Stability constants of amino acids, peptides, proteins, and other biomolecules determined by CE and related methods: Recapitulation of published data

Mobilization of electroosmotic flow markers in capillary zone electrophoresis

UV-absorbing neutral substances are commonly used as markers of mean electroosmotic flow in capillary electrophoresis for their zero electrophoretic mobility in an electric field. However, some of these markers can interact with background electrolyte components and migrate at a different velocity than the electroosmotic flow. Thus, we tested 11 markers primarily varying in their degree of methylation and type of central atom in combination with five background electrolyte cations differing in their ionic radii and surface charge density, measuring the relative electrophoretic mobility using thiourea as a reference marker. Our results from this set of experiments showed some general trends in the mobilization of the markers based on the effects of marker structure and type of background electrolyte cation on the relative electrophoretic mobility. As an example, the effects of an inadequate choice of marker on analyte identification were illustrated in the electrophoretic separation of glucosinolates. Therefore, our findings may help electrophoretists appropriately select electroosmotic flow markers for various electrophoretic systems.

Determination of thermodynamic binding constants by affinity capillary electrophoresis

A strategy to study thermodynamic binding constants by affinity capillary electrophoresis (ACE) is presented. In order to simplify mathematical treatment, analogy with acid-base dissociation equilibrium is proposed: instead of ligand concentration [X], negative logarithm of ligand concentration (or activity), pX = -log[X], is used. On this base, and taking into account ionic activities, a general procedure for obtaining thermodynamic binding constants is proposed. In addition, the method provides electrophoretic mobilities of the free analyte and analyte-ligand complex, even when binding constants are low and thus, the complexed analyte fraction is also low. This is useful as a base to rationally analyze a diversity of situations, i.e., different mathematical dependencies are obtained when analytes and ligands with different charges are combined. Practical considerations are given for carrying out a full experimental design. Enantiomeric ACE separation based on the use of chiral selectors is addressed. 2-hydroxypropyl-β-cyclodextrin was chosen as a model ligand, and both enantiomeric forms of four pharmaceutical drugs (propranolol, pindolol, oxprenolol and homatropine methylbromide) were considered as model analytes. Practical aspects are detailed and thermodynamic binding constants as well as free and complexed analytes mobilities are determined.

Moment analysis for reaction kinetics of intermolecular interactions

Moment equations were developed on the basis of the principle of relativity for analyzing elution peak profiles measured by ACE to analytically determine the association (k_a ) and dissociation (k_d ) rate constants of intermolecular interactions. Basic equations representing the mass balance, mass transfer rate, and reaction kinetics in ACE system in a Galilean coordinate system S were transformed to those in another coordinate system S', which imaginarily moved with respect to S. Moment equations for ACE peaks in S' in the time domain were derived from the analytical solution of the modified basic equations in the Laplace domain. Moment equations for ACE peaks in S were derived from those in S' by the inverse Galilean transformation. The moment equations were used for analyzing some ACE data previously published to determine k_a and k_d values. It was demonstrated that the moment equations were effective for extracting the information about affinity kinetics of intermolecular interactions from the elution peak profiles measured by ACE. The moment equations were also used to discuss the influence of mass transfer and reaction kinetics on ACE peak profiles. Some results of the numerical calculations are also indicated in Supporting Information.

Affinity capillary electrophoresis for studying interactions in life sciences

Affinity capillary electrophoresis (ACE) analyzes noncovalent interactions between ligands and analytes based on changes in their electrophoretic mobility. This technique has been widely used to investigate various biomolecules, mainly proteins, polysaccharides and hormones. ACE is becoming a technique of choice to validate high throughput screening results, since it is very predictively working in realistic and relevant media, e.g. in body fluids. It is highly recommended to incorporate ACE as a powerful analytical tool to properly prepare animal testing and preclinical studies. The interacting molecules can be found free in solution or can be immobilized to a solid support. Thus, ACE is classified in two modes, free solution ACE and immobilized ACE. Every ACE mode has advantages and disadvantages. Each can be used for a variety of applications. This review covers literature of scopus and SciFinder data base in the period from 2016 until beginning 2018, including the keywords "affinity capillary electrophoresis", "immunoaffinity capillary electrophoresis", "immunoassay capillary electrophoresis" and "immunosorbent capillary electrophoresis". More than 200 articles have been found and 112 have been selected and thoroughly discussed. During this period, the data processing and the underlying calculations in mobility shift ACE (ms ACE), frontal analysis ACE (FA ACE) and plug-plug kinetic capillary electrophoresis (ppKCE) as mostly applied free solution techniques have substantially improved. The range of applications in diverse free solution and immobilized ACE techniques has been considerably broadened.

