Use of mobility ratios to estimate binding constants of ligands to proteins in 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.

Comparison of the Performance of Different Bile Salts in Enantioselective Separation of Palonosetron Stereoisomers by Micellar Electrokinetic Chromatography

Bile salts are a category of natural chiral surfactants which have ever been used as the surfactant and chiral selector for the separation of many chiral compounds by micellar electrokinetic chromatography (MEKC). In our previous works, the application of sodium cholate (SC) in the separation of four stereoisomers of palonosetron (PALO) by MEKC has been studied systematically. In this work, the parameters of other bile salts, including sodium taurocholate (STC), sodium deoxycholate (SDC), and sodium taurodeoxycholate (STDC) in the separation of PALO stereoisomers by MEKC were measured and compared with SC. It was found that all of four bile salts provide chiral recognition for both pairs of enantiomers, as well as achiral selectivity for diastereomers of different degrees. The structure of steroidal ring of bile salts has a greater impact on the separation than the structure of the side chain. The varying separation results by different bile salts were elucidated based on the measured parameters. A model to describe the contributions of the mobility difference of solutes in the aqueous phase and the selectivity of micelles to the chiral and achiral separation of stereoisomers was introduced. Additionally, a new approach to measure the mobility of micelles without enough solubility for hydrophobic markers was proposed, which is necessary for the calculation of separation parameters in MEKC. Under the guidance of derived equations, the separation by SDC and STDC was significantly improved by using lower surfactant concentrations. The complete separation of four stereoisomers was achieved in less than 3.5 min by using 4.0 mM of SDC. In addition, 30.0 mM of STC also provided the complete resolution of four stereoisomers due to the balance of different separation mechanisms. Its applicability for the analysis of a small amount of enantiomeric impurities in the presence of a high concentration of the effective ingredient was validated by a real sample.

Study on the effects of humic and fulvic acids on quantum dot nanoparticles using capillary electrophoresis with laser-induced fluorescence detection

The increasing production and use of quantum dot (QD) nanoparticles have caused concerns on the possibility of contaminating the aquatic and terrestrial ecosystems with wastes that may contain QDs. Therefore, studies on the behavior of QDs upon interaction with components of the natural environment have become of interest. This study investigated the fluorescence and electrophoretic mobility of carboxylic or amine polyethylene glycol (PEG)-functionalized CdSe/ZnS QDs in the presence of two aquatic humic substances (HS), Suwannee River humic and fulvic acids, using capillary electrophoresis with laser-induced fluorescence detection. Results showed initial enhancement in fluorescence of QDs at the onset of the interaction with HS, followed by fluorescence quenching at longer exposure with HS (>30 min). It was also observed that the electrophoretic mobility of QDs increases with increasing concentration of HS, suggesting an increase in the ratio in charge to hydrodynamic size of the nanoparticles. To determine if the QDs degraded upon interaction with HS, the QD-HS mixtures were dialyzed to separate free Cd2+ from intact QDs, followed by analysis of the solutions using inductively coupled plasma-mass spectrometry. Results suggested that degradation of QDs in the presence of HS did not occur within the period of incubation.

Determination of binding constants by affinity capillary electrophoresis, electrospray ionization mass spectrometry and phase-distribution methods

Many methods for determining intermolecular interactions have been described in the literature in the past several decades. Chief among them are methods based on spectroscopic changes, particularly those based on absorption or nuclear magnetic resonance (NMR) [especially proton NMR ((1)H NMR)]. Recently, there have been put forward several new methods that are particularly adaptable, use very small quantities of material, and do not place severe requirements on the spectroscopic properties of the binding partners. This review covers new developments in affinity capillary electrophoresis, electrospray ionization mass spectrometry (ESI-MS) and phasetransfer methods.

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

The stability (affinity, association, binding, complexation, formation) constant characterizes binding interaction between the analyte and the complexing agent. Knowledge of the stability constant makes possible the prediction and estimation of the binding behavior of constituents (amino acids, peptides, proteins, drugs, antibiotics, enzymes, enantiomers) to their partners, and the finding of a suitable partner for the given analyte to form a stable complex. The present paper summarizes the stability constant determination methods and the approaches used to evaluate the experimental data. Further, the paper recapitulates the published stability constant values determined, mainly, by capillary electrophoretic methods, taken from the Web of Science database covering the last decade. Details of the experimental conditions employed for the determination of the stability constants are also given. The review attempts to give a critical evaluation of the problems that accompany the determination of stability constant and discusses their solution.

