Application of capillary isotachophoresis in peptide analysis

Recent developments in capillary and microchip electroseparations of peptides (2021-mid-2023)

This review article brings a comprehensive survey of developments and applications of high-performance capillary and microchip electromigration methods (zone electrophoresis in a free solution or in sieving media, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography) for analysis, micropreparation, and physicochemical characterization of peptides in the period from 2021 up to ca. the middle of 2023. Progress in the study of electromigration properties of peptides and various aspects of their analysis, such as sample preparation, adsorption suppression, electroosmotic flow regulation, and detection, are presented. New developments in the particular capillary electromigration methods are demonstrated, and several types of their applications are reported. They cover qualitative and quantitative analysis of synthetic or isolated peptides and determination of peptides in complex biomatrices, peptide profiling of biofluids and tissues, and monitoring of chemical and enzymatic reactions and physicochemical changes of peptides. They include also amino acid and sequence analysis of peptides, peptide mapping of proteins, separation of stereoisomers of peptides, and their chiral analyses. In addition, micropreparative separations and physicochemical characterization of peptides and their interactions with other (bio)molecules by the above CE methods are described.

Peptide mapping of proteins by capillary electromigration methods

This review article provides a wide overview of important developments and applications of capillary electromigration methods in the area of peptide mapping of proteins in the period 1997-mid-2022, including review articles on this topic. It deals with all major aspects of peptide mapping by capillary electromigration methods: i) precleavage sample preparation involving purification, preconcentration, denaturation, reduction and alkylation of protein(s) to be analyzed, ii) generation of peptide fragments by off-line or on-line enzymatic and/or chemical cleavage of protein(s), iii) postcleavage preparation of the generated peptide mixture for capillary electromigration separation, iv) separation of the complex peptide mixtures by one-, two- and multidimensional capillary electromigration methods coupled with mass spectrometry detection, and v) a large application of peptide mapping for variable purposes, such as qualitative analysis of monoclonal antibodies and other protein biopharmaceuticals, monitoring of posttranslational modifications, determination of primary structure and investigation of function of proteins in biochemical and clinical research, characterization of proteins of variable origin as well as for protein and peptide identification in proteomic and peptidomic studies.

Recent developments in capillary and microchip electroseparations of peptides (2019-mid 2021)

The review provides a comprehensive overview of developments and applications of high performance capillary and microchip electroseparation methods (zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography) for analysis, microscale isolation, and physicochemical characterization of peptides from 2019 up to approximately the middle of 2021. Advances in the investigation of electromigration properties of peptides and in the methodology of their analysis, such as sample preparation, sorption suppression, EOF control, and detection, are presented. New developments in the individual CE and CEC methods are demonstrated and several types of their applications are shown. They include qualitative and quantitative analysis, determination in complex biomatrices, monitoring of chemical and enzymatic reactions and physicochemical changes, amino acid, sequence, and chiral analyses, and peptide mapping of proteins. In addition, micropreparative separations and determination of significant physicochemical parameters of peptides by CE and CEC methods are described.

Antibody-free detection of amyloid beta peptides biomarkers in cerebrospinal fluid using capillary isotachophoresis coupled with mass spectrometry

This study reports a capillary isotachophoresis (ITP) - electrospray ionization mass spectrometry (ESI-MS) method for the determination of several amyloid β (Aβ) peptides, which are biomarkers of Alzheimer's disease (AD) in cerebrospinal fluids (CSF). For the first time, these peptides have been detected directly from CSF by MS without recourse to an immunocapture-based sample pre-treatment. The antibody-free approach is based on the marriage between capillary ITP, a powerful on-line electrokinetic preconcentration technique, and MS for simultaneous detection of different Aβ peptides. To ensure a good performance, the ITP process of fluorescently labelled Aβ peptides was for the first time implemented and verified with laser induced fluorescent detection, prior to methodology transfer to MS detection. Better detection sensitivity was achieved with labelled Aβ peptides for both detection modes. Using hydroxyl ions as the terminating and acetate as the leading ions, our method allows efficient ITP preconcentration under alkaline conditions of the slowly migrating Aβ peptides to reach quantifiable concentration down to 50 pM. The developed ITP-MS approach allows reliable quantification of different fluorescently derivatized Aβ peptides, i.e. Aβ 1-42, Aβ 1-40 and Aβ 1-38 down to sub nM ranges in CSF samples from AD and non-demented (healthy control) patients. Good agreement (<20% deviation) for the determination of Aβ 1-42/Aβ 1-40 ratio in CSF was achieved between results obtained with this new ITP-MS and our recently developed method based on large volume sample stacking coupled to CE. Discrimination of one AD patient from two healthy controls was successfully made with the Aβ 1-42/Aβ 1-40 ratio obtained by the developed ITP-MS method for the tested cerebrospinal fluid samples.

