The mechanism of the reaction between Au(III) and PADA in Sodium dodecylsulphate

The PADA/SDS system provides an excellent tool to selectively extract metal ions using the ligand-modified micellar-enhanced ultrafitration (LM-MEUF) technique. Application of this method to the extraction of Au(III) has required a detailed knowledge of the conditions under which the interaction of the metal ion with the extractor are optimal. For this purpose the kinetics and the equilibria of the reaction between tetrachloro-aurate ion and PADA have been investigated in water/SDS medium, exploring wide ranges of pH values and NaCl concentrations. Addition of PADA to the water/SDS medium results in the full adsorption of the ligand on the micelle and, in the presence of Au(III), the resulting Au-PADA complex is fully retained on the SDS surface. The binding process is, in fact, a ligand displacement reaction where PADA interacts with different Au(III) chloro-aquo complexes, displacing Cl− or OH− or H2O molecules, depending on pH. The reaction is biphasic and its mechanism is discussed. Experiments at different SDS concentrations show that the reaction of complex formation is retarded on going from pure water to a water/SDS mixture with [SDS] just above the cmc, while for further increases of the SDS content the reaction rate tends to stay constant. The obtained results enabled to establish that the reaction occurs on the SDS surface while the aquo-species, Au(H2O)Cl3 and Au(OH)3(H2O), which are not involved in the binding process in pure water, play an important kinetic role in the H2O/SDS medium

The kinetics of gold(III) extraction by pyridine-2-azo-p-dimethylaniline in water and in micellar systems

The kinetics of reaction between AuCl4−and the azo-dye pyridine-2-azo-p-dimethylaniline (PADA) havebeen investigated in water and in the presence of DTAC micelles, using classical spectrophotometryand the stopped-flow technique. PADA reacts with different chloro/hydroxo gold(III) complexes, in turnformed as the pH and Cl−concentration were changed, according to a network of parallel paths. In aqueoussolution, at low pH values, a fast step is observed which is ascribed to the ligand induced expulsion ofa labile water molecule from the reacting species Au(H2O)Cl3which forms at low pH values. At highervalues of pH, the reaction is much slower because, in the key step, PADA has to replace the more inertCl−ions in the gold coordination shell. In the presence of DTAC a remarkable catalytic effect is observed,owing to the absorption and attraction of the reactants on the micelle surface. Moreover, DTAC favors theformation of aquochloro aurates, thus inducing a change in the gold(III) speciation compared to that inwater. The analysis of the data suggests that the aquo species Au(H2O)Cl3and Au(H2O)2Cl2+play a majorrole in the reaction mechanism

Gold(III) extraction and recovery and gold(III)/copper(II) separation using micelles

Gold extraction from aqueous solutions has been performed by micellar enhanced ultrafiltration (MEUF), using SDS and DTAC micelles. DTAC entraps 92-100% of the metal, which is present in form of AuCl4-, owing to electrostatic interaction. Using SDS the extraction of gold is unsuccessful. However, addition of PADA, as a metal extractor agent, according to the ligand modified micellar enhanced ultrafiltration (LM-MEUF) procedure, makes the extraction yield to approach 100% also with SDS. The recovery of gold entrapped in the micellar pseudo-phase has also been investigated. Different stripping agents have been used, the most efficient of them being a mixture of NaCl and NH3 which allowed the metal to be expelled from the micelle with a yield of 85%. Finally, Au(III)/Cu(II) separation is achieved with DTAC or SDS between pH 3 and 5. Almost 100% of Au(III) is retained on DTAC micelles while all Cu(II) remains in the aqueous medium, while the opposite occurs in SDS

Intercalation of Zn(II) and Cu(II) complexes of the cyclic polyamine Neotrien into DNA: equilibria and kinetics

The equilibria and kinetics of the interaction of the Zn(II) and Cu(II) complexes of the macrocyclic polyamine 2,5,8,11-tetraaza[12]-[12](2,9)[1,10]-phenanthrolinophane (Neotrien) with calf thymus DNA have been investigated at pH = 7.0 and T = 25 degreesC by spectrophotometry, spectrofluorimetry and stopped-flow method. At low dye/polymer ratios both complexes bind to DNA according to the excluded site model. At high dye/polymer ratios the binding displays cooperative features. The logarithm of the binding constant depends linearly on - log[NaCl]. The kinetic results suggest the D + S reversible arrow D, S reversible arrow DS mechanism where the metal complexes (D) react with the DNA sites (S) leading to fast formation of an externally bound form (D, S) which, in turn, is converted into internally bound complex (DS) by intercalation. The binding constants, evaluated as ratios of rate constants, agree with those obtained from equilibrium binding experiments, thus confirming the validity of the proposed model. Fluorescence titrations, where the metal-Neotrien complexes were added to DNA previously saturated with ethidium bromide (EB), show that both complexes displace EB from the DNA cavities. The reverse process, i.e. the addition of excess ethidium to the DNA/metal Neotrien systems, leads to fluorescence recovery for DNA/ZnNeotrien but not for DNA/CuNeotrien. This observation suggests that the binding of CuNeotrien induces deep alterations in the DNA structure. Experiments with Poly(dA-dT) . Poly(dA-dT) and Poly(dG-dC) . Poly(dG-dC) reveal that CuNeotrien mainly affects the structure of the latter polynucleotide. (C) 2004 Elsevier Inc. All rights reserved

