Guidelines in selecting ligand concentrations for the determination of binding constants by affinity capillary electrophoresis

Analysis of drug interactions with serum proteins and related binding agents by affinity capillary electrophoresis: A review

Biomolecules such as serum proteins can interact with drugs in the body and influence their pharmaceutical effects. Specific and precise methods that analyze these interactions are critical for drug development or monitoring and for diagnostic purposes. Affinity capillary electrophoresis (ACE) is one technique that can be used to examine the binding between drugs and serum proteins, or other agents found in serum or blood. This article will review the basic principles of ACE, along with related affinity-based capillary electrophoresis (CE) methods, and examine recent developments that have occurred in this field as related to the characterization of drug-protein interactions. An overview will be given of the various formats that can be used in ACE and CE for such work, including the relative advantages or weaknesses of each approach. Various applications of ACE and affinity-based CE methods for the analysis of drug interactions with serum proteins and other binding agents will also be presented. Applications of ACE and related techniques that will be discussed include drug interaction studies with serum agents, chiral drug separations employing serum proteins, and the use of CE in hybrid methods to characterize drug binding with serum proteins.

Evaluation of three neutral capillary coatings for the determination of analyte-cyclodextrin binding constants by affinity capillary electrophoresis. Application to N,N'-disubstituted piperazine derivatives

The performances of three neutral static coatings (hydroxypropyl cellulose, polyethylene oxide and poly(N,N-dimethylacrylamide) have been evaluated in order to determine the binding constants of the complexes formed between four polycationic compounds (piperazine derivatives) and four cyclodextrins of pharmaceutical interest (β-CD, HP-β-CD, Me-β-CD and sulfobutyl ether-β-CD) by affinity capillary electrophoresis. The physically-adsorbed poly(N,N-dimethylacrylamide) coating proves to be the more efficient to mask the silanol groups of the capillary wall since the lowest electroosmotic flow was measured for this coating. Moreover, it drastically reduces the adsorption of the compounds since it allows a correct repeatability of their migration time, higher efficiencies of the peaks and no baseline shift. Then, it was verified for four complexes that this coating allows a correct determination of the binding constants avoiding the CD adsorption which is responsible of an undervaluation of binding constants. The highest binding constants are obtained using the anionic sulfobutyl ether-β-CD (SBE-β-CD). The structure of the complex formed between the tacrine derivative and the SBE-β-CD was further investigated through 2D ROESY NMR experiments and structure-binding constant relationships. Results suggest that the inclusion in the SBE-β-CD cavity occurs through the aliphatic ring portion of the tacrine moiety.

Drug-protein binding: a critical review of analytical tools

The extent of drug binding to plasma proteins, determined by measuring the free active fraction, has a significant effect on the pharmacokinetics and pharmacodynamics of a drug. It is therefore highly important to estimate drug-binding ability to these macromolecules in the early stages of drug discovery and in clinical practice. Traditionally, equilibrium dialysis is used, and is presented as the reference method, but it suffers from many drawbacks. In an attempt to circumvent these, a vast array of different methods has been developed. This review focuses on the most important approaches used to characterize drug-protein binding. A description of the principle of each method with its inherent strengths and weaknesses is outlined. The binding affinity ranges, information accessibility, material consumption, and throughput are compared for each method. Finally, a discussion is included to help users choose the most suitable approach from among the wealth of methods presented.

Evaluation of indole-based probes for high-throughput screening of drug binding to human serum albumin: Analysis by high-performance affinity chromatography

There has been growing interest in the use of rapid and selective separation methods such as high-performance affinity chromatography (HPAC) or affinity capillary electrophoresis (ACE) for the characterization of drug-protein interactions. L-Tryptophan is commonly used in these and other methods as a site-selective probe for examining the binding of small solutes and drugs at Sudlow site II on the protein HSA. However, solutions of L-tryptophan can be unstable and are generally prepared fresh daily for these studies. In this report, HPAC was used to examine other indole compounds as possible replacements for L-tryptophan as a site-selective probe for use in the high-throughput screening of drug binding to HSA; the implications of these results in the use of such compounds in ACE were also considered. The probe candidates that were tested included indole-3-acetic acid, indole-3-carboxylic acid, indole-3-butyric acid, indole-3-propionic acid, indole-3-methanol, 3-acetylindole, and 3-methylindole. All of these compounds were found by (1)H NMR and UV-Vis spectroscopy to be stable for up to 3 wk at room temperature when kept in a pH 7.4, 0.067 M phosphate buffer. The binding of these compounds was examined by using columns that contained immobilized HSA. 3-Acetylindole was found to be the best candidate in this group for use as an alternative probe to L-tryptophan for Sudlow site II. This probe had the same binding site and a similar affinity to L-tryptophan but was more stable in aqueous solution, making it suitable for high-throughput screening of drug-HSA binding in both HPAC and ACE.

