G. López-Pérez

Thermodynamic and structural study of phenanthroline derivative ruthenium complex/DNA interactions: probing partial intercalation and binding properties.

The binding of [Ru(PDTA-H(2))(phen)]Cl (PDTA = propylene-1,2-diaminetetra-acetic acid; phen = 1,10 phenanthroline) with ctDNA (=calf thymus DNA) has been investigated through intrinsic and induced circular dichroism, UV-visible absorption and fluorescence spectroscopies, steady-state fluorescence, thermal denaturation technique, viscosity and electrochemical measurements. The latter indicate that the cathodic and anodic peak potentials of the ruthenium complex shift to more positive values on increasing the DNA concentration, this behavior being a direct consequence of the interaction of both the reduced and oxidized form with DNA binding. From spectrophotometric titration experiments, the equilibrium binding constant and the number of monomer units of the polymer involved in the binding of one ruthenium molecule (site size) have been quantified. The intrinsic circular dichroism (CD) spectra show an unwinding and a conformational change of the DNA helix upon interaction of the ruthenium complex. Quenching process, thermal denaturation experiments and induced circular dichroism (ICD) are consistent with a partial intercalative binding mode.

Non-linear QSAR modeling by using multilayer perceptron feedforward neural networks trained by back-propagation

The use of multilayer perceptrons (MLP) feedforward neural networks trained by back-propagation (BP) for non-linear QSAR model building is presented and explained in detail through a case study. This method was compared with others often used in this field, such as multiple linear regression (MLR), partial least squares (PLS) and quadratic PLS (QPLS). The case study deals with a series of 18 alpha adrenoreceptors agonists belonging to three different classes (alpha-1, alpha-2 and alpha-1,2) according to their different pharmacological effects. Each of them is described by 15 chemical features (the X block). Six pharmacological responses were also measured for each one to build the matrix of biological responses (the Y block). The results obtained indicated a slightly better performance of MLP against the other procedures, when using the correlation coefficient of the observed versus predicted response plots as an indicator of the goodness of the fit.

Performing procrustes discriminant analysis with HOLMES.

Program HOLMES devised by target factor analysis has been updated for performing procrustes discriminant analysis (PDA). Computational details are outlined. The equivalence between PDA and partial least squares-discriminant analysis (PLS-DA) is established. Application of the PDA is illustrated by two case studies taken from literature.

Holmes, a program for performing Procrustes Transformations

Abstract The program Holmes 2000, intended for performing Procrustes Transformations, like in Target Factor Analysis (TFA) and Procrustes Discriminant Analysis (PDA), is presented and described in terms of quality and performance for chemometric purposes.

Estimation of activity coefficients at different temperatures by using the mean spherical approximation

A method of estimating activity coefficients for a variety of electrolytes at different temperatures is presented. The MSA approximation is used to calculate the activity coefficients from experimental data available in the literature. This strategy provides suitable results within a wide range of temperatures, electrolyte stoichiometries and concentrations of investigated solutions.

Reorientation of Thiols during 2D Self-Assembly: Interplay between Steric and Energetic Factors

Reorientation of thiols during their 2D self-assembly is well established; however, little is known about its energetics and the factors that control its onset. We have developed a new strategy to determine the critical reorientational surface concentration (crsc) of thiols at the substrate/solution interface, which makes use of a cathodic stripping protocol. Its application to distinct homologous series of alkylthiols shows that the magnitude of the crsc and its variation with the molecular size is strongly dependent on the nature of the terminal group. Methyl-terminated alkylthiols reorient close to the saturation coverage of the lying-down phase, thus following their molecular size trend; whereas reorientation of alkylthiols bearing a negatively charged end group starts well below the monolayer coverage of the lying-down phase, with its onset being almost independent of the molecular size. Hydroxy-terminated alkylthiols show an intermediate behavior. A theoretical approach is developed to determine the reorientation equilibrium constant from the crsc value. The standard free energy of reorientation has been found to vary linearly with the alkyl chain length, and to increase upon replacing the terminal methyl group by a negatively charged one. A quantitative correlation between the reorientation equilibrium constant and the hydrophobicity of the molecule has been established. Overall, these findings have allowed us to disentangle the role of steric and energetic factors in the onset of the reorientation process of alkylthiols, demonstrating that their interplay can be finely tuned by varying either the alkyl chain length or the nature of the terminal group.

