Measurement of antibody-antigen dissociation constants using fast capillary electrophoresis with laser-induced fluorescence detection

Interaction studies between human papillomavirus virus-like particles and laminin 332 by affinity capillary electrophoresis assisted by bio-layer interferometry

Human papillomavirus (HPV) interacts, in vitro, with laminin 332 (LN332), a key component of the extracellular matrix. In this study, we performed bio-layer interferometry (BLI) and affinity capillary electrophoresis (ACE) to investigate the binding properties of this interaction. Virus-like particles (VLPs), composed of the HPV16 L1 major capsid protein, were used as HPV model and LN332 as the VLPs binding partner. Using BLI, we quantitatively determined the kinetics of the interaction, via the measurement of VLP binding and release from LN332 immobilized onto the surface of aminopropylsilane biosensors. We found an averaged kon of 1.74 x 104 M-1s-1 and an averaged koff of 1.50 x 10-4 s-1. Furthermore, an ACE method was developed to study the interaction under physiological conditions, where the interactants are moving freely in solution, without any fluorescence labeling. Specifically, a constant amount of HPV16-VLPs was preincubated with increasing LN332 concentrations and then the samples were injected in the capillary electrophoresis instrument. A shift in the migration time of the HPV16-VLP/LN332 complexes, carrying an increasing number of LN332 molecules bound per VLP, was observed. The mobility of the complexes was found to decrease with increasing LN332 concentrations in the sample. It was used to quantify stability constant. From BLI and ACE approaches, we reported an apparent equilibrium dissociation constant in the nanomolar range (8.89 nM and 17.7 nM, respectively) for the complex between HPV16-VLPs and LN332.

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

Progress in on-line, at-line, and in-line coupling of sample treatment with capillary and microchip electrophoresis over the past 10 years: A review

The review presents an evaluation of the development of on-line, at-line and in-line sample treatment coupled with capillary and microchip electrophoresis over the last 10 years. In the first part, it describes different types of flow-gating interfaces (FGI) such as cross-FGI, coaxial-FGI, sheet-flow-FGI, and air-assisted-FGI and their fabrication using molding into polydimethylsiloxane and commercially available fittings. The second part deals with the coupling of capillary and microchip electrophoresis with microdialysis, solid-phase, liquid-phase, and membrane based extraction techniques. It mainly focuses on modern techniques such as extraction across supported liquid membrane, electroextraction, single drop microextraction, head space microextraction, and microdialysis with high spatial and temporal resolution. Finally, the design of sequential electrophoretic analysers and fabrication of SPE microcartridges with monolithic and molecularly imprinted polymeric sorbents are discussed. Applications include the monitoring of metabolites, neurotransmitters, peptides and proteins in body fluids and tissues to study processes in living organisms, as well as the monitoring of nutrients, minerals and waste compounds in food, natural and wastewater.

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.

Dual detection high-speed capillary electrophoresis for simultaneous serum protein analysis and immunoassays

Serum protein electrophoresis (SPE) separates serum proteins into bands whose shape and amplitude can alert clinicians to a range of disorders. This is followed by more specific immunoassays to quantify important antigens and confirm a diagnosis. Here we develop a high-speed capillary electrophoresis (HSCE) platform capable of simultaneous SPE and immunoassay measurements. A single laser excitation source is focused into the detection zone of the capillary to measure both refractive index (SPE) and fluorescence signals (immunoassays). The refractive index signal measures characteristic SPE profiles for human serum separated in 100 mM boric acid (pH 10), 100 mM arginine (pH 11), and 20 mM CHES (pH 10). For the immunoassay, the fluorescence electropherograms reveal that CHES provides the optimal buffer for measuring the immunocomplex and separating it from the free antigen. Immunoassays in CHES yield a LOD of 23 nM and a LOQ of 70 nM for the detection of fluorescein. The high pH reduces protein adsorption but reduces antibody affinity. Preliminary studies carried out in 50 mM barbital at pH 8 show improved stability of the immunocomplex and better separation for immunoassay quantification. Further optimization will open new capabilities for measuring orthogonal diagnostic signals in seconds with HSCE.

