Separation of proteins by sodium dodecylsulfate capillary electrophoresis in hydroxypropylcellulose sieving matrix with laser-induced fluorescence detection

In Migratio Noncovalent Fluorophore Labeling of Proteins by Propidium Iodide in Sodium Dodecyl Sulfate Capillary Gel Electrophoresis

Sodium dodecyl sulfate capillary gel electrophoresis is one of the frequently used methods for size-based protein separation in molecular biology laboratories and the biopharmaceutical industry. To increase throughput, quite a few multicapillary electrophoresis systems have been recently developed, but most of them only support fluorescence detection, requiring fluorophore labeling of the sample proteins. To avoid the time-consuming derivatization reaction, we developed an on-column labeling approach utilizing propidium iodide for the first time in SDS-CGE of proteins, a dye only used before for nucleic acid analysis. As a key ingredient of the gel-buffer system, the oppositely migrating positively charged propidium ligand in migratio complexes with the SDS-proteins, therefore, supports in situ labeling during the electrophoretic separation process, not requiring any extra pre- or postcolumn derivatization step. A theoretical treatment is given to shed light on the basic principles of this novel online labeling process, also addressing the influence of propidium iodide on the electroosmotic flow, resulting in reduced retardation. The concept of propidium labeling in SDS-CGE was first demonstrated using a commercially available protein sizing ladder ranging from 6.5 to 200 kDa with different isoelectric points and post-translational modifications. Considering the increasing number of protein therapeutics on the market next, we focused on the labeling optimization of a therapeutic monoclonal antibody and its subunits, including the addition of the nonglycosylated heavy chain. Peak efficiency and resolution were compared between noncovalent and covalent labeling. The effect of ligand concentration on the effective and apparent electrophoretic mobility, the resulting peak area, and the resolution were all evaluated in view of the theoretical considerations. The best detection sensitivity for the intact monoclonal antibody was obtained by using 200 μg/mL propidium iodide in the separation medium (LOD 2 μg/mL, 1.35 × 10-8 M) with excellent detection linearity over 3 orders of magnitude. On the other hand, the resolution between the biopharmaceutical protein test mixture components containing the intact and subunit fragments of the therapeutic monoclonal antibody was very good in the ligand concentration range of 50-200 μg/mL, but using the local maximum at 100 μg/mL for the nonglycosylated/glycosylated heavy chain pair is recommended. The figures of merit, including precision, sensitivity, detection linear range, and resolution for a sample mixture in hand, can be optimized by varying the propidium iodide concentration in the gel-buffer system, as demonstrated in this paper.

Protein separation by capillary gel electrophoresis: a review

Capillary gel electrophoresis (CGE) has been used for protein separation for more than two decades. Due to the technology advancement, current CGE methods are becoming more and more robust and reliable for protein analysis, and some of the methods have been routinely used for the analysis of protein-based pharmaceuticals and quality controls. In light of this progress, we survey 147 papers related to CGE separations of proteins and present an overview of this technology. We first introduce briefly the early development of CGE. We then review the methodology, in which we specifically describe the matrices, coatings, and detection strategies used in CGE. CGE using microfabricated channels and incorporation of CGE with two-dimensional protein separations are also discussed in this section. We finally present a few representative applications of CGE for separating proteins in real-world samples.

Buffers to suppress sodium dodecyl sulfate adsorption to polyethylene oxide for protein separation on capillary polymer electrophoresis

Although polyethylene oxide (PEO) offers several advantages as a sieving polymer in SDS capillary polymer electrophoresis (SDS-CPE), solution properties of PEO cause deterioration in the electrophoresis because PEO in solution aggregates itself, degrades into smaller pieces, and forms polymer-micelle complexes with SDS. We examined protein separation on SDS-CPE with PEO as a sieving matrix in four individual buffer solutions: Tris-CHES, Tris-Gly, Tris-Tricine, and Tris-HCl buffers. The solution properties of PEO as a sieving matrix in those buffers were examined by dynamic light scattering (DLS) and by surface tension. Preferential SDS adsorption onto PEO disturbed protein-SDS complexation and impaired the protein separation efficiency. Substantial adsorption of SDS to PEO was particularly observed in Tris-Gly buffer. The Tris-CHES buffer prevented SDS from adsorbing onto the PEO. Only Tris-CHES buffer achieved separation of six proteins. This study demonstrated efficient protein separation on SDS-CPE with PEO.

