine Peroxiredoxin from Mycobacterium tuberculosis: Kinetics, Acidity Constants, and Conformational Dynamics

Drug resistance and virulence of Mycobacterium tuberculosis are partially related to the pathogen`s antioxidant systems. Peroxide detoxification in this bacterium is achieved by the heme-containing catalase peroxidase and different two-cysteine peroxiredoxins. M. tuberculosis genome also codifies for a putative one-cysteine peroxiredoxin, alkyl hydroperoxide reductase E (MtAhpE). Its expression was previously demonstrated at a transcriptional level, and the crystallographic structure of the recombinant protein was resolved under reduced and oxidized states. Herein, we report that the conformation of MtAhpE changed depending on its single cysteine redox state, as reflected by different tryptophan fluorescence properties and changes in quaternary structure. Dynamics of fluorescence changes, complemented by competition kinetic assays, were used to perform protein functional studies. MtAhE reduced peroxynitrite 2 orders of magnitude faster than hydrogen peroxide (1.9 x 10(7) M(-1) s(-1) vs 8.2 x 10(4) M(-1) s(-1) at pH 7.4 and 25 degrees C, respectively). The latter also caused cysteine overoxidation to sulfinic acid, but at much slower rate constant (40 M(-1) s(-1)). The pK(a) of the thiol in the reduced enzyme was 5.2, more than one unit lower than that of the sulfenic acid in the oxidized enzyme. The pH profile of hydrogen peroxide-mediated thiol and sulfenic acid oxidations indicated thiolate and sulfenate as the reacting species. The formation of sulfenic acid as well as the catalytic peroxidase activity of MtAhpE was demonstrated using the artificial reducing substrate thionitrobenzoate. Taken together, our results indicate that MtAhpE is a relevant component in the antioxidant repertoire of M. tuberculosis probably involved in peroxide and specially peroxynitrite detoxification.

Inhibition of myeloperoxidase-mediated protein nitration by tempol: Kinetics, mechanism, and implications

Despite the therapeutic potential of tempol (4-hydroxy-2,2,6,6-tetra-methyl-1-piperidinyloxy) and related nitroxides as antioxidants, their effects on peroxidase-mediated protein tyrosine nitration remain unexplored. This posttranslational protein modification is a biomarker of nitric oxide-derived oxidants, and, relevantly, it parallels tissue injury in animal models of inflammation and is attenuated by tempol treatment. Here, we examine tempol effects on ribonuclease (RNase) nitration mediated by myeloperoxidase (MPO), a mammalian enzyme that plays a central role in various inflammatory processes.. Some experiments were also performed with horseradish peroxidase (HRP). We show that tempol efficiently inhibits peroxidase-mediated RNase nitration. For instance, 10 mu M tempol was able to inhibit by 90% the yield of 290 mu M 3-nitrotyrosine produced from 370 mu M RNase. The effect of tempol was not completely catalytic because part of it was consumed by recombination with RNase-tyrosyl radicals. The second-order rate constant of the reaction of tempol with MPO compound I and 11 were determined by stopped-flow kinetics as 3.3 x 10(6) and 2.6 x 10(4) M-1 s(-1), respectively (pH 7.4, 25 degrees C); the corresponding HRP constants were orders of magnitude smaller. Time-dependent hydrogen peroxide and nitrite consumption and oxygen production in the incubations were quantified experimentally and modeled by kinetic simulations. The results indicate that tempol inhibits peroxidase-mediated RNase nitration mainly because of its reaction with nitrogen dioxide to produce the oxammonium cation, which, in turn, recycles back to tempol by reacting with hydrogen peroxide and superoxide radical to produce oxygen and regenerate nitrite. The implications for nitroxide antioxidant mechanisms are discussed.