Segmentation-based analysis of single-cell immunoblots

From genomics to transcriptomics to proteomics, microfluidic tools underpin recent advances in single-cell biology. Detection of specific proteoforms-with single-cell resolution-presents challenges in detection specificity and sensitivity. Miniaturization of protein immunoblots to single-cell resolution mitigates these challenges. For example, in microfluidic western blotting, protein targets are separated by electrophoresis and subsequently detected using fluorescently labeled antibody probes. To quantify the expression level of each protein target, the fluorescent protein bands are fit to Gaussians; yet, this method is difficult to use with noisy, low-abundance, or low-SNR protein bands, and with significant band skew or dispersion. In this study, we investigate segmentation-based approaches to robustly quantify protein bands from single-cell protein immunoblots. As compared to a Gaussian fitting pipeline, the segmentation pipeline detects >1.5× more protein bands for downstream quantification as well as more of the low-abundance protein bands (i.e., with SNR ∼3). Utilizing deep learning-based segmentation approaches increases the recovery of low-SNR protein bands by an additional 50%. However, we find that segmentation-based approaches are less robust at quantifying poorly resolved protein bands (separation resolution, Rs < 0.6). With burgeoning needs for more single-cell protein analysis tools, we see microfluidic separations as benefitting substantially from segmentation-based analysis approaches.

Toward rapid infectious disease diagnosis with advances in surface-enhanced Raman spectroscopy

In a pandemic era, rapid infectious disease diagnosis is essential. Surface-enhanced Raman spectroscopy (SERS) promises sensitive and specific diagnosis including rapid point-of-care detection and drug susceptibility testing. SERS utilizes inelastic light scattering arising from the interaction of incident photons with molecular vibrations, enhanced by orders of magnitude with resonant metallic or dielectric nanostructures. While SERS provides a spectral fingerprint of the sample, clinical translation is lagged due to challenges in consistency of spectral enhancement, complexity in spectral interpretation, insufficient specificity and sensitivity, and inefficient workflow from patient sample collection to spectral acquisition. Here, we highlight the recent, complementary advances that address these shortcomings, including (1) design of label-free SERS substrates and data processing algorithms that improve spectral signal and interpretability, essential for broad pathogen screening assays; (2) development of new capture and affinity agents, such as aptamers and polymers, critical for determining the presence or absence of particular pathogens; and (3) microfluidic and bioprinting platforms for efficient clinical sample processing. We also describe the development of low-cost, point-of-care, optical SERS hardware. Our paper focuses on SERS for viral and bacterial detection, in hopes of accelerating infectious disease diagnosis, monitoring, and vaccine development. With advances in SERS substrates, machine learning, and microfluidics and bioprinting, the specificity, sensitivity, and speed of SERS can be readily translated from laboratory bench to patient bedside, accelerating point-of-care diagnosis, personalized medicine, and precision health.

High-Speed Capillary Electrophoresis Using a Thin-Wall Fused-Silica Capillary Combined with Backscatter Interferometry

High-speed capillary electrophoresis (HSCE) is implemented using a 10 cm total length fused-silica capillary (50 μm i.d., 80 μm o.d.) combined with refractive index (RI) detection using backscatter interferometry (BSI). The short capillary length reduces analysis time while the ultrathin wall (15 μm) efficiently dissipates heat from the separation channel, mitigating the deleterious effects of Joule heating. The separation capillary is mounted on a temperature-controlled heat sink that stabilizes the temperature to ±0.004 °C. This temperature stabilization improves separation efficiency and enhances RI detection. Ohm's Law plots confirm the superior heat dissipation of the HSCE capillary compared to a similarly prepared conventional CE capillary (50 μm i.d., 363 μm o.d.). The speed and efficiency of HSCE combined with universal RI detection is illustrated through the separation of K⁺, Ba²⁺, Mg²⁺, Na⁺, Li⁺, and Tris⁺ in approximately 30 s, with efficiencies greater than 500 000 plates/m. Run-to-run repeatability is explored using nine consecutive electrokinetic injections of a K⁺, Na⁺, and Li⁺ mixture. The average migration times and %RSD for K⁺, Na⁺, and Li⁺ were measured to be 22.04 s (1.59%), 26.81 s (1.38%), and 29.80 s (2.21%), respectively. Finally, we show that the BSI signal is sensitive to the separation voltage through the Kerr mechanism. This leads to peaks in the electropherogram from the injection process that are useful for precisely defining the start of each separation and quantifying the amount of sample injected onto the capillary.