Poly(2-ethyl-2-oxazoline) as a sieving matrix for SDS-CE

The use of poly(2-ethyl-2-oxazoline) with a number-average molecular weight (of polymer) of 500,000 g/mol in a sieving matrix for SDS CE size separation of proteins is investigated using polymer solutions with concentrations between 6 and 12% w/v. Optimal separation efficiency is obtained using 10% w/v, with an average separation efficiency of 150,000 (12,000) theoretical plates observed for myoglobin (weight-average molecular weight=18 kDa) and 8,800,000 (2,400,000) theoretical plates for carbonic anhydrase (weight-average molecular weight=30 kDa) for 14 capillaries with an 80 cm effective length. Some adsorbed polymers were found to be effective in reducing EOF when applied to a clean capillary; however, a covalently attached polyacrylamide coating was found to be much less troublesome after initial wall treatment. Two different buffer systems, (i) 25 mM HEPES and (ii) 20 mM Tris with 40 or 60 mM tricene, were tried with 0.05 or 0.10% w/v SDS added. Tris-tricene gave generally better current stability than HEPES but with no observed improvement in separation efficiency. This polymer has all the desired characteristics for an SDS-CE sieving matrix using LIF detection, including hydrolytic stability, optical clarity, low viscosity, acceptable hydrophilicity, and commercial availability.

Characterizing intercellular signaling peptides in drug addiction

Intercellular signaling peptides (SPs) coordinate the activity of cells and influence organism behavior. SPs, a chemically and structurally diverse group of compounds responsible for transferring information between neurons, are broadly involved in neural plasticity, learning and memory, as well as in drug addiction phenomena. Historically, SP discovery and characterization has tracked advances in measurement capabilities. Today, a suite of analytical technologies is available to investigate individual SPs, as well as entire intercellular signaling complements, in samples ranging from individual cells to entire organisms. Immunochemistry and in situ hybridization are commonly used for following preselected SPs. Discovery-type investigations targeting the transcriptome and proteome are accomplished using high-throughput characterization technologies such as microarrays and mass spectrometry. By integrating directed approaches with discovery approaches, multiplatform studies fill critical gaps in our knowledge of drug-induced alterations in intercellular signaling. Throughout the past 35 years, the National Institute on Drug Abuse has made significant resources available to scientists that study the mechanisms of drug addiction. The roles of SPs in the addiction process are highlighted, as are the analytical approaches used to detect and characterize them.

Rapid and variable-volume sample loading in sieving electrophoresis microchips using negative pressure combined with electrokinetic force

A rapid and variable-volume sample loading scheme for chip-based sieving electrophoresis was developed by negative pressure combined with electrokinetic force. This was achieved by using a low-cost microvacuum pump and a single potential supply at a constant voltage. Both 12% linear polyacrylamide (LPA) with a high viscosity of 15000 cP and 2% hydroxyethylcellulose (HEC) with a low viscosity of 102 cP were chosen as the sieving materials to study the behavior and the versatility of the proposed method. To reduce the hydrodynamic resistance in the sampling channel, sieving material was only filled in the separation channel between the buffer waste reservoir (BW) to the edge of the crossed intersection. By applying a subambient pressure to the headspace of sample waste reservoir (SW), sample and buffer solution were drawn immediately from sample reservoir (S) and buffer reservoir (B) across the intersection to SW. At the same time, the charged sample in the sample flow was driven across the interface between the sample flow and the sieving matrix into the sieving material filled separation channel by the applied electric field. The injected sample plug length is in proportion with the loading time. Once the vacuum in SW reservoir was released to activate electrophoretic separation, flows from S and B to SW were immediately terminated by the back flow induced by the difference of the liquid levels in the reservoirs to prevent sample leakage during the separation stage. The sample consumption was about 1.7 x 10(2) nL at a loading time of 1 s for each cycle. Only 0.024 s was required to transport bias-free analyte to the injection point. It is easy to freely choose the sample plug volume in this method by simply changing the loading time and to inject high quality sample plug with non-distorted shape into the separation channel. The system has been proved to possess an exciting potential for improving throughput, repeatability, sensitivity and separation performance of chip-based sieving electrophoresis.

