The Essential Guide to 2D Gel Electrophoresis

Two-dimensional gel electrophoresis (2D gel electrophoresis, 2-DE or 2D-PAGE) separates the proteins in a complex sample twice, first by isoelectric point and then by molecular weight, so that each protein lands as a discrete spot on a single gel. Depending on gel size and pH range, a 2D gel can resolve more than 5,000 proteins at once and detect less than a nanogram of protein in a spot, which is why it remains the standard method for comparing protein expression, isoforms and post-translational modifications across samples. This guide explains how it works, how to run it, where it is used, and how it compares with one-dimensional SDS-PAGE.

What is 2D gel electrophoresis?

2D gel electrophoresis is a technique for separating proteins in two independent dimensions on one gel. In the first dimension, isoelectric focusing (IEF) sorts proteins by their isoelectric point (pI), the pH at which each protein carries no net charge. In the second dimension, SDS-PAGE sorts them by molecular weight. Because the two properties are unrelated, proteins that share one property are almost always separated by the other, and the result is a two-dimensional map of spots in which each spot is, in most cases, a single protein or a single isoform.

The method was introduced by Patrick O’Farrell in 1975. The modern form, using immobilized pH gradient (IPG) strips in place of carrier-ampholyte tube gels, was developed by Angelika Görg and colleagues and solved the reproducibility problems of the original technique. Görg, Weiss and Dunn’s 2004 review in Proteomics remains the standard reference for the workflow described on this page.

You will see the technique written several ways. 2D gel electrophoresis, 2D electrophoresis, 2-DE, 2D-PAGE and 2D SDS-PAGE all refer to the same method. IEF/SDS-PAGE describes the two dimensions explicitly. 2D-DIGE is a fluorescent multiplexed variant, covered below.

How 2D gel electrophoresis works

The principle is separation by two orthogonal properties. One-dimensional SDS-PAGE separates by size only, so proteins of similar molecular weight pile up in the same band. Adding a first dimension that separates by charge spreads those proteins out across the width of the gel before the size separation runs down its length.

First dimension: isoelectric focusing (IEF)

The sample is loaded onto an IPG strip, a thin strip of polyacrylamide gel in which a pH gradient has been fixed by covalently bound buffering groups. When voltage is applied, each protein migrates through the gradient until it reaches the pH equal to its pI. At that point it carries no net charge, stops moving, and focuses into a sharp band. Proteins that drift away from their pI regain charge and are pulled back, which is why IEF produces such tight separation.

IPG strips are available in broad ranges such as pH 3 to 10 for a first survey of a sample, and in narrow ranges such as pH 4 to 7 or one-unit “zoom” gradients for resolving crowded regions. Narrow-range IPGs can resolve proteins whose pI differ by as little as 0.001 pH units, and gradients now extend from pH 2.5 to 12, which brought very acidic and very alkaline proteins into reach.

Second dimension: SDS-PAGE

After focusing, the strip is equilibrated in a buffer containing SDS, which coats every protein with negative charge in proportion to its length and unfolds it, and a reducing agent that breaks disulfide bonds. The strip is then laid along the top edge of a polyacrylamide slab gel and a second voltage is applied at right angles to the first. Proteins now migrate down the gel by molecular weight alone, smaller proteins moving faster through the polyacrylamide mesh.

The result is the familiar 2D gel image: pI increasing left to right, molecular weight decreasing top to bottom, and each protein resolved to a spot whose intensity reflects its abundance.

1D vs 2D gel electrophoresis

One-dimensional SDS-PAGE and two-dimensional gel electrophoresis use the same second-dimension chemistry. The difference is whether an isoelectric focusing step comes first, and that one difference changes what each method is for.

Use 1D SDS-PAGE when you know which protein you are looking for and want to check its size, purity or amount. Use 2D gel electrophoresis when you want to compare many proteins between samples without knowing in advance which ones will change, or when you need to see a modification that changes charge but not size, such as phosphorylation.

Step-by-step 2D gel electrophoresis protocol

Every lab tunes its own protocol to its samples and equipment. The values below are typical starting points taken from published manufacturer protocols, referenced at the end of the page, and are given so that you can see the shape of a real run rather than as a recipe to follow blindly.