Clinical and pharmaceutical applications of affinity ligands in capillary electrophoresis: A review

Affinity capillary electrophoresis (ACE) is a separation technique that combines a biologically-related binding agent with the separating power and efficiency of capillary electrophoresis. This review will examine several classes of binding agents that have been used in ACE and applications that have been described for the resulting methods in clinical or pharmaceutical analysis. Binding agents that will be considered are antibodies, aptamers, lectins, serum proteins, carbohydrates, and enzymes. This review will also describe the various formats in which each type of binding agent has been used in CE, including both homogeneous and heterogeneous methods. Specific areas of applications that will be considered are CE-based immunoassays, glycoprotein/glycan separations, chiral separations, and biointeraction studies. The general principles and formats of ACE for each of these applications will be examined, along with the potential advantages or limitations of these methods.

PEGylation and surface functionalization of liposomes containing drug nanocrystals for cell-targeted delivery

Liposomal formulations have important therapeutic applications in anti-cancer treatments but current formulations suffer from serious side effects, high dosage requirements and prolonged treatment. In this study, PEGylated azide-functionalized liposomes containing drug nanocrystals were investigated with the aim of increasing the drug payload and achieving functionalization for targeted delivery. Liposomes were characterized using cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), small and ultra-small angle neutron scattering (SANS/USANS) and small and wide angle X-ray scattering (SAXS/WAXS). Cryo-TEM experiments revealed the dimensions of the nanocrystal-loaded liposomes and the change of shape from spherical to elongated after the formation of nanocrystals. Results from SANS/USANS experiments confirmed the asymmetric particle shape. SAXS/WAXS experiments confirmed that the crystalline drug only occurred in freeze-thawed samples and correlated with a new unidentified polymorphic form of ciprofloxacin. Using a small molecule dye, dibenzocyclooctyne (DBCO)-cy5, specific conjugation between DBCO groups and surface azide groups on the liposomes was confirmed; this indicates the promise of this system for tumour-targeted delivery.

Protein biochips based on macroporous polymer supports: Material properties and analytical potential

A series of rigid macroporous polymer layers differed by hydrophobic-hydrophilic properties was synthesized in situ in preliminary fabricated wells and applied as the platforms for protein biochips. Scanning electron microscopy, etalon porosimetry and BET analysis were used for materials characterization. The comparison of analytical efficiency of the developed platforms allowed for the choice of the most optimal polymer, as well as the evaluation of impact of material porous properties. The quantitative parameters of affinity interaction between two different protein pairs were calculated depending on biochip characteristics using the developed analytical protocol. Moreover, the described biochips were successfully tested to detect acetylcholinesterase via catalytic reaction followed by the formation of fluoresceine as a product. Different parameters of enzymatic reaction were calculated for the reaction on a chip and compared to those established for in solution process.

Molecular Affinity Agents for Intrinsic Surface-Enhanced Raman Scattering (SERS) Sensors

Research at the interface of synthetic materials, biochemistry, and analytical techniques has enabled sensing platforms for applications across many research communities. Herein we review the materials used as affinity agents to create surface-enhanced Raman spectroscopy (SERS) sensors. Our scope includes those affinity agents (antibody, aptamer, small molecule, and polymer) that facilitate the intrinsic detection of targets relevant to biology, medicine, national security, environmental protection, and food safety. We begin with an overview of the analytical technique (SERS) and considerations for its application as a sensor. We subsequently describe four classes of affinity agents, giving a brief overview on affinity, production, attachment chemistry, and first uses with SERS. Additionally, we review the SERS features of the affinity agents, and the analytes detected by intrinsic SERS with that affinity agent class. We conclude with remarks on affinity agent selection for intrinsic SERS sensing platforms.

Microfluidic Chip with Integrated Electrophoretic Immunoassay for Investigating Cell-Cell Interactions

Microfluidics have been used to create "body-on-chip" systems to mimic in vivo cellular interactions with a high level of control. Most such systems rely on optical observation of cells as a readout. In this work we integrated a cell-cell interaction chip with online microchip electrophoresis immunoassay to monitor the effects of the interaction on protein secretion dynamics. The system was used to investigate the effects of adipocytes on insulin secretion. Chips were loaded with 190 000 3T3-L1 adipocytes and a single islet of Langerhans in separate chambers. The chambers were perfused at 300-600 nL/min so that adipocyte secretions flowed over the islets for 3 h. Adipocytes produced 80 μM of nonesterified fatty acids (NEFAs), a factor known to impact insulin secretion, at the islets. After perfusion, islets were challenged with a step change in glucose from 3 to 11 mM while monitoring insulin secretion at 8 s intervals by online immunoassay. Adipocyte treatment augmented insulin secretion by 6-fold compared to controls. The effect was far greater than comparable concentrations of NEFA applied to the islets demonstrating that adipocytes release multiple factors that can strongly potentiate insulin secretion. The experiments reveal that integration of chemical analysis with cell-cell interaction can provide valuable insights into cellular functions.

