Miquel Esteban

Multivariate curve resolution with alternating least squares optimisation: a soft-modelling approach to metal complexation studies by voltammetric techniques

Abstract Voltammetry of metal ions in the presence of ligands is usually interpreted by means of hard-modelling approaches. Soft-modelling approaches have been scarcely applied to electroanalytical data, especially in comparison with their use in spectroscopy. In recent years, the multivariate curve resolution–alternating least squares (MCR–ALS) method, based on factor analysis, has been shown to be a powerful tool in the analysis of metal complexation processes by a variety of voltammetric techniques. In this article, an overview of the method and its applications is presented. For this purpose, either numerically simulated or experimental data corresponding to different characteristic systems are analysed. A very interesting feature of MCR–ALS is that, by using different constraints, both the pure voltammograms of the different electrochemical processes involved and the corresponding concentration profiles are easily obtained. This type of information cannot be reached by means of classical univariate data analysis techniques. From calculated concentration profiles, the model of complexation and the equilibrium stability constants are estimated.

Cadmium-binding properties of glutathione: A chemometrical analysis of voltammetric data

The complexation of glutathione (GSH) by Cd2+ has been investigated as a model for the coordination chemistry of Cd2+ by thiol-containing peptides. Experimental data obtained by differential pulse polarography (DPP) for different Cd2+-to-GSH concentration ratios at fixed pH have been globally analyzed by the recently proposed multivariate curve resolution (MCR) method. Polarograms from experiments at fixed Cd2+-to-GSH concentration ratios but changing pH values have been univariately deconvoluted at each pH value. In both cases, the obtained single DPP peaks can be associated to specific electrode processes. The results obtained in this work, together with structural data from the literature, allow us to propose a model for the complexation of Cd2+ by GSH involving the formation of Cd(GSH)2 and Cd2(GSH)2 complexes. Moreover, a scheme for the electrode process is also proposed.

Antimony film screen-printed carbon electrode for stripping analysis of Cd(II), Pb(II), and Cu(II) in natural samples.

An in-situ antimony film screen-printed carbon electrode (in-situ SbSPCE) was successfully used for the determination of Cu(II) simultaneously with Cd(II) and Pb(II) ions, by means of differential pulse anodic stripping voltammetry (DPASV), in a certified reference groundwater sample with a very high reproducibility and good trueness. This electrode is proposed as a valuable alternative to in-situ bismuth film electrodes, since no competition between the electrodeposited copper and antimony for surface sites was noticed. In-situ SbSPCE was microscopically characterized and experimental parameters such as deposition potential, accumulation time and pH were optimized. The best voltammetric response for the simultaneous determination of Cd(II), Pb(II) and Cu(II) ions was achieved when deposition potential was -1.2 V, accumulation time 120 s and pH 4.5. The detection and quantification limits at levels of μg L(-1) suggest that the in-situ SbSPCE could be fully suitable for the determination of Cd(II), Pb(II) and Cu(II) ions in natural samples.

Application of multivariate curve resolution to voltammetric data. Part 1. Study of Zn(II) complexation with some polyelectrolytes

Multivariate curve resolution is applied to the study of Zn(II) complexation with anions of polymethacrylic acid and polyacrylic acid in aqueous solution containing 0.01 M KNO3. Data matrices obtained by different electrochemical techniques (normal pulse polarography, reverse pulse polarography, differential pulse polarography and differential pulse anodic stripping voltammetry) and for different ligand deprotonation degrees have been analysed individually and simultaneously by a procedure which consists of using several chemometrical techniques based on factor analysis: principal component analysis, evolving factor analysis and multivariate curve resolution with alternating least-squares optimization. These techniques were used to investigate the number of species present in the systems, their concentration profiles at increasing ligand concentration and their individual voltammograms together with the global (apparent) stability constants K and the ratio of diffusion coefficients of the individual species. The comparison of the results obtained by soft (model free) and hard (model postulated a priori) modelling for the systems investigated is discussed.

Differential pulse voltammetric study of the complexation of Cd(II) by the phytochelatin (γ-GluCys)2Gly assisted by multivariate curve resolution

Abstract A multivariate curve resolution method by alternating least-squares (MCR-ALS) is applied to differential pulse voltammograms measured on the Cd(II)+(γ-GluCys) 2 Gly system as a model of metal–phytochelatin interactions at concentrations of both components in the range 10 −7 –10 −5 mol l −1 . The course of complexation is different when peptide is titrated with metal from that when metal is titrated with peptide. The combined analysis of both matrices from titrations of peptide with metal and of metal with peptide allowed the resolution of the system. The analysis of the resulting pure voltammograms and concentration profiles of the resolved components suggested the presence of four different types of bound Cd(II) and made possible the formulation of a complexation model.