LIF detection of peptides and proteins in CE

CE- and microchip-based separations coupled with LIF are powerful tools for the separation, detection and determination of biomolecules. CE with certain configurations has the potential to detect a small number of molecules or even a single molecule, thanks to the high spatial coherence of the laser source which permits the excitation of very small sample volumes with high efficiency. This review article discusses the use of LIF detection for the analysis of peptides and proteins in CE. The most common laser sources, basic instrumentation, derivatization modes and set-ups are briefly presented and special attention is paid to the different fluorogenic agents used for pre-, on- and postcapillary derivatization of the functional groups of these compounds. A table summarizing major applications of these derivatization reactions to the analysis of peptides and proteins in CE-LIF and a bibliography with 184 references are provided which covers papers published to the end of 2005.

Capillary electrophoresis-based separation techniques for the analysis of proteins

CE offers the advantages of high speed, great efficiency, as well as the requirement of minimum amounts of sample and buffer for the analysis of proteins. In this review, we summarize the CE-based techniques coupled with absorption, LIF, and MS detection systems for the analysis of proteins mostly within the past 5 years. The basic principle of each technique and its advantages and disadvantages for protein analysis are discussed in brief. Advanced CE techniques, including on-column concentration techniques and high-efficiency multidimensional separation techniques, for high-throughput protein profiling of complex biological samples and/or of single cells are emphasized. Although the developed techniques provide improved peak capacity, they have not become practical tools for proteomics, mainly because of poor reproducibility, low-sample lading capacity, and low throughput due to ineffective interfaces between two separation dimensions and that between separation and MS systems. In order to identify the complexities and dynamics of the proteomes expressed by cells, tissues, or organisms, techniques providing improved analytical sensitivity, throughput, and dynamic ranges are still demanded.

Imaging mass spectrometry: fundamentals and applications to drug discovery

Imaging mass spectrometry (IMS) encompasses a variety of techniques that enable the chemical imaging of analytes, which range in size from atoms and small molecules to intact proteins, directly from biological tissues. IMS is transforming specific areas in biological research with its unique combination of chemical and spatial information. Innovations in instrumentation and imaging protocols will make this approach invaluable at many stages of the drug discovery process, including pharmacological target screening and evaluating the distribution of drug and drug metabolites in cells and tissues. The fundamentals and unique methodology of IMS are discussed, along with exciting new applications to drug discovery science.

Laser-induced fluorescence detection schemes for the analysis of proteins and peptides using capillary electrophoresis

Over the past few years, a large number of studies have been prepared that describe the analysis of peptides and proteins using capillary electrophoresis (CE) and laser-induced fluorescence (LIF). These studies have focused on two general goals: (i) development of automatic, selective and quick separation and detection of mixtures of peptides or proteins; (ii) generation of new methods of quantitation for very low concentrations (nm and subnanomolar) of peptides. These two goals are attained with the use of covalent labelling reactions using a variety of dyes that can be readily excited by the radiation from a commonly available laser or via the use of noncovalent labelling (immunoassay using a labelled antibody or antigen or noncovalent dye interactions). In this review article, we summarize the works which were performed for protein and peptide analysis via CE-LIF.

Replaceable Cross-Linked Polyacrylamide for High Performance Separation of Proteins

An alternative sieving matrix, replaceable cross-linked polyacrylamide (rCPA), was developed for sodium dodecyl sulfate capillary gel electrophoresis (SDS-CGE) separation of proteins. This rCPA could be conveniently pressurized into separation capillaries under a pressure of 80 psi. SDS-CGE separations using this matrix generated high resolutions for a wide range (approximately 4 kD to approximately 300 kD) of proteins. When compared to the most frequently used sieving matrixes, the rCPA permitted the highest resolutions with comparable or increased separation speed for protein separations.