A novel monitoring approach of antibody-peptide binding using "bending" capillary electrophoresis

Recently, the in-capillary electrophoresis assay has been applied to variety kinds of analyses owing to its multiple functional integrating features, including mixing of samples, reaction process of the mixtures, and the separation and detection in one capillary system. However, the micro-reactor still has its limitations to the currently available applications, especially the mixing step of the samples inside the capillary could not be well controlled automatically or manually. Herein, we have developed a novel capillary electrophoresis assay for the detection of antibody-peptide binding inside a bending capillary. Its efficacy was monitored using an anti-FLAG M2 antibody and its ligand conjugated with FAM dye (FAM-DYKD). The antibody and the peptide were mixed inside the bending capillary with sequential injections. It was found that the numbers of semi-circle on the capillary interfered by the antibody and peptide binding dynamic. Additionally, an online competition assay was performed, which further validated the efficacy of the bending capillary device on monitoring the dynamic binding between the antigen and antibody. In summary, our data suggests that the novel assay is a practical approach in monitoring the antibody-antigen complex formation at a nano-scale. It could be applied to detect any biomolecule-biomolecule interaction as a general strategy.

Capillary electrophoresis-laser-induced fluorescence (CE-LIF)-based immunoassay for quantifying antibodies against cyclic citrullinated peptides

CE-LIF based immunoassay to quantify antibodies against cyclic citrullinated peptides in rheumatoid arthritis patients.

Dynamic biochemical tissue analysis detects functional L-selectin ligands on colon cancer tissues

A growing body of evidence suggests that L-selectin ligands presented on circulating tumor cells facilitate metastasis by binding L-selectin presented on leukocytes. Commonly used methods for detecting L-selectin ligands on tissues, e.g., immunostaining, are performed under static, no-flow conditions. However, such analysis does not assay for functional L-selectin ligands, specifically those ligands that promote adhesion under shear flow conditions. Recently our lab developed a method, termed dynamic biochemical tissue analysis (DBTA), to detect functional selectin ligands in situ by probing tissues with L-selectin-coated microspheres under hemodynamic flow conditions. In this investigation, DBTA was used to probe human colon tissues for L-selectin ligand activity. The detection of L-selectin ligands using DBTA was highly specific. Furthermore, DBTA reproducibly detected functional L-selectin ligands on diseased, e.g., cancerous or inflamed, tissues but not on noncancerous tissues. In addition, DBTA revealed a heterogeneous distribution of functional L-selectin ligands on colon cancer tissues. Most notably, detection of L-selectin ligands by immunostaining using HECA-452 antibody only partially correlated with functional L-selectin ligands detected by DBTA. In summation, the results of this study demonstrate that DBTA detects functional selectin ligands to provide a unique characterization of pathological tissue.

High speed capillary zone electrophoresis-mass spectrometry via an electrokinetically pumped sheath flow interface for rapid analysis of amino acids and a protein digest

While capillary zone electrophoresis (CZE) has been used to produce very rapid and efficient separations, coupling these high-speed separations with mass spectrometry (MS) has been challenging. Now, with much faster and sensitive mass spectrometers, it is possible to take full advantage of the CZE speed and reconstruct the fast migrating peaks. Here are three high-speed CZE-MS analyses via an electrokinetically pumped sheath-flow interface. The first separation demonstrates CZE-ESI-MS of an amino acid mixture with a 2-min separation, >50,000 theoretical plates, low micromolar concentration detection limits, and subfemtomole mass detection limits (LTQ XL mass spectrometer). The second separation with our recently improved third-generation CE-MS interface illustrates a 20 amino acid separation in ∼7min with an average over 200,000 plate counts, and results in almost-baseline resolution of structural isomers, leucine and isoleucine. The third separation is of a BSA digest with a reproducible CZE separation and mass spectrometry detection in 2min. CZE-MS/MS analysis of the BSA digest identified 31 peptides, produced 52% sequence coverage, and generated a peak capacity of ∼40 across the 1-min separation window (Q-Exactive mass spectrometer).