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.

Estimation of binding constants of receptors and ligands by affinity capillary electrophoresis

An estimation method for determination of binding constants of receptors to ligands by affinity capillary electrophoresis was evaluated. On the basis of the theories of pseudostationary phase or so-called dynamic stationary phase, the retention factor (k) was used to represent the interaction between the receptor and ligand. k could be easily deduced from the migration times of the ligand and the receptor. Then, with the linear relationship of k versus the concentration of ligand in the running buffer, the binding constant K(b) was calculated from the slope and intercept. In order to test its feasibility, the calculation method was demonstrated using three model systems: the interactions between vancomycin and N-acetyl-D-Ala-D-Ala, ristocetin and N-acetyl-D-Ala-D-Ala, and carbonic anhydrase B and an arylsulfonamide. Estimated binding constants were compared with those determined by other techniques. The results showed that this estimation method was reliable. This calculation method offers a simple and easy approach to estimating binding constants of ligands to receptors.

Capillary electrophoresis of signaling molecules

The emerging field of quantitative systems biology uses high-throughput bioanalytical measurements to gain a deeper understanding of biological phenomena. With the advent of instrumentation platforms, capillary electrophoresis spans a very wide range of biological applications. This short article focuses on the exploitation of capillary electrophoresis for the systems-level analysis of cell signaling molecules.

Study on the interactions between anti-HIV-1 active compounds with trans-activation response RNA by affinity capillary electrophoresis

The study on the interactions between two anti-human immunodeficiency virus type 1 (anti-HIV-1) active compounds with trans-activation response (TAR) RNA by affinity capillary electrophoresis (ACE) with UV absorbance detection is presented. The results showed that the novel active molecules could interact with TAR RNA and inhibit the reproduce process of HIV-1. The binding constants were estimated by the change of migration time of the analytes through the change of concentrations of TAR RNA in the buffer solution. The yielded binding constants of 8.87 x 10(3)M(-1) for active compound C(3) and 8.42 x 10(3)M(-1) for MC(3) at 20.0 degrees C, 0.626 x 10(3)M(-1) and 0.644 x 10(3)M(-1) at 37.0 degrees C, respectively. The thermodynamic parameters Delta H and DeltaS were obtained and shown that both hydrophobic and electrostatic interaction played roles in the binding processes. The results showed that the presented method was an easy and simple method to evaluate the interaction of small molecules with some bioactive materials.

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.

Estimation of binding constants for the substrate and activator of Rhodobacter sphaeroides adenosine 5'-diphosphate-glucose pyrophosphorylase using affinity capillary electrophoresis

Binding constants were determined for the activator fructose-6-phosphate (F6P) and substrate adenosine 5'-triphosphate (ATP) (in the presence and absence of F6P) to the recombinant wild-type (WT) Rhodobacter sphaeroides adenosine 5'-diphosphate-(ADP)-glucose pyrophosphorylase (ADPGlc PPase) using affinity capillary electrophoresis (ACE). In these binding studies, the capillary is initially injected with a plug of sample containing ADPGlc PPase and noninteracting standards. The sample is then subjected to increasing concentrations of F6P or ATP in the running buffer and electrophoresed. Analysis of the change in the migration times of ADPGlc PPase, relative to those of the noninteracting standards, as a function of the varying concentration of F6P or ATP yields a binding constant. The values obtained were in good agreement with kinetic parameters obtained from steady state activity assays. The method was extended to examine the F6P binding constants for the R33A and R22A enzymes and the ATP binding constants for the R8A enzyme in the presence and absence of F6P. The R33A enzyme has been shown by activity assays to be insensitive to F6P activation, indicating a defect in binding or in downstream transmission of the allosteric signal required for full activation. ACE indicated no apparent binding of F6P, supporting the former hypothesis. The R22A enzyme was shown by activity assays to have a approximately 15-fold decrease in apparent affinity for F6P compared to that of WT while ACE indicated an affinity comparable to that of WT; potential reasons for this discrepancy are discussed. The R8A enzyme as measured by activity assays exhibits reduced fold-activation by F6P compared to that of WT but increased apparent affinity for ATP in the presence of F6P. The ACE results were in good agreement with the activity assay data, confirming the increased affinity for ATP in the presence of F6P. This method demonstrates the quantitative ability of ACE to study different binding sites/ligand interactions in allosteric enzymes.