Post-column fluorescence derivatization of proteins and peptides in capillary electrophoresis with a sheath flow reactor and 488 nm argon ion laser excitation

We report the use of a sheath flow reactor for post-column fluorescence derivatization of proteins. The derivatization reaction employed naphthalene-2,3-dicarboxaldehyde (NDA) and beta-mercaptoethanol, which were added in the sheath buffer. The labeled proteins were detected by laser-induced fluorescence with an argon-ion laser beam at 488 nm. The performance of this detection scheme was evaluated by separation of some protein standards. A column efficiency of 450,000 plates/m was obtained without stacking. The limits of detection for those standard proteins were determined to be from 8 to 32 nM. Excellent linear relationship was obtained with correlation coefficient of 0.9998 for alpha-lactalbumin concentration ranging from 3.91 x 10(-7) to 1.25 x 10(-5) M. Separation of protein standards at low pH was also demonstrated by reversing the electroosmotic flow (EOF) with addition of cetyltrimethylammonium bromide (CTAB) to the running buffer. Different separation selectivity was achieved, but the sensitivity is poorer than that at high pH. This post-column derivatization detection system was applied successfully to analyze the protein extract from HT29 human colon cancer cells as well as tryptic peptides.

MitoTracker Green labeling of mitochondrial proteins and their subsequent analysis by capillary electrophoresis with laser-induced fluorescence detection

MitoTracker Green (MTG) is a mitochondrial-selective fluorescent label commonly used in confocal microscopy and flow cytometry. It is expected that this dye selectively accumulates in the mitochondrial matrix where it covalently binds to mitochondrial proteins by reacting with free thiol groups of cysteine residues. Here we demonstrate that MTG can be used as a protein labeling reagent that is compatible with a subsequent analysis by capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). Although the MTG-labeled proteins and MTG do not seem to electrophoretically separate, an enhancement in fluorescence intensity of the product indicates that only proteins with free thiol groups are capable of reacting with MTG. In addition we propose that MTG is a partially selective label towards some mitochondrial proteins. This selectivity stems from the high MTG concentration in the mitochondrial matrix that favors alkylation of the available thiol groups in this subcellular compartment. To that effect we treated mitochondria-enriched fractions that had been prepared by differential centrifugation of an NS-1 cell lysate. This fraction was solubilized with an SDS-containing buffer and analyzed by CE-LIF. The presence of a band with fluorescence stronger than MTG alone also indicated the presence of an MTG-protein product. Confirming that MTG is labeling mitochondrial proteins was done by treating the solubilized mitochondrial fraction with 5-furoylquinoline-3-carboxaldehyde (FQ), a fluorogenic reagent that reacts with primary amino groups, and analysis by CE-LIF using two separate detection channels: 520 nm for MTG-labeled species and 635 nm for FQ-labeled species. In addition, these results indicate that MTG labels only a subset of proteins in the mitochondria-enriched fraction.

Heterogeneity of protein labeling with a fluorogenic reagent, 3-(2-furoyl)quinoline-2-carboxaldehyde

Fluorogenic reagents are used for protein labeling when high-sensitivity fluorescence detection is required. Similar to traditional labeling with activated fluorescent dyes, such as fluorescein isothiocyanate, a fluorogenic reaction is expected to change the physical-chemical properties of proteins. Knowledge of these changes may be essential for efficient separation and identification of labeled proteins. Here we studied the effect of labeling of myoglobin with a fluorogenic reagent on the acid-base properties of the protein. The fluorogenic reagent used was 3-(2-furoyl)quinoline-2-carboxaldehyde (FQ). In slab-gel isoelectric focusing, we found that the labeling reaction generated at least six species with pI values lower than that of non-labeled myoglobin. These species can be identified as products of progressive labeling of myoglobin with one to six FQ molecules. The same series of FQ-labeled species were observed when the reaction products were analyzed by capillary zone electrophoresis. The comparison of experimental and theoretical pI values allowed us to elucidate the labeling pattern--the number of FQ molecules corresponding to each labeled product detected by isoelectric focusing.

Capillary electrophoresis of proteins 1999-2001

This review article with 223 references describes recent developments in capillary electrophoresis (CE) of proteins and covers papers published during last two years, from the previous review (V. Dolnik, Electrophoresis 1999, 20, 3106-3115) through Spring 2001. It describes the topics related to CE of proteins including modeling of the electrophoretic properties of proteins, sample pretreatment, wall coatings, improving selectivity, detection, special electrophoretic techniques, and applications.