Clinical applications of capillary electrophoresis based immunoassays

Immunoassays have long been an important set of tools in clinical laboratories for the detection, diagnosis, and treatment of disease. Over the last two decades, there has been growing interest in utilizing CE as a means for conducting immunoassays with clinical samples. The resulting method is known as a CE immunoassay. This approach makes use of the selective and strong binding of antibodies for their targets, as is employed in a traditional immunoassay, and combines this with the speed, efficiency, and small sample requirements of CE. This review discusses the variety of ways in which CE immunoassays have been employed with clinical samples. An overview of the formats and detection modes that have been employed in these applications is first presented. A more detailed discussion is then given on the type of clinical targets and samples that have been measured or studied by using CE immunoassays. Particular attention is given to the use of this method in the fields of endocrinology, pharmaceutical measurements, protein and peptide analysis, immunology, infectious disease detection, and oncology. Representative applications in each of these areas are described, with these examples involving work with both traditional and microanalytical CE systems.

Development of a capillary electrophoresis platform for identifying inhibitors of protein-protein interactions

Methods for identifying chemical inhibitors of protein-protein interactions (PPIs) are often prone to discovery of false positives, particularly those caused by molecules that induce protein aggregation. Thus, there is interest in developing new platforms that might allow earlier identification of these problematic compounds. Capillary electrophoresis (CE) has been evaluated as a method to screen for PPI inhibitors using the challenging system of Hsp70 interacting with its co-chaperone Bag3. In the method, Hsp70 is labeled with a fluorophore, mixed with Bag3, and the resulting bound and free Hsp70 are separated and detected by CE with laser-induced fluorescence detection. The method used a chemically modified CE capillary to prevent protein adsorption. Inhibitors of the Hsp70-Bag3 interaction were detected by observing a reduction in the bound-to-free ratio. The method was used to screen a library of 3443 compounds, and the results were compared to those from a flow cytometry protein interaction assay. CE was found to produce a lower hit rate with more compounds that were reconfirmed in subsequent testing, suggesting greater specificity. This finding was attributed to the use of electropherograms to detect artifacts such as aggregators and to differences in protein modifications required to perform the different assays. Increases in throughput are required to make the CE method suitable for primary screens, but at the current stage of development it is attractive as a secondary screen to test hits found by higher-throughput methods.

Single-molecule measurements of the binding between small molecules and DNA aptamers

Aptamers that bind small molecules can serve as basic biosensing platforms. Evaluation of the binding constant between an aptamer and a small molecule helps to determine the effectiveness of the aptamer-based sensors. Binding constants are often measured by a series of experiments with varying ligand or aptamer concentrations. Such experiments are time-consuming, material nonprudent, and prone to low reproducibility. Here, we use laser tweezers to determine the dissociation constant for aptamer-ligand interactions at the single-molecule level from only one ligand concentration. Using an adenosine 5'-triphosphate disodium salt (ATP) binding aptamer as an example, we have observed that the mechanical stabilities of aptamers bound with ATP are higher than those without a ligand. Comparison of the change in free energy of unfolding (ΔG(unfold)) between these two aptamers yields a ΔG of 33 ± 4 kJ/mol for the binding. By applying a Hess-like cycle at room temperature, we obtained a dissociation constant (K(d)) of 2.0 ± 0.2 μM, a value consistent with the K(d) obtained from our equilibrated capillary electrophoresis (CE) (2.4 ± 0.4 μM) and close to that determined by affinity chromatography in the literature (6 ± 3 μM). We anticipate that our laser tweezers and CE methodologies may be used to more conveniently evaluate the binding between receptors and ligands and also serve as analytical tools for force-based biosensing.