Quantitative and label-free technique for measuring protease activity and inhibition using a microfluidic cantilever array

We report the use of a SiN x based gold coated microcantilever array to quantitatively measure the activity and inhibition of a model protease immobilized on its surface. Trypsin was covalently bound to the gold surface of the microcantilever using a synthetic spacer, and the remaining exposed silicon nitride surface was passivated with silanated polyethylene glycol. The nanoscale cantilever motions induced by trypsin during substrate turnover were quantitatively measured using an optical laser-deflection technique. These microcantilever deflections directly correlated with the degree of protease turnover of excess synthetic fibronectin substrate ( K M = 0.58 x 10 (-6) M). Inhibition of surface-immobilized trypsin by soybean trypsin inhibitor (SBTI) was also observed using this system.

Complexation of tauro- and glyco-conjugated bile salts with three neutral β-CDs studied by ACE

Complexation of the bile salts (BS) taurocholate, tauro-beta-muricholate, taurodeoxycholate, taurochenodeoxycholate, glycocholate, glycodeoxycholate, and glycochenodeoxycholate common in rat, dog, and man with natural beta-CD and the chemically modified beta-CDs 2-hydroxypropyl-beta-CD and 2-O-methyl-beta-CD was studied using mobility shift ACE. The CDs were selected due to their frequent use in preformulation and drug formulation as oral excipients for the solubilization of drug substances with low aqueous solubility. ACE was demonstrated to be a feasible and efficient technique for investigation of the interactions between BS and beta-CDs. All the investigated BS possessed affinity for the three CDs with stability constants ranging from 2x10(3) to 4x10(5) M(-) (1). The requirements and assumptions related to the use of ACE for estimating high affinity stability constants were discussed. The extent and pattern of hydroxylation significantly influenced the affinity of the glyco- and tauro-conjugated BS toward the beta-CDs (chenodeoxycholates >> deoxycholates > cholates) whereas the nature of the beta-CD derivatization and BS conjugation played a minor role only. The results indicate that displacement of drug substances from beta-CD inclusion complexes is likely to occur in the small intestine where BS are present potentially influencing drug bioavailability.

Comparative studies of the interaction between ferulic acid and bovine serum albumin by ACE and surface plasmon resonance

Affinity capillary electrophoresis (ACE) was used to study the interaction between ferulic acid (FA) and BSA. The interaction between FA and BSA was facilitated by injecting FA into a BSA-containing running buffer. Both mobility ratio and mobility shift assays were performed to deduce the binding constant (K(b)). However, the K(b )value obtained with the mobility ratio assay was only approximately 20% of that extracted from the mobility shift assay. The former assay yielded a K(b) value (5.6 +/- 0.4 x 10(4) M(-1)), which compares well with the result obtained with surface plasmon resonance (SPR) (5.1 +/- 0.6 x 10(4) M(-1)). The discrepancy between the mobility ratio and mobility shift assays suggests that the data extrapolation from the mobility ratio should be more reliable for cases when both changes in the EOF and viscosity of the running buffer are important. The work demonstrates that ACE, a solution-based technique, and SPR, a technique addressing interfacial processes, are highly complementary to each other and the comparative studies are confirmatory and allow binding constants to be accurately determined.

Quantitative studies of allosteric effects by biointeraction chromatography: analysis of protein binding for low-solubility drugs