A separation voltage polarity switching method for higher sample loading capacity and better separation resolution in transient capillary isotachophoresis separation

A separation voltage polarity switching transient capillary isotachophoresis (PS-tCITP) was developed to overcome a major sample loading volume limitation in transient capillary isotachophoresis (tCITP). The fundamental idea of PS-tCITP is to let sample ions move back and forth in a separation capillary during their initial isotachophoresis focusing stage by switching the polarity of the separation voltage, in order to both increase the sample loading volume and improve the separation efficiency as compared to the conventional tCITP method. The experimental evaluation of the novel PS-tCITP method by using two peptide standards at 2 μM concentration showed that the maximum sample loading volume could be increased from 45% of the total separation capillary volume in tCITP to 70% in PS-tCITP, which resulted in a more than 1.5 fold increase in the peptide peak intensity at a given length/volume of the separation capillary. Due to the consecutive focusing of sample volume from each polarity switching of the separation voltage, the separation time window at a given sample loading volume was also increased significantly in PS-tCITP as compared to tCITP. Experiment comparison between tCITP and PS-tCITP at 45% sample loading volume using the same setup showed that the migration time difference between the two peptide peaks increased from 0.3 min in tCITP to 0.363 min in PS-tCITP with similar peak widths and heights, resulting in roughly a 21% improvement in separation resolution. The performance advantages of PS-tCITP separation over tCITP separation were further verified by using a mixture of six peptide standards.

Recent developments in capillary and microchip electroseparations of peptides (2015-mid 2017)

The review brings a comprehensive overview of recent developments and applications of high performance capillary and microchip electroseparation methods (zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography) to analysis, microscale isolation, purification, and physicochemical and biochemical characterization of peptides in the years 2015, 2016, and ca. up to the middle of 2017. Advances in the investigation of electromigration properties of peptides and in the methodology of their analysis (sample preseparation, preconcentration and derivatization, adsorption suppression and EOF control, and detection) are described. New developments in particular CE and CEC methods are presented and several types of their applications to peptide analysis are reported: qualitative and quantitative analysis, determination in complex (bio)matrices, monitoring of chemical and enzymatical reactions and physical changes, amino acid, sequence and chiral analysis, and peptide mapping of proteins. Some micropreparative peptide separations are shown and capabilities of CE and CEC methods to provide important physicochemical characteristics of peptides are demonstrated.

From micro to macro: conversion of capillary electrophoretic separations of biomolecules and bioparticles to preparative free-flow electrophoresis scale

This invited contribution in the special issue of Electrophoresis published in celebration of the 30th Anniversary of this journal reflects the impact of our milestone paper [Prusík, Z., Kasicka, V., Mudra, P., Stepánek, J., Smékal, O., Hlavácek, J., Electrophoresis 1990, 11, 932-936] in the area of conversion of microscale analytical and micropreparative CE separations of biomolecules and bioparticles into (macro)preparative free-flow electrophoresis (FFE) scale on the basis of a correlation between CE and FFE methods. In addition to the survey of advances in the relatively narrow field of CE-FFE correlation and CE-FFE conversion, a comprehensive review of the recent developments of micropreparative CE and (macro)preparative FFE techniques is also presented and applications of these techniques to micro- and (macro)preparative separations and purifications of biomolecules and bioparticles are demonstrated. The review covers the period since the year of publication of the above paper, i.e. ca. the last 20 years.

Selective enrichment and ultrasensitive identification of trace peptides in proteome analysis using transient capillary isotachophoresis/zone electrophoresis coupled with nano-ESI-MS

Besides the complexity in protein samples of biological origin, probably the greatest challenge presently facing comprehensive proteome analysis is related to the large variation of protein relative abundances (>6 orders of magnitude), having potential biological significance in mammalian systems. As demonstrated in this work, transient capillary ITP/zone electrophoresis (CITP/CZE) provides selective analyte enrichment through electrokinetic stacking and extremely high resolving power toward protein and peptide mixtures. The result of the CITP process is that major components may be diluted, but trace compounds are concentrated. The on-column transition of CITP to CZE minimizes additional band broadening while providing superior analyte resolution. Online coupling of transient CITP/CZE with nano-ESI-MS allows ultrasensitive detection of trace peptides at levels of subnanomolar concentration or subfemtomole mass in complex peptide mixtures. More importantly, selective enrichment of trace peptides enables the identification and sequence analysis of low-abundance peptides co-migrated with highly abundant species at a concentration ratio of 1:500,000. The combined CITP/CZE-nano-ESI-MS system is demonstrated to be at least one to two orders of magnitude more sensitive than that attained in conventional electrophoretic and chromatographic-based proteome technologies over a wide dynamic concentration range, potentially allowing comprehensive analysis of protein profiles within a small cell population and limited tissue samples using conventional mass spectrometers. Furthermore, the speed of CITP/CZE separation and the lack of column equilibration in CITP/CZE not only improve the throughput of proteome analysis, but also facilitate its seamless integration with other separation technologies in a multidimensional protein identification platform.