Measuring aptamer equilibria using gradient micro free flow electrophoresis

Gradient micro free flow electrophoresis (muFFE) was used to observe the equilibria of DNA aptamers with their targets (IgE or HIVRT) across a range of ligand concentrations. A continuous stream of aptamer was mixed online with an increasing concentration of target and introduced into the muFFE device, which separated ligand-aptamer complexes from the unbound aptamer. The continuous nature of muFFE allowed the equilibrium distribution of aptamer and complex to be measured at 300 discrete target concentrations within 5 min. This is a significant improvement in speed and precision over affinity capillary electrophoresis (ACE) assays. The dissociation constant of the aptamer-IgE complex was estimated to be 48 +/- 3 nM. The high coverage across the range of ligand concentrations allowed complex stoichiometries of the aptamer-HIVRT complexes to be observed. Nearly continuous observation of the equilibrium distribution from 0 to 500 nM HIVRT revealed the presence of complexes with 3:1 (aptamer/HIVRT), 2:1, and 1:1 stoichiometries.

Analysis of proteins in solution using affinity capillary electrophoresis

Analysis of protein interactions by means of capillary electrophoresis (CE) has unique challenges and rewards. The choice of analysis conditions, especially involving electrophoresis buffers, are crucial and not universal for protein analysis. If conditions for analysis can be worked out, it is possible to utilize CE quantitatively and qualitatively to characterize protein-ligand binding involving unmodified molecules in solution and taking place under physiological conditions. This chapter deals with the most important practical considerations in capillary electrophoretic affinity approaches, affinity CE (ACE). The text emphasizes the most critical factors for successful analyses and has application examples illustrating various types of information offered by ACE-based studies. Also included are step-by-step accounts of the two main classes of experimental design: the pre-equilibration ACE (in the form of CE-frontal analysis (CE-FA)) and mobility shift ACE together with examples of their use. The ACE approaches for binding assays of proteins should be considered when the biological material is scarce, when any kind of labeling is not possible or desired, when the interacting molecules are the same size and when rapid and simple method development is a priority.

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.

Protein-aptamer binding studies using microchip affinity capillary electrophoresis

The use of traditional CE to detect weak binding complexes is problematic due to the fast-off rate resulting in the dissociation of the complex during the separation process. Additionally, proteins involved in binding interactions often nonspecifically stick to the bare-silica capillary walls, which further complicates the binding analysis. Microchip CE allows flexibly positioning the detector along the separation channel and conveniently adjusting the separation length. A short separation length plus a high electric field enables rapid separations thus reducing both the dissociation of the complex and the amount of protein loss due to nonspecific adsorption during the separation process. Thrombin and a selective thrombin-binding aptamer were used to demonstrate the capability of microchip CE for the study of relatively weak binding systems that have inherent limitations when using the migration shift method or other CE methods. The rapid separation of the thrombin-aptamer complex from the free aptamer was achieved in less than 10 s on a single-cross glass microchip with a relatively short detection length (1.0 cm) and a high electric field (670 V/cm). The dissociation constant was determined to be 43 nM, consistent with reported results. In addition, aptamer probes were used for the quantitation of standard thrombin samples by constructing a calibration curve, which showed good linearity over two orders of magnitude with an LOD for thrombin of 5 nM at a three-fold S/N.

On-line sample preconcentration by sweeping with dodecyltrimethylammonium bromide in capillary zone electrophoresis

On-line sample preconcentration of oligonucleotides with a new sweeping carrier was developed by using dodecyltrimethylammonium bromide (DTAB) below the critical micelle concentration (CMC). The sweeping results with DTAB below and above the CMC were compared. The use of DTAB below the CMC benefits the preconcentration of the oligonucleotides, while the use of DTAB above the CMC is good for hydrophobic small molecules. The factors affecting the sweeping results were optimized and this method was evaluated by constructing calibration curves for thrombin aptamers. The sweeping scheme produced a 112-fold sensitivity enhancement for the oligonucleotides relative to that run in a running buffer without DTAB. The sweeping method developed here can be a good reinforcement of the preconcentration scheme by sweeping when less-hydrophobic analytes or large negatively-charged molecules need to be preconcentrated.

Bioanalytical interaction studies executed by preincubation affinity capillary electrophoresis

The versatility of CE is beneficial for the study of many types of molecular interactions, because different experimental designs can be made to suit the characteristics of a particular interaction. A very versatile starting point is the preequilibration type of affinity CE that has been used extensively for characterizing biomolecular interactions in the last 15 years. We review this field here and include a comprehensive overview of the existing preincubation ACE modes including their advantages and limitations as well as the methodological developments and applications within the bioanalytical field.