A new chromatographic method was developed for characterizing allosteric interactions between an immobilized binding agent and low-solubility compounds. This approach was illustrated by using it to characterize the interactions between tamoxifen and warfarin during their binding to the protein human serum albumin (HSA), with beta-cyclodextrin being employed as a solubilizing agent for these drugs. It was confirmed in this work through several experiments that warfarin had a single binding site on HSA with an association equilibrium constant of (2-5) x 10(5) M(-1) (average, 3.9 x 10(5) M(-1)) at 37 degrees C, in agreement with previous reports. It was also found that tamoxifen had a single major binding site on HSA, with an association equilibrium constant of (3-4) x 10(7) M(-1) (average, 3.5 x 10(7) M(-1)) at 37 degrees C. When warfarin was used as a mobile-phase additive in competition studies with tamoxifen, this had a positive allosteric effect on tamoxifen/HSA binding, giving a coupling constant of 2.3 (+/-0.3). Competitive studies using tamoxifen as a mobile-phase additive indicated that tamoxifen had a negative allosteric effect on warfarin/HSA binding, providing a coupling constant of 0.79 (+/-0.03). A unique feature of the technique described in this report was its ability to independently examine both directions of the warfarin/tamoxifen allosteric interaction. This approach is not limited to warfarin, tamoxifen, and HSA but can also be used to study other solutes and binding agents.

Chiral resolution of tryptophan derivatives by CE using canine serum albumin and bovine serum albumin as chiral selectors

This work deals with the application of BSA and canine serum albumin (CSA) for enantioseparation of tryptophan derivatives with CE. The aim of this work was the investigation of the influence of different functional groups of tryptophan derivatives on enantioseparation. CSA as a chiral selector was tested to compare its selector properties with those of BSA. The enantiomers of the tryptophan derivatives were separated by adding BSA or CSA to the BGE. The influence of pH, temperature, BSA and CSA concentration and organic modifiers was investigated. It was found that the stereoselectivity for the different tryptophan derivatives is dependent on the albumin species. It turned out that the different functional groups of the derivatives showed a significant influence on stereoselectivity.

Lipid vesicles in capillary electrophoretic techniques: characterization of structural properties and associated membrane-molecule interactions

This paper reviews the use of lipid vesicles as model membranes in capillary electrophoresis (CE). The history and utility of CE in the characterization of microparticles is summarized, focusing on the application of colloidal electromigration theories to lipid vesicles. For instance, CE experiments have been used to characterize the size, surface properties, enclosed volumes, and electrophoretic mobilities of lipid vesicles and of lipoprotein particles. Several techniques involving small molecules or macromolecules separated in the presence of lipid vesicles are discussed. Interactions between the analytes and the lipid vesicles - acting as a pseudostationary phase or coated stationary phase in electrokinetic chromatography (EKC) - can be used to obtain additional information on the characteristics of the vesicles and analytes, and to study the biophysical properties of membrane-molecule interactions in lipid vesicles and lipoproteins. Different methods of determining binding constants by EKC are reviewed, along with the relevant binding constant calculations and a discussion of the application and limitations of these techniques as they apply to lipid vesicle systems.

Characterization of drug interactions with soluble beta-cyclodextrin by high-performance affinity chromatography

This study examined the use of an immobilized human serum albumin (HSA) column to study solution-phase reactions between drugs and beta-cyclodextrin (beta-CD). Chromatographic equations were developed to characterize the binding of chemicals to a soluble ligand (beta-CD) in the presence of an independent immobilized ligand (HSA). Situations considered included the presence of both a homogeneous and heterogeneous immobilized ligand, as well as complex interactions between the chemical of interest and soluble ligand. Three drugs (warfarin, tamoxifen, and phenytoin) were examined by this approach. This method involved injecting a small amount of each drug onto an HSA column in the presence of various concentrations of beta-CD in the mobile phase. By measuring the change in the drug's retention factor as the concentration of beta-CD was varied, it was possible to determine the stability constant between the injected drug and beta-CD. With this approach, warfarin and beta-CD were found to have 1:1 interactions with a stability constant of 5.2 x 10(2) M(-1) at 37 degrees C and pH 7.4, a result in close agreement with previous literature values. Tamoxifen and phenytoin were also found to have 1:1 interactions with beta-CD and had stability constants of 0.9-1.2 x 10(4) and 6-9 x 10(2) M(-1) respectively. With these latter solutes, the effects of secondary binding to the chromatographic support had to be considered. The theory and methods described in this report are not limited to these drugs and beta-CD but can be applied to other analytes and soluble ligands.

