Alberto Rojas-Hernández

Spectrophotometric and electrochemical determination of the formation constants of the complexes Curcumin–Fe(III)–water and Curcumin–Fe(II)–water

The formation of complexes among the Curcumin, Fe(III) and Fe(II) was studied in aqueous media within the 5-11 pH range by means of UV-Vis spectrophotometry and cyclic voltammetry. When the reaction between the Curcumin and the ions present in basic media took place, the resulting spectra of the systems Curcumin-Fe(III) and Curcumin-Fe(II) presented a similar behaviour. The cyclic voltammograms in basic media indicated that a chemical reaction has taken place between the Curcumin and Fe(III) before that of the formation of complexes. Data processing with SQUAD permitted to calculate the formation constants of the complexes Curcumin-Fe(III), corresponding to the species FeCur (lob beta110 = 22.25 +/- 0.03) and FeCur(OH)- (log beta111 = 12.14 +/- 0.03), while for the complexes Curcumin-Fe(II) the corresponding formation constants of the species FeCur- (log beta110 = 9.20 +/- 0.04), FeHCur (log beta111 = 19.76 +/- 0.03), FeH2Cur+ (log beta112 = 28.11 +/- 0.02).

Spectrophotometric study on the stability of dopamine and the determination of its acidity constants

The interest in determining the acidity constants of the catecholamines stems from the fact that they play rather an important biological role. The present work reveals the effect of different parameters such as oxygen, light, analysis time and pH on the dopamine oxidation process, where oxygen has an effect on the dopamine oxidation of 40% and up to 20% is attributed to exposure to light as a function of the pH. The application of adequate control on the said parameters (which ensured stability of the dopamine) facilitated the determination of the corresponding three acidity constants, 9.046+/-0.147, 10.579+/-0.148 and 12.071+/-0.069.

Searching for Computational Strategies to Accurately Predict pKas of Large Phenolic Derivatives

Twenty-two reaction schemes have been tested, within the cluster-continuum model including up to seven explicit water molecules. They have been used in conjunction with nine different methods, within the density functional theory and with second-order Møller-Plesset. The quality of the pKa predictions was found to be strongly dependent on the chosen scheme, while only moderately influenced by the method of calculation. We recommend the E1 reaction scheme [HA + OH(-) (3H2O) ↔ A(-) (H2O) + 3H2O], since it yields mean unsigned errors (MUE) lower than 1 unit of pKa for most of the tested functionals. The best pKa values obtained from this reaction scheme are those involving calculations with PBE0 (MUE = 0.77), TPSS (MUE = 0.82), BHandHLYP (MUE = 0.82), and B3LYP (MUE = 0.86) functionals. This scheme has the additional advantage, compared to the proton exchange method, which also gives very small values of MUE, of being experiment independent. It should be kept in mind, however, that these recommendations are valid within the cluster-continuum model, using the polarizable continuum model in conjunction with the united atom Hartree-Fock cavity and the strategy based on thermodynamic cycles. Changes in any of these aspects of the used methodology may lead to different outcomes.

Construction of Multicomponent Pourbaix Diagrams Using Generalized Species

A method for the construction of predominance-zone diagrams of multicomponent and multiphase systems of chemical species in a single oxidation state, which consider the formation of mixed and polynuclear complexes (of the type M i L j X k ) is presented. Such diagrams may be constructed by defining generalized species, complexation coefficients (with multiplicative structure) and generalized equilibria of complexation, dismutation, polynucleation, and phase formation, with their conditional constants associated. By having these diagrams for each oxidation state of the same component, multidimensional Pourbaix diagrams are constructed. The predominance-zone diagrams of *Eu(0), *Eu(II), and *Eu(III) in the presence of citric acid (H 3 Cit) and nitrilotriacetic acid (H 3 NTA) in aqueous solution, are constructed by using this approach

Determination of pKa values of tenoxicam from 1H NMR chemical shifts and of oxicams from electrophoretic mobilities (CZE) with the aid of programs SQUAD and HYPNMR

In this work it is explained, by the first time, the application of programs SQUAD and HYPNMR to refine equilibrium constant values through the fit of electrophoretic mobilities determined by capillary zone electrophoresis experiments, due to the mathematical isomorphism of UV-vis absorptivity coefficients, NMR chemical shifts and electrophoretic mobilities as a function of pH. Then, the pK(a) values of tenoxicam in H(2)O/DMSO 1:4 (v/v) have been obtained from (1)H NMR chemical shifts, as well as of oxicams in aqueous solution from electrophoretic mobilities determined by CZE, at 25 degrees C. These values are in very good agreement with those reported by spectrophotometric and potentiometric measurements.

Determination of pKa's for thymol blue in aqueous medium: evidence of dimer formation.

