Evolution of the theoretical description of the isoelectric focusing experiment: II. An open system isoelectric focusing experiment is a transient,bidirectional isotachophoretic experiment

The carrier ampholytes-based (CA-based) isoelectric focusing (IEF) experiment evolved from Svensson's closed system IEF (constant spatial current density, absence of convective mixing, counter-balancing electrophoretic and diffusive fluxes yielding a steady state pH gradient) to the contemporary open system IEF (absence of convective mixing, large cross-sectional area electrode vessels, lack of counter-balancing electrophoretic- and diffusive fluxes leading to transient pH gradients). Open system IEF currently is described by a two-stage model: In the first stage, a rapid IEF process forms the pH gradient which, in the second stage, is slowly degraded by isotachophoretic processes that move the most acidic and most basic CAs into the electrode vessels. An analysis of the effective mobilities and the effective mobility to conductivity ratios of the anolyte, catholyte, and the CAs indicates that in open system IEF experiments a single process, transient bidirectional isotachophoresis (tbdITP) operates from the moment current is turned on until it is turned off. In tbdITP, the anolyte and catholyte provide the leading ions and the pI 7 CA or the reactive boundary of the counter-migrating H₃O⁺ and OH⁻ ions serves as the shared terminator. The outcome of the tbdITP process is determined by the ionic mobilities, pK_a values, and loaded amounts of all ionic and ionizable components: It is constrained by both the transmitted amount of charge and the migration space available for the leading ions. tbdITP and the resulting pH gradient can never reach steady state with respect to the spatial coordinate of the separation channel.     

High-performance liquid chromatographic enantioseparation of cyclic β-aminohydroxamic acids on zwitterionic chiral stationary phases based on Cinchona alkaloids

Cyclic β-aminohydroxamic acid enantiomer pairs were stereoselectively separated by high-performance liquid chromatography on the recently developed Cinchona alkaloid-based zwitterionic chiral stationary phases Chiralpak ZWIX(+)™, ZWIX(−)™, ZWIX(+A) and ZWIX(−A). The results of variation of the applied chromatographic conditions, such as the bulk solvent composition, the concentrations and ratio of the acid and base additives, the presence of water as mobile phase additive and the counter-ion concentration furnished a better understanding of the retention mechanism. A thermodynamic study in the temperature range 5–50 °C revealed enthalpy-controlled enantiodiscrimination in all cases. The structure–selectivity relationships clearly indicated the importance of the strereochemistry of the functional groups. From an enantiorecognition aspect, the diexo position of the functional groups always proved more favorable than the diendo position. The elution sequence was determined in all cases and was found to reversed when ZWIX(+)™ was changed to ZWIX(−)™ or ZWIX(+A) to ZWIX(−A).     

A bridge to coordination isomer selection in lanthanide(III) DOTA-tetraamide complexes

Interest in macrocyclic lanthanide complexes such as DOTA is driven largely through interest in their use as contrast agents for MRI. The lanthanide tetraamide derivatives of DOTA have shown considerable promise as PARACEST agents, taking advantage of the slow water exchange kinetics of this class of complex. We postulated that water exchange in these tetraamide complexes could be slowed even further by introducing a group to sterically encumber the space above the water coordination site, thereby hindering the departure and approach of water molecules to the complex. The ligand 8O2-bridged DOTAM was synthesized in a 34% yield from cyclen. It was found that the lanthanide complexes of this ligand did not possess a water molecule in the inner coordination sphere of the bound lanthanide. The crystal structure of the ytterbium complex revealed that distortions to the coordination sphere were induced by the steric constraints imposed on the complex by the bridging unit. The extent of the distortion was found to increase with increasing ionic radius of the lanthanide ion, eventually resulting in a complete loss of symmetry in the complex. Because this ligand system is bicyclic, the conformation of each ring in the system is constrained by that of the other; in consequence, inclusion of the bridging unit in the complexes means only a twisted square, antiprismatic coordination geometry is observed for lanthanide complexes of 8O2-bridged DOTAM.     