Study of lipid and apolipoprotein binding interactions using vesicle affinity capillary electrophoresis

Vesicle affinity capillary electrophoresis (VCE), a newly developed technique, was designed to assess the effect of physicochemical properties of apolipoprotein (apo) on the binding to lipoproteins, under physiological conditions (phosphate-saline buffer system at pH 7.4 and 37 degrees C), using vesicle as a model. The technique results in similar lipid binding properties of apo CIII (CIII) and its peptides compared to other techniques. It also offers a fast and more sensitive tool in determining the lipid affinity of apos in a unique system simulating the dynamic binding properties of apo in vivo. A noncompetitive binding model is used to determine the multiple binding properties of CIII and its peptides to vesicle. The VCE binding constants are dependent on temperature, physicochemical properties of the protein (hydrophobicity and charge), and nature of the vesicle. The vesicles used in the VCE experiments described here have been fully characterized and found to be stable under different temperatures (4 and 37 degrees C) and voltage conditions. Migration behavior of CIII and related peptides is reported in terms of relative mobility in order to correct for variability in viscosity at different vesicle concentrations. The VCE method provides very precise data on the migration time from 0.1 to 3.3% RSD at the highest concentration of vesicle. The model and current data have been used to determine VCE binding constants and protein-to-lipid binding ratios. The model predicts that higher lipid affinity (K(B)), protein-lipid binding ratio (n), and lower protein concentration result in a shift of the binding isotherm toward a lower concentration range of vesicle. A higher vesicle mobility, reflecting the size and charge of the vesicle, results in a larger separation window between the migration time of the free protein and the complex. The value of VCE for structure-function studies and drug design for peptides and proteins that are strongly bound to lipids has been illustrated.

Drug affinity to immobilized target bio-polymers by high-performance liquid chromatography and capillary electrophoresis

This review addresses the use of high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE) as affinity separation methods to characterise drugs or potential drugs-bio-polymer interactions. Targets for the development of new drugs such as enzymes (IMERs), receptors, and membrane proteins were immobilized on solid supports. After the insertion in the HPLC system, these immobilized bio-polymers were used for the determination of binding constants of specific ligands, substrates and inhibitors of pharmaceutical interest, by frontal analyses and zonal elution methods. The most used bio-polymer immobilization techniques and methods for assessing the amount of active immobilized protein are reported. Examples of increased stability of immobilized enzymes with reduced amount of used protein were shown and the advantages in terms of recovery for reuse, reproducibility and on-line high-throughput screening for potential ligands are evidenced. Dealing with the acquisition of relevant pharmacokinetic data, examples concerning human serum albumin binding studies are reviewed. In particular, papers are reported in which the serum carrier has been studied to monitor the enantioselective binding of chiral drugs and the mutual interaction between co-administered drugs by CE and HPLC. Finally CE, as merging techniques with very promising and interesting application of microscale analysis of drugs' binding parameters to immobilized bio-polymers is examined.

Investigations of cyclophilin interactions with oligopeptides containing proline by affinity capillary electrophoresis

Affinity capillary electrophoresis using mobility-shift analysis was utilized to characterize the binding of peptide ligands to cyclophilins, which are members of the enzyme family of peptidyl-prolyl cis/trans isomerases. Peptides derived from the human immunodeficiency virus capsid protein p24 exhibited different affinities to the isoenzymes cyclophilin18 and cyclophilin20. For the interaction of the peptide hormone bradykinin with cyclophilin18, a dissociation constant of 1.4 +/- 0.1 mM was determined. Finally, the affinity of cyclophilin20 to peptides from a cellulose-bound peptide library scanning the sequence of Drosophila melanogaster protein cappuccino was investigated. The affinities of selected peptides to cyclophilin20 and a green fluorescent fusion protein with cyclophilin20 were compared.

Evalution of capillary electrophoresis-frontal analysis for the study of low molecular weight drug-human serum albumin interactions

Capillary electrophoresis frontal analysis was applied to 12 low molecular weight compounds including 8 drug substances displaying a range of different properties with respect to binding affinity, binding location, structure, lipophilicity, charge at physiological pH, and electrophoretic mobility. It was found that capillary electrophoresis frontal analysis can be used as a general method to study and quantify drug-human serum albumin interactions. The binding parameters obtained were consistent with literature values. Dextran was in some cases added to the run buffer to improve separation of the drug and human serum albumin plateau peaks. Results indicate that mobility differences between free and complexed human serum albumin give rise to only minor errors. Capillary electrophoresis frontal analysis was also found applicable to the study of human serum albumin drug displacement reactions. Low sensitivity of the UV-detection system was found to be the major limitation of capillary electrophoresis frontal analysis. The method is simple, and minimal effort has to be put into method development, which makes it well suited for screening in early drug development.

