Photophysical and structural impact of phosphorylated anions associated to lanthanide complexes in water

A new ligand, LC, bis-[(6'-carboxy-2,2'-bipyridine-6-yl)]phenylphosphine oxide, in which the tridentate 6-carboxy-2,2'-bipyridyl arms are directly linked to a phenylphosphine oxide fragment, has been synthesized. The corresponding [Ln.LC]Cl.xH2O complexes (Ln = Eu, x = 4, and Tb, x = 3) were isolated from solutions containing equimolar amounts of LC and hydrated LnCl3 salts and characterized by elemental analysis, mass spectrometry, and infrared spectroscopy. The interactions of the Eu complex with various anions (AMP(2-), ADP3-, ATP,4- HPO4(2-), and NO3-) were studied by titration experiments, using UV-vis, luminescence spectroscopy, and excited-state lifetime measurements. The results are in keeping with strong interactions with the ADP3-, ATP4-, and phosphate anions in TRIS/HCl buffer (0.01 M, pH = 7.0), as revealed by the determination of the conditional stepwise association constants. These values are higher than the one determined for ligand LB, bis[(6'-carboxy-2,2'-bipyridine-6-methyl-yl)]-n-butylamine (Delta log K approximately 1-2). The interaction of complexes [Ln.LB]+ and [Ln.LC]+ with nitrate, monohydrogenophosphate, methyl phosphate (MeP2-), methyldiphosphate (MeDP3-), and methyltriphosphate (MeTP4-) anions was investigated by means of quantum mechanical (QM) calculations. The results, combined with data on the photophysical impact of the sequential competitive binding of anions to the Eu complexes in water, suggest that LB is too flexible to ensure a good coordination pocket, while the molecular structure of ligand LC stabilizes both the formation of the lanthanide complexes and its adducts with ATP.     

Liquid-junction potential effects in measurements of sodium ion activity in unbuffered aqueous solutions

Measurements of pNa with the glass-membrane ion-selective electrode in unbuffered aqueous solutions require care in the constitution of the measuring cell, because of varialbe liquid-junction potentials at the salt bridge. Of the bridge electrolytes tested, with calomel or silver/silver chloride reference electrodes, 3 M ammonium chloride in agar-agar gel was most satisfactory. Calibration graphs were of almost theoretical slope in the range 10^?3?5 × 10^?1 M sodium ion.     

Modeling of the adsorption behavior and the chromatographic band profiles of enantiomers : Behavior of methyl mandelate on immobilized cellulose

The adsorption isotherms of (−)- and (+)-methyl mandelate from a hexane-isopropanol (90:10) solution were measured on a chromatographic column packed with 4-methylcellulose tribenzoate coated on silica. These isotherms are accounted for by a bi-Langmuir isotherm model, the two Langmuir terms having widely different initial slopes and saturation capacities, but each term having the same saturation capacity for the two enantiomers. The competitive isotherms were also measured. They are in excellent agreement with the prediction of a competitive bi-Langmuir model based on the single-component isotherms. The individual band profiles are in agreement with the profiles calculated from these isotherms. Thus, a simplified competitive isotherm can be used to model a separation on a chiral stationary phase the recognition mechanism of which is not well identified and the adsorption behavior of which is certainly not ideal.     

Csaba Horváth and preparative liquid chromatography

Few chromatographers have been interested in furthering preparative liquid chromatography. The pioneers, Tswett, Kuhn and Lederer, A.J.P. Martin, Tiselius, isolated fractions but as an intermediate step in the analysis of their samples. The progress in electronics and sensors, and in their miniaturization has lead to the paradoxical situation that the analysts never see the transient pure fractions that their detector quantitates. Yet, over the last 25 years, preparative liquid chromatography has become an important industrial process for the separation, the extraction, and/or the purification of many pharmaceuticals or pharmaceutical intermediates, including pure enantiomers, purified peptides and proteins, compounds that are manufactured at the relatively large industrial scale of a few kilograms to several hundred tons per year. This development that has strongly affected the modem pharmaceutical industry is mainly due to the pioneering work of Csaba Horváth. His work in preparative HPLC was critical at both the practical and the theoretical levels. He was the first scientist in modem times to pay serious attention to the relationships between the curvature of the equilibrium isotherms, the competitive nature of nonlinear isotherms, and the chromatographic band profiles of complex mixtures. The thermodynamics of multi-component phase equilibria and mass transfer kinetics in chromatography attracted his interest and were the focus of ground-breaking contributions. He investigated displacement chromatography, an old method invented by Tiselius that Csaba was first to implement in HPLC. This choice was explained by the essential characteristic of displacement chromatography, in that it delivers fractions that can be far more concentrated than the feed. Remarkably, once the basics of nonlinear chromatography had been mastered in his group, most of the applications that were studied by his coworkers dealt with peptides of various sizes and with proteins. Thus, all the applications of preparative HPLC in the biotechnologies derive directly from Csaba's work. Although displacement did not pan out as a general method, the reasons are related more to practical constraints of the production of pharmaceuticals and to the long period of cheap energy that might be ending now. This report reviews Csaba's work in nonlinear chromatography.     