Synthesis of 16-E-([aryl]idene)-3-methoxy-estrones by a palladium catalysed Mizoroki-Heck reaction

16-E-Arylidene-3-methoxy-estrones were synthesized using a palladium catalysed Mizoroki-Heck reaction. This methodology tolerates various functional groups and gives the corresponding products in yields between 14 and 99%, which were strongly dependent on the electronic character of the aryl halides.     

Chemistry of an Unexplored Heterocyclic Ring System: Versatile Synthesis of 5‐Aryl‐2,3,4‐benzothiadiazepine 2,2‐dioxides

The article gives insight into the synthetic approach of a practically new heterocyclic ring system, 2,3,4‐benzothiadiazepines. Starting from the easily available phthalides, an efficient synthesis of 5‐aryl‐2,3,4‐benzothiadiazepine 2,2‐dioxides is described here. Owing to their close structural similarity to 2,3‐benzodiazepines and 2,3‐benzodiazepine‐4‐ones, two well‐known bioactive compound families, the new derivatives can be of importance in medicinal chemistry.     

The Accuracy of Merging Approximation in Generalized St. Petersburg Games

Merging asymptotic expansions of arbitrary length are established for the distribution functions and for the probabilities of suitably centered and normalized cumulative winnings in a full sequence of generalized St. Petersburg games, extending the short expansions due to Csörg? (Acta Sci. Math. (Szeged) 73:297–331, 2007). These expansions are given in terms of suitably chosen members from the classes of subsequential semistable infinitely divisible asymptotic distribution functions and certain derivatives of these functions. The length of the expansion depends upon the tail parameter. Both uniform and nonuniform bounds are presented.     

Concentration and fractionation by isoelectric trapping in a micropreparative-scale multicompartmental electrolyzer having orthogonal pH gradients. Part 2

A micropreparative-scale multicompartmental electrolyzer called ConFrac has been developed and tested for isoelectric trapping separations. ConFrac can be operated in pass-by-pass mode or recirculating mode, using either asymmetrical feeding (feed enters only the anodic or the cathodic flow-through compartment) or symmetrical feeding (feed enters both the anodic and the cathodic flow-through compartment). Symmetrical feeding results in higher processing rates and is the preferred operation mode. Residence time in the flow-through compartments is set as a compromise between processing rate and temperature rise in the effluent. Ampholytic components have been isolated from 5 to 50 mL volumes of micromolar feed solutions and hundredfold concentrated into 100-μL collection compartments. Samples containing ampholytic analytes in highly conducting salt solutions were readily desalted and fractionated in ConFrac in one operation. pH transients formerly observed in other isoelectric trapping devices were observed in ConFrac as well. The pH transients were caused by the unequal ion transmission rates of the anodic- and cathodic-side buffering membranes.     

Analytical and numerical computation of error propagation of model parameters

Linear and nonlinear regression analyses based on the least-squares method play a fundamental role in the evaluation of scientific data. A large number of valuable papers have dealt with various applications of the least-squares method in the chemical literature. They, however, usually contain tremendous formulas for computing the error estimates of the estimated parameters. This paper presents a simple numerical solution based on the well-known simplex method to this problem. Elaborate enzyme kinetic data published earlier have been chosen to test the simplex method extended with error estimation. The capability of the numerical method is demonstrated by the revision of the originally calculated error propagation. Our program might prove useful in handling either chemical or biological data.     

pH-oscillations in a closed chemical system of CaSO3-H2O2-HCO3(-)

Long-lasting large amplitude periodic change of the pH is measured in an aqueous suspension of CaSO(3)-H(2)O(2)-HCO(3)(-) at 2.0-10.0 °C in a closed reactor. The amplitude can be as large as 2 pH units between pH 5 and 7. The observed phenomenon is explained and simulated by taking into account a slow dissolution of CaSO(3), which serves as a continuous supply of HSO(3)(-) for a H(+)-producing autocatalytic composite reaction between H(2)O(2) and HSO(3)(-). Protonation of HCO(3)(-) to form CO(2) in a reversible reaction provides for the necessary negative feedback in [H(+)].