Identification of drug-binding sites on human serum albumin using affinity capillary electrophoresis and chemically modified proteins as buffer additives

A technique based on affinity capillary electrophoresis (ACE) and chemically modified proteins was used to screen the binding sites of various drugs on human serum albumin (HSA). This involved using HSA as a buffer additive, following the site-selective modification of this protein at two residues (tryptophan 214 or tyrosine 411) located in its major binding regions. The migration times of four compounds (warfarin, ibuprofen, suprofen and flurbiprofen) were measured in the presence of normal or modified HSA. These times were then compared and the mobility shifts observed with the modified proteins were used to identify the binding regions of each injected solute on HSA. Items considered in optimizing this assay included the concentration of protein placed into the running buffer, the reagents used to modify HSA, and the use of dextran as a secondary additive to adjust protein mobility. The results of this method showed good agreement with those of previous reports. The advantages and disadvantages of this approach are examined, as well as its possible extension to other solutes.

Antigen-antibody interactions in capillary electrophoresis

Immunoreactions in combination with separations by capillary electrophoresis (CE) are increasingly being used to quantitate specific analytes in biological fluids. Both competitive and non-competitive approaches have been used for the purpose and, in selected cases, now compare favorably with conventional quantitative immunoassays with respect to concentration limits of detection. CE is also a useful method to evaluate antigen-antibody binding on-line and offers unique possibilities for binding constant estimates, also for weakly binding antibodies and antibody fragments. In this review we cover recent developments in the use of antigen-antibody interactions in conjunction with CE and conclude that continued development of miniaturization, on-line preconcentration and more sensitive detection schemes will contribute to the further dissemination of CE-based immunoassays building on already established affinity CE approaches.

Methods for the determination of binding constants by capillary electrophoresis

Apparent equilibrium constants for molecular association (e.g., association constants, binding constants, dissociation constants, partition coefficients) can be determined with a variety of different capillary electrophoresis (CE) approaches. In many cases, the investigated association behavior is between a smaller molecule or ion (i.e., the solute, drug, or analyte of interest) and a larger entity (e.g., proteins, micelles, polymers, chiral selectors such as cyclodextrins, etc.). Each experimental approach has advantages and disadvantages. Frequently, it is the nature of the system being evaluated that determines the optimal experimental approach. Six different CE-based techniques for evaluating binding constants are reviewed. Examples of each method, and recent references on its use are given.

Application of a green fluorescent fusion protein to study protein-protein interactions by electrophoretic methods

A screening procedure for protein-protein interactions in cellular extracts using a green fluorescent protein (GFP) and affinity capillary electrophoresis (ACE) was established. GFP was fused as a fluorescent indicator to the C-terminus of a cyclophilin (rDmCyp20) from Drosophila melanogaster. Cyclophilins (Cyps) belong to the ubiquitously distributed enzyme family of peptidyl-prolyl cis/trans isomerases (PPlases) and are well known as cellular targets of the immunosuppressive drug cyclosporin A (CsA). The PPlase activity of the GFP fused rDmCyp20 as well as the high affinity to CsA remain intact. Using native gel electrophoresis and ACE mobility-shift assays, it was demonstrated that the known moderate affinity of Cyp20 to the capsid protein p24 of HIV-1 was detectable in the case of rDmCyp20 fused to the fluorescent tag. For the p24 / rDmCyp20-GFP binding an ACE method was established which allowed to determine a dissociation constant of Kd = 20+/-1.5 x 10(-6) M. This result was verified by size-exclusion chromatography and is in good agreement with published data for the nonfused protein. Moreover the fusion protein was utilized to screen rDmCyp20-protein interactions by capillary electrophoresis in biological matrices. A putative ligand of rDmCyp20 in crude extracts of embryonic D. melanogaster was discovered by mobility-shift assays using native gel electrophoresis with fluorescence imaging and ACE with laser-induced fluorescence detection. The approach seems applicable to a wide range of proteins and offers new opportunities to screen for moderate protein-protein interactions in biological samples.

Chromatographic and electrophoretic studies of protein binding to chiral solutes

Protein interactions are important in determining the transport, metabolism and/or activity of many chiral compounds within the body. This review examines data that have been obtained on these interactions by various chromatographic and electrophoretic methods, especially those based on either high-performance liquid chromatography or capillary electrophoresis. Zonal elution, frontal analysis and vacancy methods are each considered, as are approaches that employ either soluble or immobilized proteins. There are a variety of different items that can be learned about a solute-protein system through these techniques. This includes information on the binding constants and number of binding sites for a solute-protein system, as well as the thermodynamic parameters, rate constants, interaction forces and binding site structure for the protein and solute. Numerous examples are provided throughout this review, as taken from the literature and from work performed within the author's laboratory.