Bond functions for AB initio calculations on polyatomic molecules hydrocarbons

Transferable gaussian CH and CC bond functions and a C lone-pair function were optimized and compared to conventional polarization functions. The exponents (α) and position (γ,β) of these functions are α = 0.9, γ = 0.27 for CH bonds; α = 1.2, γ = 0.50 for CC bonds; α = 0.3, β = 0.47 for C lone-pair.     

Repeatability and reproducibility of retention data and band profiles on six batches of monolithic columns

Chromatographic data were acquired for eight different mixtures, under five different sets of experimental conditions, for a total of 30 neutral, acidic and basic test compounds, on a series of six Chromolith Performance columns from Merck. These columns are made of a C₁₈ chemically bonded silica monolith. Each column belonged to a different production batch, so the data reported here characterize their batch-to-batch reproducibility. The parameters studied in this work were the retention times, the retention and separation factors, the hydrophobic and the steric selectivities, the column efficiencies, and the tailing factors for all 30 compounds.     

Effect of the pH, the concentration and the nature of the buffer on the adsorption mechanism of an ionic compound in reversed-phase liquid chromatography. II. Analytical and overloaded band profiles on Symmetry-C18 and Xterra-C18

In a previous report, the influence of the pH, the concentration, and the nature of the buffer on the retention and overloading behavior of propranolol (pKa = 9.45) was studied on Kromasil-C18 at 2.75 < pH < 6.75, using four buffers (phosphate, acetate, phthalate, and succinate), at three concentrations, 6, 20, and 60 mM. The results showed that the propranolol cation was eluted as an ion-pair with the buffer counter-anion. A similar study was carried out with Symmetry-C18 and Xterra-C18. Two additional buffers, formate and citrate, were also used. Propranolol elution band profiles were recorded for a small (less than 1 microg) and a large (375 microg) sample size. The results are similar to those obtained with Kromasil and confirm earlier conclusions. The buffer concentration, not its pH, controls the retention time of propranolol, in agreement with the chaotropic model. The retention factor depends also on the nature of the buffer, particularly on its valence, and on the hydrophobicity of the basic anion. With the monovalent anions HCOO- (pH 3.75), H2PO4- (pH 2.75), HOOC-Ph-COO- (pH 2.75), HOOC-CH2-CH2-COO- (pH 4.16), CH3COO- (pH 4.75) and HOOC-CHCOOH-COO- (pH 3.14), at moderate loadings, and for the two larger buffer concentrations, the band profiles are well accounted for by a simple bi-Langmuir isotherm model (no adsorbate-adsorbate interactions). By contrast, these profiles are accounted for by a bi-Moreau isotherm model (i.e., with significant adsorbate-adsorbate interactions) with the bivalent anions -OOC-Ph-COO- (pH 4.75), -OOC-CH2-CH2-COO- (pH 5.61), HPO4(2-) (pH 6.75), and HOOC-CHCOO(-)-COO- (pH 4.77) and with the trivalent anion -OOC-CHCOO(-)-COO- (pH 6.39). The best values of the isotherm parameters were determined using the inverse method. The saturation capacity and the equilibrium constant on the low-energy sites increase with increasing buffer concentration, a result consistent with the formation in the mobile phase of a hydrophobic complex between the propranolol cation and the buffer anion. With bivalent and trivalent anions, adsorbate-adsorbate interactions are strong on the low-energy sites but they remain negligible on the high-energy sites. The density of the high energy sites is lower and the equilibrium constant on the low-energy sites are both higher with the bivalent and the trivalent buffer anions than with the univalent buffer anions. These results are consistent with the formation of a 2:1 and a 3:1 propranolol-buffer complex with the bivalent and the trivalent anions, respectively.     

Determination of the single component and competitive adsorption isotherms of the 1-indanol enantiomers by the inverse method

The inverse method of isotherm determination consists in calculating the numerical values of the coefficients of an isotherm model that give a set of chromatographic profiles in best possible agreement with the set of experimental profiles available. This method was applied to determine the adsorption isotherms of the 1-indanol enantiomers on a cellulose tribenzoate chiral stationary phase. Both single-component and competitive isotherms were determined by using no more than one or two overloaded band profiles. The isotherms determined from the overloaded band profiles agreed extremely well with the isotherms determined by frontal analysis. Several isotherm models were used and tested. The best-fit isotherm was selected by means of statistical evaluation of the results. The results show that the adsorption is best characterized with a model describing heterogeneous adsorption with bimodal adsorption energy distribution.     

Organic Syntheses Without Solvent: Preparation of Alkoxyphthalimides and of Alkoxylamines

Alkoxyphthalimides are prepared by alkylation of N-hydroxyphthalimide under solid-liquid phase transfer catalysis without solvent. When conversion of alkoxyphthalimides into alkoxylamines is nearly complete, neat hydrazine hydrate is added at room temperature.