Influence of the ionic strength of acidic background electrolytes on the separation of proteins by capillary electrophoresis

Electromigration Dispersion in Sodium Dodecyl Sulfate Capillary Gel Electrophoresis of Proteins

The electromigration dispersion of the light- and heavy-chain subunit peaks of the therapeutic monoclonal antibody omalizumab was investigated in sodium dodecyl sulfate capillary gel electrophoresis (SDS-CGE) using borate cross-linked dextran sieving matrices. Increasing boric acid content (340-640 mM) caused electromigration dispersion shifts for both low (2%)- and high (10%)-dextran-concentration gels in all gel-buffer compositions. In case of the heavy-chain fragment, elevated borate concentrations resulted in decreasing tailing and increasing fronting with the use of higher- and lower-dextran-concentration gels, respectively. The light-chain fragment, on the other hand, exhibited increased fronting with increasing borate concentration for both dextran concentrations examined in this study. Increase of the glycerol ingredient level in the gel-buffer system caused the same effect as the increasing borate concentration in both dextran concentrations. The detected electromigration dispersion was considered as the result of the formation of monomeric and dimeric glycerol-borate complexes as co-ionic constituents, migrating slower than that of the unconjugated tetrahydroxyborate. In addition, complexation of the tetrahydroxyborate anion with the glucose building blocks of the dextran polymer decreased its mobility to practically zero, contributing to further decrease in the resultant effective mobility of the co-ionic species. We suggest that the observed fronting and/or tailing peak shapes of the monoclonal antibody fragments in SDS-CGE at increasing boric acid concentrations can be considered as the result of multiple effects including changes in pH, sieving matrix pore size, viscosity, and the mobility variation of the co-ionic borate adducts with the gel-buffer ingredients. While electromigration dispersion-mediated band broadening, in general, can be minimized via matching the effective mobility of the co-ionic species to the analyte molecules of interest, in case of borate cross-linked dextran gels, optimization of the boric acid concentration required special consideration of its gel cross-linking function. For the light- and heavy-chain fragments of the IgG analyte, best peak shapes were attained with the use of 10% dextran/340 mM boric acid and 10% dextran/640 mM boric acid-containing gel-buffer systems, respectively. Based on this observation, here we introduce the concept of borate-gradient-mediated transient mobility matching in SDS-CGE of proteins. This novel approach resulted in close to optimal peak shapes for the distantly migrating IgG subunits within a single run, as well as unraveled the long-sought possible solution to perform capillary pore-size-gradient gel electrophoresis.

On-line separation and quantification of virus antigens of different serotypes in multivalent vaccines by capillary zone electrophoresis: A case study for quality control of foot-and-mouth disease virus vaccines

Accurate quantification of effective antigens of different serotypes is crucial for quality control of multivalent vaccines but challenging. A simple and rapid capillary zone electrophoresis (CZE) method was developed for on-line separation and quantification of foot-and-mouth disease virus (FMDV) antigens in monovalent and bivalent FMDV vaccines. The FMDV peak identity in CZE was demonstrated by the study of FMDV dissociation combined with high performance size exclusion chromatography (HPSEC) analysis. After optimizing CZE conditions including UV detecting wavelength, injection volume, and separation voltage, both serotype A and O FMDV showed good reproducibility (RSD <5%) and linear responses (R²=0.999) between the peak area and FMDV content in the concentration range of 15-400 μg/mL. The two serotypes of FMDV with similar size had different migration time in CZE according to their different zeta potential, which allows them to be separated and quantified, with accuracy of <10% relative error. CZE was then successfully applied for antigen quantification of commercial O monovalent and A/O bivalent FMDV vaccines. Compared with HPSEC, CZE was not only able to quantify each serotype of FMDV, but also free from interference of nucleic acids impurities. In summary, the CZE can be a simple, rapid, and reliable tool for quality control of monovalent and bivalent FMDV vaccines. The CZE method can also be further extended to the quality control of other multivalent virus and virus like particle vaccines.

Mechanistic Study of Protein Adsorption on Mesoporous TiO2 in Aqueous Buffer Solutions

Protein adsorption is of fundamental importance for bioseparation engineering applications. In this work, a series of mesoporous TiO₂ with various geometric structures and different aqueous buffer solutions were prepared as platforms to investigate the effects of the surface geometry and ionic strength on the protein adsorptive behavior. The surface geometry of the TiO₂ was found to play a dominant role in the protein adsorption capacity when the ionic strength of buffer solutions is very low. With the increase in ionic strength, the effect of the geometric structure on the protein adsorption capacity reduced greatly. The change of ionic strength has the highest significant effect on the mesoporous TiO₂ with large pore size compared with that with small pore size. The interaction between the protein and TiO₂ measured with atomic force microscopy further demonstrated that the adhesion force induced by the surface geometry reduced with the increase in the ionic strength. These findings were used to guide the detection of the retention behavior of protein by high-performance liquid chromatography, providing a step forward toward understanding the protein adsorption for predicting and controlling the chromatographic separation of proteins.