Multiple-step ligand injection affinity capillary electrophoresis for determining binding constants of ligands to receptors

This work demonstrates the use of multiple-step ligand injection affinity capillary electrophoresis (ACE) using two model systems: vancomycin from Streptomyces orientalis and carbonic anhydrase B (CAB, EC 4.2.1.1). In this technique a sample plug of receptor and non-interacting standards is injected by pressure and electrophoresed in a buffer containing a given concentration of ligand. The sequence is repeated for all concentrations of ligand generating a single electropherogram containing a series of individual sample plugs superimposed on environments of buffer containing increasing concentrations of ligand. Analysis of the change in the relative migration time ratio, RMTR, relative to the non-interacting standards, as a function of the concentration of the ligand, yields a value for the binding constant. A competitive assay using the technique is also demonstrated using neutral ligands for CAB. These values agree well with those estimated using other binding and ACE techniques. Data demonstrating the quantitative potential of this method are presented.

Identification, quantitation, and characterization of biomolecules by capillary electrophoretic analysis of binding interactions

The high resolving power of capillary electrophoresis combined with the specificity of binding interactions may be used with advantage to characterize the structure-function relationship of biomolecules, to quantitate specific analytes in complex sample matrices, and to determine the purity of pharmaceutical and other molecules. We here review recent and innovative methodologies and applications of high resolution affinity electrophoresis within the fields of binding constant determination, structure-activity studies, quantitative microassays, analysis of drug purity and protein conformation, and immobilized affinity ligands. Despite the virtues of these approaches with respect to applicability, resolving power, speed, and low sample consumption, problems remain with respect to analyte identification and low concentration limits of detection. The ongoing development of new detector technologies for capillary electrophoresis such as mass spectrometry, and possibly nuclear magnetic resonance and other spectroscopic methods, is therefore very promising for the continued increased use of affinity capillary electrophoresis.

Use of a partial-filling technique in affinity capillary electrophoresis for determining binding constants of ligands to receptors

This work evaluates the concept of a partial-filling technique in affinity capillary electrophoresis (ACE) using two model systems: vancomycin from Streptomyces orientalis and carbonic anhydrase B (CAB, EC 4.2.1.1). In this technique the capillary is first partially-filled with ligand followed by a sample of receptor and non-interacting standard and electrophoresed. Analysis of the change in the mobility ratio, M, of the receptor, relative to the non-interacting standard, as a function of the concentration of the ligand, yields a value for the binding constant. These values agree well with those estimated using other binding and ACE techniques. Data demonstrating the quantitative potential of this method is presented.

Use of mobility ratios to estimate binding constants of ligands to proteins in affinity capillary electrophoresis

This work evaluates the use of mobility ratios (M) to estimate binding constants of proteins to ligands using affinity capillary electrophoresis (ACE). This concept is demonstrated using two model systems: vancomycin (Van) from Streptomyces orientalis and carbonic anhydrase B (CAB, EC 4.2.1.1). A plot of change in M (deltaM) over the concentration of ligand [L] versus deltaM yields a more useful representation of the Scatchard plot in capillary electrophoresis (CE) than traditional plots of the change in mobility delta mu over [L] versus delta mu in a wide set of circumstances, especially when comparing electropherograms obtained in the presence of substantial variations in electroosmotic flow. Altering the voltage and/or capillary length of the CE system produced only small variations in M, but much larger changes in the more standard measures of migration used by the mu form of analysis. The use of M in the Scatchard analysis offers a new approach to estimating binding constants of ligands to proteins using ACE.

Chiral separations in capillary electrophoresis using proteins as stereoselective binding agents

Proteins are receiving increased attention in capillary electrophoresis (CE) for use as ligands in chiral separations. This review examines the types of proteins that have been employed in CE for the analysis of chiral compounds and provides a summary of applications for such an approach. Several formats for this type of analysis are discussed, including methods that use immobilized proteins (i.e., affinity capillary electrochromatography and capillary gel affinity electrophoresis) or techniques that use proteins as running buffer additives (i.e., affinity capillary electrophoresis). The role of various experimental factors in these separations are considered, and a comparison is made between the general analytical properties of CE systems that use immobilized versus solution-phase proteins.