ChromWin--a computer program for the determination of enantiomerization barriers in dynamic chromatography

Determination of physicochemical parameters of (bio)molecules and (bio)particles by capillary electromigration methods

The review provides an overview of recent developments and applications of capillary electromigration (CE) methods for the determination of important physicochemical parameters of various (bio)molecules and (bio)particles. These parameters include actual and limiting (absolute) ionic mobilities, effective electrophoretic mobilities, effective charges, isoelectric points, electrokinetic potentials, hydrodynamic radii, diffusion coefficients, relative molecular masses, acidity (ionization) constants, binding constants and stoichiometry of (bio)molecular complexes, changes of Gibbs free energy, enthalpy and entropy and rate constants of chemical reactions and interactions, retention factors and partition and distribution coefficients. For the determination of these parameters, the following CE methods are employed: zone electrophoresis in a free solution or in sieving media, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography. In the individual sections, the procedures for the determination of the above parameters by the particular CE methods are described.

Separation of Monosaccharide Anomers on Photo-Click Cysteine-Based Stationary Phase: The α/β Interconversion Process Studied by Dynamic Hydrophilic Liquid Chromatography

In High-Performance Liquid Chromatography (HPLC), the separation of reducing sugars can typically show three possible typologies of chromatographic profiles (i.e., single peak, two resolved peaks and two peaks interconnected by a plateau) due to the rate at which the relevant α/β anomers interconversion (anomerization) can take place in relation to their elution-time. By analyzing these chromatographic profiles, thermodynamic and kinetic properties of anomerization phenomenon can be extrapolated. In this work we studied the anomerization of some monosaccharides by using a recently developed photo-click cysteine-based stationary phase through dynamic hydrophilic interaction liquid chromatography (D-HILIC) conditions. In the 5–25 °C temperature range, the ΔG#α→β and ΔG#β→α barriers were found to achieve values within the interval 21.1/22.2 kcal/mol for glucose, with differences between α→β and β→α reactions of about 0.4 kcal/mol. For xylose, in the same temperature range, the ΔG#α→β and ΔG#β→α barriers are between 20.7 to 21.5 kcal/mol, with differences between α→β and β→α reactions of about 0.2 kcal/mol. The experimental data are in agreement with those reported in literature, confirming the this new stationary phase using HILIC conditions is a robust platform to measure kinetic and thermodynamic properties of the isomerization reaction.

Some thoughts about enantioseparations in capillary electrophoresis

In this overview the goal of the authors was to analyze from the historical perspective the reasons of success and failure of chiral capillary electrophoresis. In addition, the current trends are analyzed, unique advantages of capillary electrophoresis are highlighted and some future directions are discussed.

Thermodynamic and kinetic investigation of monoketo-aldehyde-peroxyhemiacetal (MKA), a stereolabile degradation product of dihydroartemisinin

Theβ⇆αepimerization process ofMKAhas been studied under a thermodynamic and kinetic point of view in several solvents. LSER analyses and molecular modeling calculations allowed an effective rationalization of the findings.

From stereodynamics to high-throughput screening of catalysed reactions

In this review we summarised recent developments in high-throughput kinetic monitoring of reactions including the dynamics of interconverting stereoisomers and the simultaneous combination of (catalysed) reactions with chemical analysis in on-column reaction chromatographic devices.

Stereodynamics of small 1,2-dialkyldiaziridines

Diaziridines are very interesting representatives of organic compounds containing stereogenic nitrogen atoms. In particular, 1,2-dialkyldiaziridines show extraordinarily high stereointegrity. The lone electron pairs of the nitrogen atoms are in trans configuration, avoiding a four-electron repulsive interaction. Furthermore, the trans configuration of the substituents at the nitrogen atoms is energetically favored because of reduced steric interactions. Therefore only two stereoisomers (enantiomers) are observed. At elevated temperatures the enantiomers are interconverting because of the limited stereointegrity of the chirotopic nitrogen atoms. The enantiomerization rate constants and the activation parameters of interconversion are of great interest. Here, we investigated the stereodynamics of a set of small 1,2-dialkyldiaziridines bearing short substituents (Me, Et, iPr, tBu), using enantioselective dynamic gas chromatography (DGC). Separation of enantiomers of all compounds, including the highly volatile 1,2-dimethyldiaziridine, was achieved using heptakis(2,3-di-O-ethyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin in 50% PS086 (w/w) as chiral stationary phase in fused silica capillaries with a length of up to 50 m. Measurements at variable temperatures were performed and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers at room temperature for 1-(tert-butyl)-2-ethyldiaziridine, ΔG(╪)(298K) = 123.8 kJ mol(-1) (ΔH(╪) = 115.5±2.9 kJ mol(-1), ΔS(╪) = -28±1 J mol(-1) K(-1)), and 1-ethyl-2-isopropyldiaziridine, ΔG(╪)(298K) = 124.2 kJ mol(-1) (ΔH(╪) = 113.1±2.4 kJ mol(-1), ΔS(╪) = -37±2 J mol(-1) K(-1)), were determined, representing some of the highest values observed for nitrogen inversion in diaziridines.