Formation constants for recrystallized thymol blue were determined in water, using the SQUAD and SUPERQUAD programs. The best model correlating spectrophotometric, potentiometric and conductimetric data was fitted with the dissociation of HL(-)=L(2-)+H(+)-log K=8.918+/-0.070 and H(3)L(2)(-)=2L(2-)+3H(+)-log K=29.806+/-0.133 with the SUPERQUAD program at variable low ionic strength (1.5x10(-4)-3.0x10(-4) M); and HL=L(2-)+H(+)-log K=8.9+/-0.000, H(3)L(2)(-) =2L(2-)+3H(+)-log K=30.730+/-0.032, H(4)L(2)=2L(2-)+4H(+)-log K=32.106+/-0.033 with SQUAD at 1.1 M ionic strength.

Equilibria among condensed phases and a multi-component solution using the concept of generalized species: Part II. Systems with polynuclear species

Abstract In this second part a general algorithm is presented which is based on the definition of total generalized species and equilibria and the combination of Charlot and Ringboms approaches; this leads to the establishment of the relative importance of the chemical species present in a multi-component-multi-reacting system that includes complexation, polynucleation and phase-formation phenomena. The definition of total generalized species [†M(τ) and †M(τ(c)] permits as expression to be obtained for the total generalized intrinsic solubility equilibria for the calculation of the saturation conditions in a solution in spite of the presence of polynuclear species both in the solution and in the condensed phases. The symbol used for the total generalized species implies that the subscripts for the polynucleation of M are included in the superscript dagger (†). The procedure used to select the most insoluble phase from the condensed-phases diagram of this system was described in Part I. In order to exemplify the scope of the proposed algorithm, graphical representations of the following systems are discussed: Be(II)H2OH, Be(II)H4PDTAH2OH, (Ca(II)H3PO4H2OH and Mg(II)H3PO4NH3H2OH, where H4PDTA is 1,3-diaminopropylene-N,N,N′,N′-tetraacetic acid.

Equilibria among condensed phases and a multi-component solution using the concept of generalized species: Part I. Systems with mixed complexes

Abstract A method for the construction of predominance-existence diagrams (PED) in saturated multi-component solutions is discussed. The utilization of generalized species and equilibria, both in the solution and in the condensed phases, allows for the analysis of the saturation conditions from an intrinsic solubility generalized equilibrium [M (τ) c ⇌ M (τ) ]. Likewise, an algorithm is proposed for the selection of the most insoluble condensed phase M (τ) (c) , from a condensed-phases diagram (CPD) dependent upon the parameters of the saturated solution. The CPD is constructed utilizing generalized phase interconversion equilibria, where the multi-conditional constants are dependent only on the buffering conditions; it is also necessary to consider the maximum number of phases that can coexist in the system (phase rule), the electroneutrality conditions for the condensed phases and the number of variables involved in the solubility equations. The consideration of mixed complexes in all phases with the proposed algorithm is simple as it is an extension of the concept of generalized species used previously. In order to exemplify the proposed method, graphical representations of the following system are discussed: Zn(II)H 2 OH, Zn(II)H 2 C 2 O 4 H 2 OH and Ca(II)H 2 C 2 O 4 H 2 SO 4 H 2 OH, where H 2 C 2 O 4 is oxalic acid.

Relationship of multidimensional predominance-zone diagrams with multiconditional constants for complexation equilibria

Abstract A generalized method for the construction of predominance-zone diagrams using conditional constants is presented. This generalization permits the construction of multidimensional diagrams using multiconditional constants for the study of chemical equilibria in aqueous solutions of multicomponent systems. The predominance-zone diagrams can be constructed by defining generalized species, complexation coefficients (with a multiplicative structure instead of the traditionally used additive structure) and generalized complexation and dismutation equilibria for multibuffered systems. Systems involving mixed complex formation were studied by using copper(II)-nitrilotriacetic acid-histidine-proton (under buffered conditions in pHist′ and pH) and mercury(II)-ethylenediaminetetraacetic acid-ammonia-proton (under buffered conditions in pNH 3 ′ and pH) as examples.

Multi-dimensional predominance-zone diagrams for polynuclear chemical species

Abstract A definition of generalized species and equilibria (including the concepts of ampholyte and disproportionation or dismutation) is proposed for the study of polynucleation phenomena in systems with multiple buffering and mixed complexes. With this method, it is possible to draw multi-dimensional predominance-zone diagrams (PZD) for such systems. Three alternative methods for the graphical representation of the PZD are discussed, which use the parameters pM, −log C and −log C m total ; the relationship among them is shown and also the fact that multi-conditional dismutation constants are the only criteria for the intrinsic stability of ampholytes. In order to exemplify the proposed method, a study of the systems Be(II)-H and Be(II)-PDTA-H is reported (where PDTA=species of 3-propylenediamino- N , N , N′ , N′ -tetraacetic acid, which is represented by H 4 PDTA).