Induced reactant adsorption in metal—polyelectrolyte systems: pulse polarographic study

Abstract A theoretical model is developed for describing the reduction, by normal-pulse polarography, of a metal ion in the presence of a macromolecular ligand, including both the ligand adsorption and the induced adsorption of the metal ion. The following basic assumptions are made: reversible charge transfer at a stationary planar electrode (static mecury drop electrode); labile complex; large excess of ligand compared with the total metal concentration; Langmuirian adsorption for both the ligand and complex species; and diffusion coefficients for ligand and complex species very different from that of the free metal. The mathematical approach is based on transforming the system of differential equations with their boundary conditions into an integral equation which has been solved numerically. A simple procedure to obtain stability constants and adsorption parameters simultaneously from the experimental data is described. This model has been used to reproduce some experimental data from the Cd(II)—polymethacrylate system.

Multivariate Curve Resolution: A Possible Tool in the Detection of Intermediate Structures in Protein Folding

Different multivariate data analysis techniques based on factor analysis and multivariate curve resolution are shown for the study of biochemical evolutionary processes like conformational changes and protein folding. Several simulated CD spectral data sets describing different hypothetical protein folding pathways are analyzed and discussed in relation to the feasibility of factor analysis techniques to detect and resolve the number of components needed to explain the evolution of the CD spectra corresponding to the process (i.e., to detect the presence of intermediate forms). When more than two components (the native and unordered forms) are needed to explain the evolution of the spectra, an iterative multivariate curve resolution procedure based on an alternating least squares algorithm is proposed to estimate the CD spectrum corresponding to the intermediate form.

Heavy Metal Binding by Tannic Acid: A Voltammetric Study

The interactions between tannic acid (TA) and the heavy metal ions Cu(II), Pb(II), Cd(II), and Zn(II), have been studied by several voltammetric techniques as a model for the metal binding of tannin substances and also to evaluate the possible effects of such compounds on the voltammetric analysis of natural samples. The study has shown quite a different behavior of the systems considered: Cd(II) is practically not complexed by TA, whereas Pb(II) and Zn(II) produce labile complexes and Cu(II) seems to form an inert complex with TA. In the cases of the Zn(II)+TA and Pb(II)+TA systems, it was possible to give a rough estimation of the stability constants and the relative diffusion coefficients of the complexes. From these values, it appears that the affinity for TA increases in the order Cd(II)<Zn(II)<Pb(II). In contrast, the possible stabilization of Cu(I) species during the voltammetric measurement hindered a quantitative study of the complexation of Cu(II) by TA.

Thermodynamics of Cd2+ and Zn2+ binding by the phytochelatin (γ-Glu-Cys)4-Gly and its precursor glutathione

Isothermal titration calorimetry (ITC) was used to study the binding of Cd(2+) and Zn(2+) by glutathione (GSH) and phytochelatins (PC(n)), the metal sequestering compounds in plants and algae. The results are compared with those obtained by differential pulse polarography (DPP) assisted by multivariate curve resolution with alternating least squares (MCR-ALS) and by electrospray ionization mass spectrometry (ESI-MS). ITC allows one to determine (i) the stoichiometries of the different complexes (and confirms those found by DPP/MCR-ALS and ESI-MS) and (ii) their binding and thermodynamic parameters. For Cd-PC(4), the sequential binding sites model with two identical sites yields the best fitting of ITC curves and confirms the presence of CdPC(4) and Cd(2)PC(4) complexes. For Zn-PC(4), exothermic formation of ZnPC(4) is reported. Conditional stability and formation constants for Cd-GSH and Zn-GSH are determined from the fitting of the proper model to experimental ITC curves. The effect of different buffers in the complexation processes shows the key role of the choice of the buffer in calorimetric study.

Reverse pulse polarography of labile metal + macromolecule systems with induced reactant adsorption: theoretical analysis and determination of complexation and adsorption parameters

A model for the interpretation of reverse pulse polarography (RPP) in metal macromolecular ligand systems is developed, including both adsorption of the macromolecule and the induced adsorption of the metal ion. The following basic assumptions are made: reversible charge transfer at a stationary planar electrode (static mercury drop electrode), labile complex, large excess of ligand compared with the total metal concentration, formation of a 1:1 complex and diffusion coefficients for ligand and complex species different from that of the free metal ion. Equations for the limiting current Ilim and for the RPP full wave are deduced for both linear and Langmuirian adsorption. It is found that adsorption does not modify the expressions for Ilim valid for the case without adsorption. Thus the stability constant K can be determined. In contrast, adsorption influences the shape and position of the RPP full wave. In the case of linear adsorption, an analytical expression has been deduced which relates the adsorption parameter KML1 to ΔE12 values, thus allowing the calculation of KML1 from RPP waves. This method has been applied to experimental results obtained for the Cd(II) polymethacrylate system.