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.

Enantiomeric Separation of Basic Drugs with Partially Filled Serum Albumin as Chiral Selector in Capillary Electrophoresis

A reliable method is presented for the chiral separation of three basic drugs (mexiletine, chlorpheniramine and propranolol) with serum albumins (human and porcine, HSA and PSA) as chiral selectors by capillary electrophoresis in combination with the partial filling technique. Based on the systematic optimization of operation variables, the chiral separation of mexiletine, chlorpheniramine and propranolol was achieved in the pH 7.4 phosphate buffer by using HSA, PSA and PSA as selectors, respectively. The chiral recognition ability of HSA and PSA was compared. HSA and PSA show a different chiral recognition ability for each of the three drugs. In addition, the association constants between enantiomeric drugs and proteins were determined to be 2.00 and 3.80 x 10(2) M(-1) for mexiletine and HSA, 0.59 and 1.12 x 10(3) M(-1) for chlorpheniramine and PSA, and 0.87 and 1.42 x 10(3) M(-1) for propranolol and PSA. The method for the chiral separation and determination of association constants possesses the advantages of simple performance, effective avoiding of the interference of the UV detection from protein, and lowering of the reagent consumption.

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.

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.

Applications of on-line weak affinity interactions in free solution capillary electrophoresis

The impressive selectivity offered by capillary electrophoresis can in some cases be further increased when ligands or additives that engage in weak affinity interactions with one or more of the separated analytes are added to the electrophoresis buffer. This on-line affinity capillary electrophoresis approach is feasible when the migration of complexed molecules is different from the migration of free molecules and when separation conditions are nondenaturing. In this review, we focus on applying weak interactions as tools to enhance the separation of closely related molecules, e.g., drug enantiomers and on using capillary electrophoresis to characterize such interactions quantitatively. We describe the equations for binding isotherms, illustrate how selectivity can be manipulated by varying the additive concentrations, and show how the methods may be used to estimate binding constants. On-line affinity capillary electrophoresis methods are especially valuable for enantiomeric separations and for functional characterization of the contents of biological samples that are only available in minute quantities.

A study of the binding between polymers and peptides, using affinity capillary electrophoresis, applied to polymeric drug delivery systems

We have investigated the potential of affinity capillary electrophoresis (ACE) to evaluate binding constants between an anionic polydispersed polymer and four peptides. Nonlinear regression and three current linearization methods, the y-reciprocal, the x-reciprocal and the double-reciprocal, were employed for the estimation of the binding constants. The x-reciprocal and the double-reciprocal plots indicated the presence of two portions of straight lines for angiopeptin, triptorelin and the thyrotropin releasing hormone (TRH), and therefore the probable existence of a second-order interaction which causes the deviation from the 1:1 model. Peptide 1 exhibited a unique binding constant of 2.4 x 10(6)M(-1). In contrast, angiopeptin, triptorelin and TRH exhibited a K(1) of 4.0 x 10(6), 5.3 x 10(6) and 20.2 x 10(6)M(-1), respectively, and a K(2) of 0.4 x 10(6), 0.5 x 10(6) and 1.4 x 10(6)M(-1), respectively. The origin of the high scattering of the data points was further investigated. Neither the viscosity, nor the adsorption of the peptides to the capillary wall appeared to be the determining factor of data scattering. Finally, a possible adsorption of the polymer leading to the electroosmotic flow instability was supposed.

