Ruben Elvas Leitão

Modeling Preferential Solvation in Ternary Solvent Systems

The theoretical solvent exchange model of Bosch and Rosés for binary solvents was extended to ternary solvent mixtures. The model was applied to ENT values for the mixture methanol/1-propanol/acetonitrile, in terms of 48 new values in a total of 79, measured at 25 degrees C over the whole range of solvent compositions. It was also applied to the mixture methanollethanol/acetone at the same temperature using 93 E(N)T values obtained from literature. Very good fits between experimental and calculated values, substantiated by external validation methods, were achieved for both sets of data. The use of the developed extended model allowed the interpretation of measured solvatochromic shifts in terms of solute-solvent and solvent-solvent interactions in the local environment of the solutes dye for the two ternary systems and the underlying binary mixtures. It also provided the identification of various complex solvent entities and the quantification of their relative concentrations in the probes cybotactic region, thus leading to new and significant physicochemical insights at a molecular level, regardless of the nonconsideration of the formation of solvent complexes in the bulk. Results clearly showed a different solvent composition in the vicinity of the solute. The further extension of the model to four and five components is also presented.

Solvent and temperature effects on ion association and mobility of 2,6-lutidinium chloride in non-aqueous solvents

Molar conductivity values (Λ m) of 2,6-lutidinium chloride at ten different concentrations ranging from 0.001 to 0.01 mol dm−3, in several solvents and temperatures are reported for: seven solvents (dimethyl sulphoxide, acetonitrile, propylene carbonate, N,N-dimethylacetamide, ethanol, 1-propanol and 1-butanol) at five different temperatures (60.00, 65.00, 70.00, 75.00 and 80.00°C) and three additional solvents (nitromethane, 2-methyl-1-propanol and 2-ethoxyethanol) at 60.00°C. Molar conductivities at infinite dilution ( ), association constants (K A) and respective errors are also determined. The temperature dependence of and K A shows positive, negative and negligible trends. versus viscosity plots reveal a breakdown of Waldens rule. K A is markedly dependent on the solvents features, revealing strong deviations from the predictions of classical dielectric theories. It is reasoned that specific ion–solvent–solvent interactions may play an important role.

Simultaneous monitoring of toxic metals on white poplar (populus) by SWASV

A tecnica electroanalitica, Voltametria de Redissolucao Anodica de Onda Quadrada (VRAOQ), foi utilizada na determinacao simultânea de quantidades traco dos metais toxicos Pb(II), Cd(II) e Cu(II) em folhas de choupo (populus), utilizadas como bio-indicador, recolhidas numa area de trafego automovel intenso da cidade de Lisboa. As folhas, apos secagem, foram submetidas a um processo de digestao acida por microondas. Para a aplicacao desta tecnica a analise das folhas foi realizado um estudo de optimizacao dos varios parâmetros voltametricos. O eletrodo de trabalho consistiu num filme de mercurio depositado numa superficie de carbono vitreo. O sistema Ag/AgCl foi utilizado como referencia e um fio de Platina como eletrodo auxiliar. Concentracoes medias (em mg de metal/kg de materia seca - folhas) de 2,6, 0,18 e 5,0 foram obtidas para o Pb(II), o Cd(II) e o Cu(II), respectivamente. O valor obtido para o chumbo coincide com o obtido pelo metodo de referencia baseado na Espectrofotometria de Absorcao Atomica em Forno de Grafite (EAAFG).

Molecular details of INH-C10 binding to wt KatG and Its S315T mutant.

Isoniazid (INH) is still one of the two most effective antitubercular drugs and is included in all recommended multitherapeutic regimens. Because of the increasing resistance of Mycobacterium tuberculosis to INH, mainly associated with mutations in the katG gene, new INH-based compounds have been proposed to circumvent this problem. In this work, we present a detailed comparative study of the molecular determinants of the interactions between wt KatG or its S315T mutant form and either INH or INH-C10, a new acylated INH derivative. MD simulations were used to explore the conformational space of both proteins, and results indicate that the S315T mutation did not have a significant impact on the average size of the access tunnel in the vicinity of these residues. Our simulations also indicate that the steric hindrance role assigned to Asp137 is transient and that electrostatic changes can be important in understanding the enzyme activity data of mutations in KatG. Additionally, molecular docking studies were used to determine the preferred modes of binding of the two substrates. Upon mutation, the apparently less favored docking solution for reaction became the most abundant, suggesting that S315T mutation favors less optimal binding modes. Moreover, the aliphatic tail in INH-C10 seems to bring the hydrazine group closer to the heme, thus favoring the apparent most reactive binding mode, regardless of the enzyme form. The ITC data is in agreement with our interpretation of the C10 alkyl chain role and helped to rationalize the significantly lower experimental MIC value observed for INH-C10. This compound seems to be able to counterbalance most of the conformational restrictions introduced by the mutation, which are thought to be responsible for the decrease in INH activity in the mutated strain. Therefore, INH-C10 appears to be a very promising lead compound for drug development.

Study of Metal NH3 Interfaces (Metal = Cu, Ni, Ag) Using Potentiostatic Curves.

Students determine kinetic and thermodynamic parameters of different metal�solution interfaces. These parameters are obtained from potentiostatic curves using the Tafel and the Arrhenius equations. The work involves the variation of type of metal, solution concentration, and temperature. The results allow the discussion of the polarizability, transfer coefficient, and activation energy of the processes, as well as the differences between them.