1. Sample preparation and solubilization

The single largest source of failed 2D gels is the sample. Proteins must be fully solubilized, denatured and reduced, and the sample must be free of salts, nucleic acids, lipids and anything else that will disturb the pH gradient. A typical rehydration buffer is 8 M urea, 2% CHAPS, 50 mM DTT and 0.2% carrier ampholytes with a trace of bromophenol blue. Thiourea (2 M) is often added for membrane and other hard-to-solubilize proteins. Salt must be kept low, because ions carry current and flatten the pH gradient; if the sample is salty, desalt or precipitate the protein first.

Quantify protein before loading. Typical loads on a broad-range strip are 20 to 50 µg for Coomassie staining and 5 to 15 µg for silver; on a narrow-range strip, 100 to 200 µg for Coomassie and 50 to 100 µg for silver.

2. IPG strip rehydration

Dried IPG strips are rehydrated in the sample-containing buffer so that the protein enters the gel as the strip swells. Volumes scale with strip length: 125 µl for a 7 cm strip, 185 µl for 11 cm, 300 µl for 17 cm. Passive rehydration runs overnight, typically 11 to 16 hours, at room temperature. Some protocols apply a low voltage during rehydration to help large proteins enter the gel.

3. Isoelectric focusing

The rehydrated strip is focused in a dedicated IEF unit. Programs step the voltage up to limit current at the start, when the sample is still salty, and then hold at high voltage for a set number of volt-hours. A typical program for a 7 cm strip is 250 V for 20 minutes, a linear ramp to 4,000 V over 2 hours, then 4,000 V held to a total of 10,000 volt-hours. Longer strips run higher: 8,000 V to 20,000 Vh for 11 cm, 10,000 V to 40,000 Vh for 17 cm. Focused strips can be run immediately or stored frozen.

4. Equilibration

Before the second dimension, the strip is soaked twice, ten minutes each, in an SDS equilibration buffer. The base buffer is 6 M urea, 2% SDS, 0.375 M Tris-HCl pH 8.8 and 20% glycerol. The first soak adds 2% DTT to reduce disulfide bonds. The second replaces DTT with iodoacetamide to alkylate the freed cysteines, which prevents them re-forming and sharpens spots. Skipping or shortening equilibration is a common cause of vertical streaking.

5. SDS-PAGE

The equilibrated strip is placed on the top of a slab gel, sealed in place with agarose, and run. Gradient gels such as 8 to 16% are common because they resolve a wide molecular weight range on one gel; single-percentage gels are used when the proteins of interest fall in a narrow range. Run conditions scale with gel size: for example 200 V constant for about 40 minutes on a 7 cm mini gel, or 16 mA per gel for 30 minutes then 24 mA per gel for roughly five hours on a 17 cm large-format gel. Large gels are run cooled, because heat distorts the spot pattern and the distortion differs from gel to gel.

6. Staining and imaging

Spots are visualized with a stain chosen for the sensitivity and linearity the experiment needs. Coomassie is the simplest and is compatible with mass spectrometry. Silver is more sensitive but less linear and less MS-friendly. Fluorescent stains such as SYPRO Ruby combine good sensitivity with a wide linear range. In 2D-DIGE the proteins are labeled with fluorescent dyes before the run, so no post-staining is needed.

Image capture matters as much as the run. Use 16-bit images, avoid saturation, keep resolution and file size consistent across a dataset, and never edit the image before analysis. Our separate guide covers this in detail.

7. Image analysis and quantification

Comparing spots across gels is where a 2D experiment succeeds or fails. Every gel is slightly different, so spots must be aligned across the set, detected consistently, quantified, normalized and compared with appropriate statistics. Software that detects spots separately on each gel and then tries to match them produces missing values wherever a spot is found on one gel and missed on another, and missing values undermine the statistics. SameSpots takes the opposite approach, aligning all images first and then detecting one spot pattern across the whole experiment, so that every spot is measured on every gel.

How to analyze 2D gels

SameSpots 2D gel analysis software

8. Spot picking and protein identification

Spots that differ significantly between conditions are excised, digested with trypsin, and identified by mass spectrometry, most often peptide mass fingerprinting or MS/MS. Analysis software exports a picking list with spot coordinates for a robotic picker or a manual template.