Use of gold nanoparticles as crosslink agent to form chitosan nanocapsules: study of the direct interaction in aqueous solutions.

A systematic study of the interaction between free anionic gold nanoparticles and chitosan in a solution is presented. A spectroscopic study of the interaction between 10nm gold nanoparticles and low molecular weight chitosan is reported as a function of the concentration and pH of the polymer in a solution. Zeta potential measurements and TEM images indicate the effective aggregation of the nanoparticles in the presence of chitosan. At the same time, anionic gold nanoparticles act as crosslink agents to form chitosan nanocapsules with an average molecular size of 260nm. The changes of the surface plasmon band due to the adsorption of the polymer on the nanoparticle surface allow using of the citrate gold nanoparticles as sensors of the polymer for analytical purposes. The limit of detection for chitosan biopolymer is 69nM. The optimum pH for the interaction between the biopolymer and the nanoparticles is found at a value of 6.4, obtained from spectrophotometric measurements and applying a deconvolution analysis of the experimental data. A simple model based on molecular surface electrostatic interactions is proposed to understand the pH dependence of the investigated system.

Influence of temperature on the reduction kinetics of Zn2+ at a mercury electrode

The reduction of Zn 2 at a mercury electrode has been studied by the a.c. impedance technique in the 263/318 K temperature range. Kinetic data have been analyzed in terms of a mechanism consisting of two consecutive charge transfer steps. The apparent activation enthalpies and charge transfer coefficients of both steps are found to be temperature independent. Differential capacity curves and potentials of zero charge of the NaClO4 supporting electrolyte solutions were measured at each temperature of interest. Double layer effects on the Zn 2 reduction rate constants were analyzed within the framework of the unequal distances of closest approach theory, and the corrected rate constants were found to be independent of supporting electrolyte concentration when allowance for changes in the Zn 2 activity coefficient was made. The implications of the activation parameter values on the nature of the rate determining steps are also discussed. # 2002 Elsevier Science B.V. All rights reserved.

Aliphatic Alcohols Facilitate Interfacial Reorientation of Thiols: Correlation with Alcohol Adsorptivity

The influence of a series of aliphatic alcohols on the reorientation of alkylthiols during their self-assembly has been studied by cathodic stripping voltammetry. The presence of an aliphatic alcohol in the deposition solution is shown to lower the critical reorientational surface concentration of alkylthiols, making it less sensitive to molecular size. The use of a series of alcohols differing in their molecular length or branching reveals that the onset of thiol reorientation correlates well with the extent of alcohol adsorption. A theoretical model is developed to account for this effect, whose crux is the competition between the alcohol molecule and the alkyl chain of the thiol for adsorption sites. The analytical expression derived for the critical reorientational surface concentration reveals that the effect of adding alcohol can be rationalized in terms of an apparent reorientation equilibrium constant, which embodies both the bulk concentration and the adsorption equilibrium constant of the relevant alcohol. Overall, these findings corroborate that the interfacial reorientation of n-alkanethiols is sterically controlled and demonstrate that its onset can be finely tuned by addition of a suitable coadsorbate.

Modification of a Commercial SMDE Electrode to Improve the Precision of Electrochemical Measurements

A modification of the commercial PARC 303A SMDE has been developed to improve the precision of electrochemical measurements by using an easy integrated electrical design. The original remote operation for the Dislodge/Dispense cycle has been modified allowing an entire time control of the signals involved in this process. Highly reproducible experimental data can be obtained using this procedure.