Cyclic band compression in toroidal capillary electrophoresis delivers an unlimited number of theoretical plates with a quadratic growth in time and a constant peak capacity

Analytical instruments able to provide extremely high sensitivities, separation efficiencies, and peak capacities are important for both applied sciences and basic research. It is even more interesting if this can be achieved within organic, aqueous, and physiological solutions without restricting the operation parameters, such as buffer pH, temperature, ionic strength, and background electrolyte composition. Toroidal capillary electrophoresis offers this potential, as was recently proposed and demonstrated. In this platform, the analytes perform continuous round trips inside a fused-silica capillary having a torus-like shape. In the present work, the equations of the number of plates and peak capacity are deduced when on-column cyclic thermal band compression is applied. They are expressed as a function of the number of turns performed by the analyte, axial length of the toroid, number of microholes (reservoirs), compression factor, number of compression events performed per turn, and applied voltage. It was found that the variances of the bands reach a steady state, regardless of the number of dispersion mechanisms present. Consequently, the number of theoretical plates grows indefinitely as the square of time. The expression of peak capacity shows a well-defined limiting value that remains constant over time.

Number of theoretical plates achievable by a toroidal capillary electrophoresis system

The ability of a method and instrument to separate very similar compounds is related to the "plate number," a number indicating performance. The resolution between two neighboring peaks is proportional to the square root of the plate number. Currently available commercial capillary electrophoresis instruments easily reach plate numbers of a few million. In the present work, a capillary electrophoresis system with a toroidal platform is proposed and theoretically studied with the goal of extending the achievable plate number. In this new system, electrophoresis occurs in a nonstop continuous circulating mode within a closed loop capillary (toroid). Plate numbers upwards of one billion are theoretically predicted. This could resolve hundreds of unseparated mixtures of stereoisomers and other analytes that remain without a method for their analysis.

Twenty years of development of dual and multi-selector models in capillary electrophoresis: a review

It has been 20 years since Lurie et al. first published their model of electromigration of an analyte under simultaneous interaction with two cyclodextrins as chiral selectors. Since then, the theory of (enantio)separation in dual and complex mixtures of (chiral) selectors is well understood. In spite of this, a trial-and-error approach still prevails in analytical practice. Such a situation is likely caused by the fact that the entire theory is spread over numerous papers and the relations between various models are not always clear. The present review condenses the theory for the first time. Available mathematical models and feasible practical approaches are summarized and their advantages and limitations discussed.

Complexation of buffer constituents with neutral complexation agents: part II. Practical impact in capillary zone electrophoresis

This article elucidates the practical impact of the complexation of buffer constituents with complexation agents on electrophoretic results, namely, complexation constant determination, system peak development, and proper separation of analytes. Several common buffers, which were selected based on the pH study in Part I of this paper series (Riesová, M.; Svobodová, J.; Tošner, Z.; Beneš, M.; Tesařová, E.; Gaš, B. Anal. Chem., 2013, DOI: 10.1021/ac4013804); e.g., CHES, MES, MOPS, Tricine were used to demonstrate behavior of such complex separation systems. We show that the value of a complexation constant determined in the interacting buffers environment depends not only on the analyte and complexation agent but it is also substantially affected by the type and concentration of buffer constituents. As a result, the complexation parameters determined in the interacting buffers cannot be regarded as thermodynamic ones and may provide misleading information about the strength of complexation of the compound of interest. We also demonstrate that the development of system peaks in interacting buffer systems significantly differs from the behavior known for noncomplexing systems, as the mobility of system peaks depends on the concentration and type of neutral complexation agent. Finally, we show that the use of interacting buffers can totally ruin the results of electrophoretic separation because the buffer properties change as the consequence of the buffer constituents' complexation. As a general conclusion, the interaction of buffer constituents with the complexation agent should always be considered in any method development procedures.