Stereodynamics of tetramezine

The antidepressant drug tetramezine [1,2-bis-(3,3-dimethyldiaziridin-1-yl)ethane] consists of two bridged diaziridine moieties with four stereogenic nitrogen centers, which are stereolabile and, therefore, are prone to interconversion. The adjacent substituents at the nitrogen atoms of the diaziridines moieties exist only in an antiperiplanar conformation, which results in a coupled interconversion. Therefore, three stereoisomers exist (meso form and two enantiomeric forms), which epimerize when the diaziridine moieties are regarded as stereogenic units due to the coupled interconversion. Here, we have investigated the epimerization between the meso and enantiomeric forms by dynamic gas chromatography. Temperature-dependent measurements were performed, and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers of the epimerization were found to be ΔG(≠) = 100.7 kJ mol(-1) at 25°C and ΔG(≠) = 104.5 kJ mol(-1) at 37°C, respectively. The activation enthalpy and entropy were determined to be ΔH(≠) = 70.3 ± 0.4 kJ mol(-1) and ΔS(≠) = -102 ± 2 J mol(-1) K(-1) .

The sensitivity of the cis/trans-isomerization of enalapril and enalaprilat to solvent conditions

The cis- and trans-isomers of enalapril and enalaprilat can be resolved by HPLC and by capillary electrophoresis. The isomeric content of enalapril is perturbed by the ionization of both its carboxyl and amine groups, while the isomeric content of enalaprilat is only perturbed by the ionization of its amine group. Increasing the hydrophobicity of the analyte solvent, as reflected in its molar polarization, increases the Z (cis) content of enalapril and markedly decreases the kinetics for isomerization. Far UV circular dichroic measurements suggest that the increase in Z (cis) content of enalapril is due to protonation of its carboxylate group. Taken together, the in-vitro properties of enalapril and enalaprilat suggest that the in-vivo transformation of the prodrug enalapril to the inhibitor enalaprilat and its delivery to angiotensin-converting enzyme should not be significantly limited by cis/trans-isomerization.

Elaborate treatment of retention in chemoselective chromatography--the retention increment approach and non-linear effects

The retention increment approach is described which quantifies the association equilibria of a selectand and a selector in complexation and inclusion chromatography. A thermodynamic treatment of enantioselectivity based on retention phenomena in chromatography including entropy/enthalpy compensation and the isoenantioselective temperature is advanced. Kinetic parameters of enantiomerization are discussed. Non-linear effects, both existent and elusive, are described and proposed, respectively. The enantioselectivity pertaining to catalysis vs. chromatography is compared and a unified nomenclature is proposed. Through an educational effort, this account is aimed at providing a deeper insight into chemoselective aspects of chromatography thereby stimulating further research of both established and speculative phenomena of the most intriguing manifestation of chemoselectivity, that is, of enantioselectivity.

Gas chromatographic high-throughput screening techniques in catalysis

Discovering highly efficient catalysts is of great scientific and economical interest. Advances in high-throughput assays in combination with sophisticated analytical techniques have increased the rapidity with which catalysts can be identified and optimized. Understanding how kinetics in the mechanism of catalysis is controlled by structural parameters is essential for a directed design of catalysts. To identify such rate-controlling elementary steps and to develop and refine models, comprehensive experimental kinetic data of a broad variety of substrates are necessary. In the present article concepts of high-throughput screening techniques in catalysis using gas chromatography are reviewed in a survey covering the period from 1998 to 2007. To cover also the origins of concepts and groundbreaking experiments in this research area milestones going back to 1950 are also reviewed. The first part of the review will focus on off-line gas chromatographic analysis, the second part on on-line gas chromatographic analysis covering sequential, parallelized and high-throughput multiplexing gas chromatography. The third part presents recent advances in the integration of chemical transformation and analysis in gas chromatography. The present review article describes the state-of-the-art, scope and limitations, and applications of these different high-throughput screening approaches.

Enantioseparations by using capillary electrophoretic techniques. The story of 20 and a few more years

This paper provides the author's insight on the past, present and future of performing enantioseparations using capillary electrophoretic (CE) techniques. These techniques are discussed from the historical point of view, as well as based on their potential as the separation techniques of today and the future. The overview covers mechanistic as well as practical aspects of CE techniques.