2D protein spot excision and identification

2D-DIGE: the multiplexed version

Two-dimensional difference gel electrophoresis (2D-DIGE) labels up to three samples with spectrally distinct fluorescent dyes, Cy2, Cy3 and Cy5, mixes them, and runs them on the same gel. Because the samples share one gel, gel-to-gel variation is removed from the comparison, and a pooled internal standard labeled with Cy2 on every gel allows accurate quantitation across a whole experiment. DIGE costs more per gel but needs fewer gels for the same statistical power. Whether it is worth the extra cost depends on the experiment, and we compare the two approaches in a separate article.

2D-DIGE vs 2D gel electrophoresis

Applications of 2D gel electrophoresis

Differential expression proteomics. Comparing spot intensities between conditions, treated and untreated, healthy and diseased, wild type and mutant, to find proteins whose abundance changes. This is still the largest use of the technique.

Biomarker discovery. Screening body fluids or tissue for proteins that mark a disease or a response to treatment, in clinical, veterinary and agricultural research.

Isoforms and post-translational modifications. Phosphorylation, glycosylation and other modifications change a protein’s charge or mass, so they move a spot. 2D gels show these as trains of spots in a way that peptide-based LC-MS/MS cannot, because 2D gels separate intact proteins.

Host cell protein (HCP) analysis. In biologics manufacturing, 2D gels and 2D Western blots are used to assess how well an anti-HCP antibody covers the host cell proteome, a regulatory requirement for ELISA validation. TotalLab’s SpotMap 2D is built for this assay.

SpotMap 2D HCP coverage software

Microbiology and plant science. Proteome maps of bacteria, fungi and plants under stress, infection or growth conditions, where the organism’s genome may be well enough known to identify spots by peptide mass fingerprinting.

Teaching. Because the whole workflow is visible on one gel, 2D electrophoresis is widely used in undergraduate and master’s proteomics practicals.

Advantages and limitations

Advantages

It separates intact proteins, so isoforms, modifications and degradation products appear as distinct spots rather than being lost at the peptide level. It resolves thousands of proteins in one run with no need for a labeling step or a mass spectrometer to see the result. Quantitation is relative and visual: a difference between two samples can be seen on the gel before any software is involved. And the equipment is inexpensive relative to LC-MS/MS, which is why the technique remains common in university and hospital laboratories.

Limitations

Very hydrophobic proteins, very large or very small proteins, and very acidic or basic proteins are under-represented, although IPG ranges and solubilization chemistry have narrowed these gaps. Low-abundance proteins are masked by abundant ones unless the sample is prefractionated. The method is labor-intensive, taking a day or two per gel set, and gel-to-gel variation means that replicate gels and careful image analysis are essential. A spot is not always a single protein, so identification by mass spectrometry is still needed to be sure.

Common problems and how to fix them

Horizontal streaking. Usually a first-dimension problem: too much salt, incomplete solubilization, or too much protein for the strip. Desalt the sample, check the urea and detergent, and reduce the load.

Vertical streaking. Usually a second-dimension problem: incomplete equilibration, too little SDS, or a poor contact between the strip and the slab gel. Run both equilibration steps for the full time and make sure the strip is sealed flat against the gel.

Few or faint spots. Underloading, an inappropriate stain for the amount of protein, or sample loss during preparation. Quantify before loading and match the stain to the load.

Missing high molecular weight proteins. Large proteins enter IPG strips poorly during passive rehydration. Try active rehydration at low voltage, or cup loading at the anodic end of the strip.

Poor reproducibility between gels. Some variation is inherent to the technique. Cast gels in batches, run replicates together, control temperature during the second dimension, and use analysis software that aligns gels before detecting spots so that the variation is corrected rather than propagated into the results.

Image problems. Saturated spots cannot be quantified, and 8-bit images lose dynamic range. Capture at 16 bits, below saturation, at consistent resolution. Image quality control in SameSpots checks each image for these problems on import.

Frequently asked questions

Q: What is 2D gel electrophoresis?
A: A technique that separates proteins by isoelectric point in a first dimension (isoelectric focusing) and by molecular weight in a second dimension (SDS-PAGE), producing a map of spots in which each spot is typically a single protein or isoform.

Q: What is the difference between 1D and 2D gel electrophoresis?
A: 1D SDS-PAGE separates proteins by molecular weight only and produces bands in a lane. 2D gel electrophoresis adds a first separation by isoelectric point and produces spots on a map, resolving hundreds to thousands of proteins instead of tens and distinguishing isoforms and modifications that co-migrate in 1D.