Enantioselective analysis of cetirizine in pharmaceuticals by cyclodextrin-mediated capillary electrophoresis

The present paper demonstrates the potential of cyclodextrin (CD)-mediated CE for the chiral analysis of a drug of zwitterionic nature, viz. cetirizine (CET). Various separation mechanisms were applied and several parameters affecting the separation were studied, including the type and concentration of chiral selector, coselector, and carrier ion, and pH of buffer. The optimal separation conditions were based on a medium buffer pH (approximately 5.2) (migration velocity of CET molecule was near to zero) and a highly substituted CD derivative, sulfated-beta-CD, serving as an analyte carrier in the anionic regime of the separation with suppressed electroosmotic flow. In this way, a baseline enantioseparation, reasonable separation efficiency, and short analysis time could be easily achieved. Acceptable validation criteria for sensitivity, linearity, precision, accuracy, and robustness were obtained using a hydrodynamically closed CE separation system. The proposed method was successfully applied to the enantioselective assay of CET in pharmaceutical formulations using fexofenadine (FEX) as an internal standard.

pH-mediated acid stacking with reverse pressure for the analysis of cationic pharmaceuticals in capillary electrophoresis

When using capillary electrophoresis (CE) for the analysis of biological samples, it is often necessary to employ techniques to overcome peak-broadening that results from having a high-conductivity sample matrix. To improve the concentration detection limits and separation efficiency of cationic pharmaceuticals in CE, pH-mediated acid stacking was performed to electrofocus the sample, improving separation sensitivity for the analyzed cations by 60-fold. However, this method introduces a large titrated acid plug into the capillary. To overcome the limitations this low-conductivity plug poses to stacking, the plug was removed prior to the separation step by applying reverse pressure to force it out of the anode of the capillary. Employing this technique allows for roughly twice the volume of sample to be injected. A maximum sample injection time of 240 s was attainable with baseline peak resolution compared to a maximum sample injection time of 120 s without reverse pressure, leading to a twofold decrease in the limits of detection of the analytes used. Separation efficiency overall is also improved when utilizing the reverse pressure step. For example, a 60 s sample injection time results in 94,000 theoretical plates as compared to 60,500 theoretical plates without reverse pressure. This reverse-pressure method was used for detection and quantitation of several cationic pharmaceuticals that were prepared in Ringer's solution to simulate microdialysis sampling conditions.

Separation of proteins by zone electrophoresis on-line coupled with isotachophoresis on a column-coupling chip with conductivity detection

This feasibility study deals with the separations of proteins by an on-line combination of zone electrophoresis (ZE) with isotachophoresis (ITP) on a poly(methylmethacrylate) column-coupling (CC) chip with integrated conductivity detection. ITP and ZE provided specific analytical functions while performing the cationic mode of the separation. ITP served, mainly, for concentrations of proteins and its concentrating power was beneficial in reaching a low dispersion transfer (injection) of the proteinous constituents, loaded on the CC chip in a 960 nL volume, into the ZE separation stage. This was complemented by an electrophoretically driven removal of the sample constituents migrating in front of the focused proteins from the separation system before the ZE separation. On the other hand, ZE served as a final separation (destacking) method and it was used under the separating conditions providing the resolutions and sensitive conductivity detections of the test proteins. In this way, ITP and ZE cooperatively contributed to low- or sub-microg/mL concentration detectabilities of proteins and their quantitations at 1-5 microg/mL concentrations. However, a full benefit in concentration detectabilities of proteins, expected from the use of the ITP-ZE combination, was not reached in this work. Small adsorption losses of proteins and detection disturbances in the ZE stage of separation, very likely due to trace constituents concentrated by ITP, appear to set limits in the detection of proteins in our experiments. The ITP-ZE separations were carried out in a hydrodynamically closed separation compartment of the chip with suppressed hydrodynamic and electroosmotic flows of the electrolyte solutions. Such transport conditions, minimizing fluctuations of the migration velocities of the separated constituents, undoubtedly contributed to highly reproducible migrations of the separated proteins (fluctuations of the migration time of a particular protein were typically 0.5% RSD in repeated ITP-ZE runs).