Molecular aptamers for real-time protein-protein interaction study

Protein-protein interactions play critical roles in cellular functions, but current techniques for real-time study of these interactions are limited. We report the real-time monitoring of protein-protein interactions without labeling either of the two interacting proteins; this procedure poses minimum effects on the binding properties of the proteins. Our strategy uses a protein/aptamer complex to probe the interactions in a competitive assay where the binding of an aptamer to its target protein is altered by a second protein that interacts with the target protein. Two signal transduction strategies, fluorescence resonance energy transfer (FRET) and fluorescence anisotropy, have been designed to study the interactions of human alpha-thrombin with different proteins by using two aptamers specific for two binding sites on alpha-thrombin. Our method has been shown to be simple and effective, does not require labeling of proteins, makes use of easily obtainable aptamers, provides detailed protein-protein interaction information and has excellent sensitivity for protein detection and protein-protein interaction studies. The FRET and the fluorescent anisotropy approaches complement each other in providing insight into the kinetics, mechanisms, binding sites and binding dynamics of the interacting proteins.

Effects of the limited analyte solubility on its mobility and zone shape: Electrophoretic behavior of sanguinarine and chelerythrine around pH 7

Electrophoretic mobilities and shapes of zones of sanguinarine and chelerythrine in aqueous media around pH 7 are affected by limited solubility of their uncharged forms and by the pH-dependent chemical equilibrium between cationic and uncharged forms of these alkaloids. The sanguinarine solubility in sodium MOPS of pH 7.4 was estimated at 50 micromol x L(-1). Sanguinarine zones in this buffer have the shape of tailed peak with concentration-independent mobility if the injected sanguinarine concentration exceeds this solubility limit only slightly. The chelerythrine solubility is higher because of lower dissociation constants of its cations. Precipitation of sanguinarine and chelerythrine with the phosphate anions decelerates their electrophoretic transport in phosphate buffer. Sanguinarine solubility is 5 micromol x L(-1) at the most in 13 mmol x L(-1) sodium phosphate buffer of pH 7.4. Acidifying of the sample up to pH 3 decreases the tailing of the peaks of sanguinarine and chelerythrine and contributes to the rise of sharp maxima of their migrating zones. Any capillary coating deteriorates the peak shape.

Analysis of vasopressin using capillary electrophoresis with laser-induced fluorescence detector based on competitive immunoassay

A competitive immunoassay based on CE-LIF has been developed for the determination of vasopressin in biological mixtures. Vasopressin participates in the hormonal control of water metabolism and the constriction of arterioles in humans. Thus, detection of vasopressin is important in diagnosing pathological conditions and physiological water metabolism. The peptides were fluorescently tagged with FITC and purified by HPLC. The purified product was then mixed with the cerebrospinal fluid sample followed with the addition of anti-vasopressin antibody. It was possible to separate antibody-bound and free FITC-tagged vasopressin within 10 min by CE-LIF analysis using uncoated fused-silica capillary with high reproducibility.

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.

Immunoaffinity capillary electrophoresis: determination of binding constant and stoichiometry for antibody-antigen interaction

Affinity capillary electrophoresis (ACE) can provide both qualitative and quantitative information on molecular interactions and affords the advantages of very low sample consumption, high mass sensitivity, short analysis time, and the use of automated instrumentation. It has been applied clinically and biochemically to the determination of the binding constant and to the measurement of the binding stoichiometry for interactions between antibodies (Ab's) and antigens (Ag's) in free solution. In many situations, the Ag molecule has two or multiple binding sites, each of which has a similar or different intrinsic affinity for binding independently to the combining site(s) on an Ab molecule. The multivalent binding reactions between Ab and Ag molecules often occur. The objective of this review is to describe the uses of ACE in the determination of binding constants and stoichiometry of Ab-Ag interactions (immunoaffnity capillary electrophoresis), focusing especially on multivalent Ab-Ag interaction modes. Five model binding systems developed recently using ACE techniques are described with principles and examples: (i) divalent mAb-monovalent Ag interaction, (ii) divalent mAb-(homo)polyvalent Ag interaction, (iii) cooperativity of two binding sites of mAb-monovalent Ag interaction, (iv) monovalent Fab-divalent Ag interaction, and (v) polyclonal Ab-monovalent Ag interaction. Finally, the determination of binding stoichiometry of Ab-Ag interactions by ACE is described.

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.