Q: What is the principle of 2D gel electrophoresis?
A: Separation by two independent physical properties. Because a protein’s charge (pI) and its size (molecular weight) are unrelated, two proteins that share one property are almost always separated by the other.

Q: What are the steps of 2D gel electrophoresis?
A: Sample preparation and solubilization, IPG strip rehydration, isoelectric focusing, equilibration in SDS buffer with reduction and alkylation, SDS-PAGE, staining and imaging, image analysis, and finally spot picking and identification by mass spectrometry.

Q: What is isoelectric focusing?
A: The first dimension of 2D electrophoresis. Proteins migrate through an immobilized pH gradient until they reach the pH equal to their isoelectric point, where they have no net charge and stop, focusing into sharp bands.

Q: How many proteins can a 2D gel resolve?
A: More than 5,000 on a large-format gel with a suitable pH gradient, and around 2,000 routinely. Detection sensitivity is below 1 ng of protein per spot with sensitive stains.

Q: How long does 2D gel electrophoresis take?
A: Typically one to two days: overnight strip rehydration, several hours of isoelectric focusing, equilibration, a second-dimension run of under an hour for a mini gel to around five hours for a large gel, then staining. Image analysis with SameSpots then takes under five minutes per gel.

Q: What is 2D-PAGE used for?
A: Comparing protein expression between samples, discovering biomarkers, studying isoforms and post-translational modifications, assessing host cell protein antibody coverage in biologics manufacturing, and mapping proteomes in microbiology and plant science.

Q: What are the disadvantages of 2D gel electrophoresis?
A: It is labor-intensive, under-represents very hydrophobic, very large, very small and extreme-pI proteins, masks low-abundance proteins behind abundant ones, and shows gel-to-gel variation that must be corrected during image analysis.

Q: What is the difference between 2D gel electrophoresis and 2D-DIGE?
A: 2D-DIGE is 2D gel electrophoresis with samples pre-labeled by fluorescent dyes (Cy2, Cy3, Cy5) and run together on one gel, which removes gel-to-gel variation from the comparison. The separation chemistry is the same.

Q: Which software is used to analyze 2D gels?
A: Dedicated 2D gel analysis software that aligns images, detects and quantifies spots and runs statistics. SameSpots from TotalLab is one; Melanie is another. PDQuest, DeCyder and Delta2D were widely used; PDQuest is discontinued and DeCyder is no longer developed.

References

1. Görg A, Weiss W, Dunn MJ. Current two-dimensional electrophoresis technology for proteomics. Proteomics. 2004;4(12):3665-85. https://doi.org/10.1002/pmic.200401031
(Source for: resolving more than 5,000 proteins, ~2,000 routinely; detection below 1 ng per spot; IPG range pH 2.5 to 12; narrow IPG resolution of 0.001 pH units; the 2-DE/MS workflow; the contrast with peptide-based LC-MS/MS.)

2. O’Farrell PH. High resolution two-dimensional electrophoresis of proteins. J Biol Chem. 1975;250(10):4007-21.
(Source for: the origin of the technique.)

3. Bio-Rad Laboratories. ReadyPrep 2-D Starter Kit Instruction Manual, Bulletin 4110009A. https://www.bio-rad.com/webroot/web/pdf/lsr/literature/4110009A.pdf
(Source for: rehydration buffer composition; rehydration volumes by strip length; passive rehydration time; IEF voltage programs; equilibration buffer compositions and times; 8-16% gradient gels; SDS-PAGE run conditions.)

4. Thermo Fisher Scientific. 2D Gel Electrophoresis. https://www.thermofisher.com/us/en/home/life-science/protein-biology/protein-gel-electrophoresis/protein-gels/specialized-protein-gels/2d-gel-electrophoresis.html
(Source for: protein loading amounts by strip range and stain; IPG strip pH range options; urea/thiourea and DTT concentrations; staining times.)

Analyze your 2D gels with SameSpots

Once the gels are run, the hard part is comparing them. SameSpots aligns every image in your experiment, detects one spot pattern across all of them so there are no missing values, and runs the statistics in the same window. A typical gel takes under five minutes. Bio-Rad named TotalLab its preferred alternative vendor when it discontinued PDQuest.