Comparative study of three methods for affinity measurements: capillary electrophoresis coupled with UV detection and mass spectrometry, and direct infusion mass spectrometry

We present affinity capillary electrophoresis and mass spectrometry (ACE-MS) as a comprehensive separation technique for label-free solution-based affinity analysis. The application of ACE-MS for measuring affinity constants between eight small molecule drugs [ibuprofen, s-flurbiprofen, diclofenac, phenylbutazone, naproxen, folic acid, resveratrol, and 4,4'-(propane-1,3-diyl) dibenzoic acid] and β-cyclodextrin is described. We couple on-line ACE with MS to combine the separation and kinetic capability of ACE together with the molecular weight and structural elucidation of MS in one system. To understand the full potential of ACE-MS, we compare it with two other methods: Direct infusion mass spectrometry (DIMS) and ACE with UV detection (ACE-UV). After the evaluation, DIMS provides less reliable equilibrium dissociation constants than separation-based ACE-UV and ACE-MS, and cannot be used solely for the study of noncovalent interactions. ACE-MS determines apparent dissociation constants for all reacting small molecules in a mixture, even in cases when drugs overlap with each other during separation. The ability of ACE-MS to interact, separate, and rapidly scan through m/z can facilitate the simultaneous affinity analysis of multiple interacting pairs, potentially leading to the high-throughput screening of drug candidates.

Simulation of the effects of complex- formation equilibria in electrophoresis: II. experimental verification

The complete mathematical model of electromigration in systems with complexation agents introduced in the Part I of this article (V. Hruška et al., Eletrophoresis, 2012, 33, this issue), which was implemented into our simulation program Simul 5, was verified experimentally. Three different chiral selector (CS) systems differing in the type of the CS, the magnitude of the complexation constants as well as in the experimental conditions were selected for verification. The experiments and simulations were performed at various concentrations of the CSs in order to discuss the influence of the concentration of the CS on the separation. The simulated and experimental electropherograms show very good agreement in the position, shape and amplitude of the analyte peaks. The new Simul 5 Complex offers a deep insight into electrophoretical separations that take place in systems containing complexing agents, for example into enantiomer separations. Using Simul 5 Complex we were able to predict and explain the significant electromigration dispersion of analyte peaks. It was clarified that the electromigration dispersion in these systems results directly from complexation. The new Simul 5 Complex was also shown to be a useful and powerful tool for the prediction of the results of enantioseparations.

Effects of bile salts on propranolol distribution into liposomes studied by capillary electrophoresis

The objective of this study was to study the effect of four different bile salts, cholate (C), deoxycholate (DC), taurocholate (TC), monoketocholate (MKC), on the membrane binding of a cationic model drug, propranolol, using capillary electrophoresis. The apparent distribution coefficient of propranolol in a buffer/liposome system, in the absence and presence of various concentrations of the bile salts, was measured using capillary electrophoresis frontal analysis. At bile salt concentrations which did not disrupt the liposomes, the bile salts increased the apparent distribution coefficient of propranolol in a concentration-dependent manner, to various extents (DC>C>TC>MKC). The mechanisms for these increases were inferred from studies of ion pairing between bile salts and propranolol using mobility shift affinity capillary electrophoresis and from zeta potential measurements. The bile salts ion-paired with propranolol to different extents as indicated by the estimated complexation constants (K range: 30-58 M(-1)). This was found to have a minor effect on the membrane distribution of propranolol only. The major effect is proposed to be due to the insertion of bile salt into the liposomal membranes leading to a more negatively charged membrane surface thereby providing stronger electrostatic interactions with the positively charged propranolol.