Molecular interactions of glycopeptide antibiotics investigated by affinity capillary electrophoresis and bioaffinity electrospray ionization-mass spectrometry

Many analytical approaches are available to evaluate (bio)molecular interactions, all of which have their particular advantages and disadvantages. In recent years, two relatively new techniques have emerged that may be used by the bioanalytical community to evaluate such interactions, namely affinity capillary electrophoresis (ACE) and bioaffinity electrospray ionization-mass spectrometry (ESI-MS). In this paper, we describe and evaluate the use of both these techniques for the investigation of the interactions of glycopeptide antibiotics with peptides that mimic the bacterial cell wall binding site. We focus particularly on the effect of the sugar moieties attached to the antibiotic peptide backbone and on the noncovalent dimerization of these glycopeptide antibiotics.

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.

Estimation of binding constants by capillary electrophoresis

Capillary electrophoresis (CE) has become a useful technique for measuring binding constants. This review is focused on recent trends in the estimation of binding constants by affinity CE. First, we introduce several mathematical equations in which it is assumed that the stoichiometry of the binding between drug and protein is 1:1 as a simple model. In order to calculate accurate binding constants by affinity CE, several experimental considerations are described in this review. In addition, some recent methodologies, such as partial filling technique and multiple-step ligand injection method, are introduced. Among research publications within 3 years, recent applications for determining binding constants are reviewed.

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.

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.

Recent advances in affinity capillary electrophoresis

Use of the specificity of (bio)interactions can effectively overcome the selectivity limitation faced in capillary electrophoresis (CE), and the resulting technique usually is referred to as affinity capillary electrophoresis (ACE). Despite the high selectivity of ACE, several important problems still need to be addressed. A major issue in all CE separations, including ACE, is the concentration detection limit. Using UV detection, this is usually in the order of 10(-6) M whereas laser-induced fluorescence (LIF) detection can provide detection limits down to the sub-10(-10) M range. However, a marked disadvantage of LIF is that labeling of the analytes is usually required, which might change the interaction behavior of the solutes under investigation. Additionally, labeling reactions at sub-10(-10) M concentration levels are certainly not trivial and often difficult to perform quantitatively. Alternative and universal detection approaches, particularly mass spectrometric (MS) detection, look very promising but (A) CE-MS techniques are still far from routine application. Important future progress in sensitive detection strategies is likely to increase the use of ACE in the future.

Estimation of receptor-ligand interactions by the use of a two-marker system in affinity capillary electrophoresis

The study of receptor-ligand interactions by affinity capillary electrophoresis (ACE) requires an accurate form of analysis. Here, we examine the use of two noninteracting standards (markers) in the analysis of binding constant data in ACE studies. This concept is demonstrated using two model systems: carbonic anhydrase B (CAB, EC 4.2.1.1) and arylsulfonamides, and vancomycin (Van) from Streptomyces orientalis and the dipeptide N-acetyl-d-Ala-d-Ala. In this procedure a plug of receptor and noninteracting standards is injected, and analysis of the change in the relative migration time ratio of the receptor, relative to the noninteracting standards, as a function of the concentration of the ligand yields a value for the binding constant. The findings described here demonstrate that data from ACE studies can best be analyzed using two noninteracting standards, yielding values comparable to those estimated using other binding and ACE techniques.

Using affinity capillary electrophoresis to evaluate average binding constant of 18-mer diphosphotyrosine peptide to antiphosphotyrosine Fab

We used affinity electrophoresis in capillaries to investigate the interaction between a monovalent antiphosphotyrosine antibody fragment, antigen-binding fragment (Fab), and a divalent antigen (dAg), an 18-mer diphosphopeptide phosphorylated on two-site tyrosine residues. The migration shift behavior of Fab in electrophoretic solution was observed and the quantitative expression was presented to estimate the arithmetical average value of the intrinsic affinities for two epitopes on the dAg with the Ag binding site on the Fab. In dAg excess, based on measurement of mobility changes of Fab analytes at different dAg concentrations, the experimental average dissociation constant (Kd = 27.7 microM) was calculated. It was also found that the structural variation of the two epitopes for binding specificity to the Ag-binding domain of Fab is not apparent. Moreover, the Kd values of Fab-dAg complexes were measured at higher electric fields and shown to be independent of changes in the electric field. Thus, under conditions where the total dAg concentration is in excess of the total Fab concentration, the method and quantitative expression which we developed is generally useful for the understanding of molecular interaction for an unlabeled monovalent receptor and its divalent ligand in free solution.