Design of an automated capillary electrophoresis platform for single-cell analysis

Single-cell analysis of cellular contents by highly sensitive analytical instruments is known as chemical cytometry. A chemical cytometer typically samples one cell at a time, quantifies the cellular contents of interest, and then processes and reports that data. Automation adds the potential to perform this entire sequence of events with minimal intervention, increasing throughput and repeatability. In this chapter, we discuss the design considerations for an automated capillary electrophoresis-based instrument for assay of enzymatic activity within single cells. We describe the key requirements of the microscope base and capillary electrophoresis platforms. We also provide detailed protocols and schematic designs of our cell isolation, lysis, sampling, and detection strategies. Additionally, we describe our signal processing and instrument automation workflows. The described automated system has demonstrated single-cell throughput at rates above 100cells/h and analyte limits of detection as low as 10^-20mol.

Separation and Characterization of Highly Charged Polyelectrolytes Using Free-Solution Capillary Electrophoresis

The characterization of statistical copolymers of various charge densities remains an important and challenging analytical issue. Indeed, the polyelectrolyte (PE) effective electrophoretic mobility tends to level off above a certain charge density, due to the occurrence of Manning counterion condensation. Surprisingly, we demonstrate in this work that it is possible to get highly resolutive separations of charged PE using free-solution capillary electrophoresis, even above the critical value predicted by the Manning counterion condensation theory. Full separation of nine statistical poly(acrylamide-co-2-acrylamido-2-methylpropanesulfonate) polymers of different charge densities varying between 3% and 100% was obtained by adjusting the ionic strength of the background electrolyte (BGE) in counter electroosmotic mode. Distributions of the chemical charge density could be obtained for the nine PE samples, showing a strong asymmetry of the distribution for the highest-charged PE. This asymmetry can be explained by the different reactivity ratios during the copolymerization. To shed more light on the separation mechanism, effective and apparent selectivities were determined by a systematic study and modeling of the electrophoretic mobility dependence according to the ionic strength. It is demonstrated that the increase in resolution with increasing BGE ionic strength is not only due to a closer matching of the electroosmotic flow magnitude with the PE electrophoretic effective mobility, but also to an increase of the dependence of the PE effective mobility according to the charge density.

High pH instability of quaternary ammonium surfactant coatings in capillary electrophoresis

The two-tailed cationic surfactant dioctadecyldimethyl ammonium bromide (DODAB) produces semi-permanent coatings that yield strongly reversed electroosmotic flow (EOF), for example -0.31 ± 0.01 cm² kV^-1 s^-1 at pH 3.5. Moreover, these coatings are easy to prepare, regenerable, cost effective, and yield high efficiency (520 000-900 000 plates per m) separations of cationic proteins over many runs under acidic (pH 3.5) conditions. Given the quaternary amine functionality of DODAB, we were surprised to observe that DODAB coatings become unstable at pH > 7. At pH 7.2, the EOF of a DODAB coated capillary drifted from reversed to cathodic over only 5 runs, and protein separations became severely compromised. By pH 12, no EOF reversal was observed. Electrophoretic and mass spectrometric studies demonstrate that the coating decomposition involves a surface conversion of the quaternary amine in DODAB to a variety of products, although the exact mechanism remains elusive. Regardless, the results herein demonstrate that semi-permanent coatings based on cationic two-tailed surfactants such as DODAB are limited to separations using acidic buffers.

Recent advances in protein analysis by capillary and microchip electrophoresis

This review article describes the significant recent advances in the analysis of proteins by capillary and microchip electrophoresis during the period from mid-2014 to early 2017. This review highlights the progressions, new methodologies, innovative instrumental modifications, and challenges for efficient protein analysis in human specimens, animal tissues, and plant samples. The protein analysis fields covered in this review include analysis of native, reduced, and denatured proteins in addition to Western blotting, protein therapeutics and proteomics.

On the ionic strength dependence of the electrophoretic mobility: From 2D to 3D slope-plots

Determining the charge and the nature (small ion, nanoparticle, or polyelectrolyte) of an unknown solute from its electrophoretic characteristics remains a challenging issue. In this work, we demonstrate that, if the knowledge of the effective electrophoretic mobility (μ_ep ) at a given ionic strength is not sufficient to characterize a given solute, the combination of this parameter with (i) the relative decrease of the electrophoretic mobility with the ionic strength (S), and (ii) the hydrodynamic radius (R_h ), is sufficient (in most cases) to deduce the nature of the solute and its charge. These three parameters are experimentally accessible by CZE and Taylor dispersion analysis performed on the same instrumentation. 3D representation of the three aforementioned parameters (μ_ep ; S and R_h ) is proposed to visualize the differences in the electrophoretic behavior between solutes according to their charge and nature. Surprisingly, such 3D slope plot in the case of small ions and nanoparticles looks like a "whale-tail," while polyelectrolyte contour plot represents a rather simple and monotonous map that is independent of solute size. This work also sets how to estimate the effective charge of a solute from a given experimental (S,Rh,μ ep 5 mM ) triplet, which is not possible to obtain unambiguously with only (Rh,μ ep 5 mM ) or (S,μ ep 5 mM ) doublet, where μ ep 5 mM is the effective electrophoretic mobility at 5 mM ionic strength.