Mg(II) and Ni(II) induce aggregation of poly(rA)poly(rU) to either tetra-aggregate or triplex depending on the metal ion concentration

The ability of magnesium(II) and nickel(II) to induce dramatic conformational changes in the synthetic RNA poly(rA)poly(rU) has been investigated. Kinetic experiments, spectrofluorometric titrations, melting experiments and DSC measurements contribute in shedding light on a complex behaviour where the action of metal ions (Na+, Mg2+, Ni2+), in synergism with other operators as the intercalating dye coralyne and temperature, all concur in stabilising a peculiar RNA form. Mg2+ and Ni2+ (M) bind rapidly and almost quantitatively to the duplex (AU) to give a RNA/metal ion complex (AUM). Then, by the union of two AUM units, an unstable tetraaggregate (UAUA(M2)*) is formed which, in the presence of a relatively modest excess of metal, evolves to the UAUM triplex by releasing a single AM strand. On the other hand, under conditions of high metal content, the UAUA(M2)* intermediate rearranges to give a more stable tetra-aggregate (UAUA(M2)). As concerns the role of coralyne (D), it is found that D strongly interacts with UAUA(M2). Also, in the presence of coralyne, the ability of divalent ions to promote the transition of AUD into UAUD is enhanced, according to the efficiency sequence [Ni2+]≫[Mg2+]≫[Na+]

Comportamento de ondas de ultrassom na presença de ocos na madeira

This work aimed to analyze the pattern of variation in wave lengths in presence of wood holes and to develop a model capable of describing the process. To attain that end, wood pieces were used from species pequiá (Aspidosperma desmanthum), on which circular and linear artificial holes were made and gradually enlarged. Ultrasonic tests were performed using USLab equipment and 45 kHz transducers. Measurements were taken first on the intact piece and then after each stage of artificial hole enlargement. Results demonstrated that propagation velocities of ultrasonic waves are affected by presence of holes and also that reduction in velocity is caused by changes in wave path, since waves tend to deviate from empty space and travel through matter. The circular hole type had a slightly stronger influence on velocity reduction than the linear hole type. Variation in velocity as a function of increasing percentage of hollow space relative to the intact piece can be represented by a linear model.Neste trabalho, objetivou-se estudar o comportamento da variação do percurso da onda em presença de oco na madeira bem como elaborar um modelo que o descrevesse. Para atingir esse objetivo, foram utilizadas peças de madeira da espécie pequiá (Aspidosperma desmanthum) nos quais foram confeccionados orifícios artificiais (circular e linear). Esses orifícios foram sendo aumentados gradualmente. Os ensaios de ultrassom foram realizados utilizando equipamento da marca USLab e transdutores de 45 kHz. As medições foram realizadas na peça íntegra e após cada fase de confecção do oco artificial. Os resultados permitiram confirmar que as velocidades de propagação de ondas de ultrassom são afetadas pela presença de ocos e que a redução da velocidade é provocada pela mudança de percurso da onda, que tende a se desviar do vazio e percorrer o material. O oco do tipo circular teve influência levemente superior na redução da velocidade do que o oco do tipo linear. A variação da velocidade com o aumento da porcentagem de oco em relação à peça integra pode ser representada por um modelo linear.507514Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Binding of Al(III) to synthetic RNA and metal-mediated strand aggregation

Over the last few years, focused interest in aluminum has been heightened by recent studies regarding its health effects. Its possible relation with chronic diseases makes it convenient to address more in depth the reactivity of aluminum with biologically relevant molecules. The present work investigates the interaction of the aluminum ion with two synthetic RNAs, poly(rA) and poly(rU), through a detailed thermodynamic and kinetic study. The trivalent aluminum ion was kept in solution by complexation with the cacodylate anion, even at neutral pH, thus making the study with biological molecules feasible. The results obtained by spectrophotometry, circular dichroism, viscometry and thermal stability measurements indicate that aluminium strongly interacts with single and duplex RNA structures. The kinetic experiments point out that, even though cacodylate is required to keep the metal in solution, it actually inhibits the reaction of aluminum with RNA as it converts the metal into an unreactive dimer species. Notably, further interaction occurred in an excess of the aluminum/cacodylate complex, inducing aggregation of single-stranded RNAs. An analysis of the kinetic data has shown that the modes of aggregation of the two RNAs differ and such a difference can be ascribed to the diverse polynucleotide secondary structures. The observed stabilization of multiple-stranded systems by aluminum can serve as a model for future studies due to the interest aroused by this metal in the study of non-canonical nucleic acid structures