Zone electrophoresis of proteins on a poly(methyl methacrylate) chip with conductivity detection

This work deals with zone electrophoresis (ZE) separations of proteins on a poly(methyl methacrylate) chip with integrated conductivity detection. Experiments were performed in the cationic mode of the separation (pH 2.9) with a hydrodynamically closed separation compartment and suppressed electroosmotic flow. The test proteins reached the detector in less than 10 min under these working conditions and their migration times characterized excellent repeatabilities (0.1-0.6% RSD values). The chip-to-chip agreements of the migration times, evaluated from the ZE runs performed on three chips, were within 1.5%. The conductivity detection provided for protein, loaded on the chip at 10-1000 microg/ml concentrations, detection responses were characterized by 1-5% RSD values of their peak areas. Such migration and detection performances made a frame for reproducible baseline separations of a five-constituent mixture (cytochrome c, avidin, conalbumin, human hemoglobin and trypsin inhibitor). On the other hand, a high sample injection channel/separation compartment volume ratio of the chip (500 nl/8500 nl) restricted the resolution of proteins of very close effective mobilities in spite of the fact that in the initial phase of the separation an electric field stacking was applied. A maximum macroconstituent/trace constituent ratio attainable for proteins on the chip was assessed for cytochrome c (quantifiable when its concentration in the loaded sample was 10 microg/ml) and apo-transferrin (containing a trace constituent migrating in the position of cytochrome c detectable when the load of apo-transferrin was 2000 microg/ml). This assessment indicated that a ratio of 1000:1 is attainable with the aid of conductivity detection on the present chip.

Automated free-solution isotachophoresis: instrumentation and fractionation of human serum proteins

An automated free-solution isotachophoresis system (FS-ITP) for preparative fractionation of biopolymers is described, operated in a batch mode. The dimension of the separation chamber allows an up to 1200-fold higher sample load compared to separation in capillaries of 180 microm inner diameter as used in analytical capillary isotachophoresis (C-ITP). The preparative capacity of the system is within the milligram range. The method is fully compatible with analytical C-ITP, which is essential for preparative-scale isotachophoresis with regard to optimization of electrolyte systems and the search for suitable spacers. As a model application the fractionation of human serum proteins is reported. The collected fractions were analyzed by C-ITP and agarose gel electrophoresis.

On-line solid-phase extraction-capillary electrophoresis for enhanced detection sensitivity and selectivity: application to the analysis of metallothionein isoforms in sheep fetal liver

A simple and rapid capillary electrophoresis (CE) method for analysis of metallothionein (MT) isoforms is described. A modified two-step solvent extraction procedure was used in combination with CE and an on-line solid-phase preconcentration device for sensitive and reproducible detection of MT isoforms in sheep fetal liver. Preparation of twenty samples was practicable within a working day with subsequent automated overnight analysis by solid-phase extraction (SPE)-CE. A commercially available divinylbenzene-based reversed-phase resin was found to be most suitable for the SPE device because it is resistant to extremes of pH and can be adequately regenerated between analyses. Each SPE device was readily constructed from commonly available materials and was used for the reproducible separation of over 100 biological samples before replacement. Precision of analysis within or between sample batches was < 10% and usually < 5% while detection limits were at least 28 ng/ml for standards and 272 ng/ml for biological samples. This would indicate a detection limit of about 0.5 microgram/g wet weight of tissue. Recovery of MT from tissue cytosols by solvent extraction was measured using radiolabeled MT and was found to be just over 50% increasing to almost 70% by addition of NaCl to the homogenisation buffer. The combined solvent extraction and SPE-CE methodology was applied to the analysis of MT in sheep fetal liver and the results compared favorably with those obtained by high-resolution chromatography. MT-1 levels were 2 to 4-times higher than those of MT-2 and both isoforms decreased from day 89 to day 136 of gestation. These results were compared with MT levels in fetal liver from sheep embryos that had been perturbed by temporary transfer to an advanced uterine environment. Hepatic MT levels at day 136 of gestation were 3 to 8-times higher than in normal fetal liver and significant differences were observed with both isoforms.

Application of capillary and free-flow zone electrophoresis and isotachophoresis to the analysis and preparation of the synthetic tetrapeptide fragment of growth hormone-releasing peptide

The use of high-performance electromigration separation methods, capillary zone electrophoresis (CZE) and capillary isotachophoresis (CITP) and continuous free-flow arrangements of these two separation principles, free-flow zone electrophoresis (FFZE) and free-flow isotachophoresis (FFITP), was investigated in the analysis and purification of the synthetic C-terminal tetrapeptide fragment (H2N-Ala-Trp-D-Phe-Lys.NH2) of the growth hormone-releasing peptide. CZE and CITP were used for microanalysis of peptide preparations after different steps of their purification. The homogeneity of the peptide preparations, including fractions of preparative separations, was quantified by relative zone length (CITP) and/or relative peak height (CZE). In addition, the data obtained by CZE and CITP (electrophoretic and electroosmotic flow migration velocities) were utilized for conversion of micro-scale capillary separations (nano- to picomole level) into the preparative separations realized by FFZE and FFITP with a capacity from tens to hundreds of milligrams per hour.