SDS-PAGE is a laboratory technique that separates proteins by size. The full name, sodium dodecyl sulfate polyacrylamide gel electrophoresis, describes exactly what happens: proteins are coated with a detergent called SDS, then pulled through a gel by an electric current. Smaller proteins move through the gel faster, while larger ones lag behind, producing distinct bands that researchers can visualize and analyze.
How SDS-PAGE Works
Proteins in their natural state come in wildly different shapes, and many carry different electrical charges. That makes separating them purely by size a challenge, because shape and charge both influence how fast a molecule moves through a gel. SDS-PAGE solves this by stripping away those differences before separation begins.
SDS is a detergent that unfolds (denatures) proteins into long, rod-like chains and coats them with negative charges. Each protein picks up roughly 1.4 grams of SDS per gram of protein, which works out to about one SDS molecule for every two amino acids. This coating is so uniform that every protein ends up with nearly the same ratio of negative charge to mass. Once that’s true, the only thing left to determine how fast a protein moves through the gel is its size.
The gel itself is made of polyacrylamide, a mesh-like polymer with tiny pores. When an electric field is applied, all the negatively charged proteins migrate toward the positive electrode. Small proteins slip through the pores easily and travel farther. Large proteins get slowed down by the mesh and stay closer to the top. After the run is complete, proteins are arranged in order of their molecular mass, with the smallest at the bottom of the gel.
Sample Preparation
Before proteins ever enter the gel, they need to be prepared in a special mixture called Laemmli buffer. This buffer contains SDS to coat the proteins, glycerol to make the sample dense enough to sink into the loading wells, and a small amount of blue tracking dye so you can watch the progress of the run. The sample is also heated, typically to around 95°C for a few minutes, which helps the SDS fully unfold each protein.
Most protocols also include a reducing agent that breaks disulfide bonds, the chemical bridges that hold parts of a protein’s 3D structure together. Without breaking these bonds, some proteins would remain partially folded and migrate unpredictably. The result of this preparation step is a collection of fully unfolded, uniformly charged protein chains ready for separation.
The Two-Layer Gel System
A standard SDS-PAGE gel has two distinct layers, each with a different density and pH. The top layer is the stacking gel, a low-density gel (around 4% acrylamide, pH 6.8) with large pores. Its job is to compress all the proteins into a thin, concentrated band before they enter the separation zone. This is what gives SDS-PAGE its sharp, well-defined bands, even when a sample is dilute. The stacking gel needs to be at least twice the height of the sample in the well to work properly.
Below it sits the resolving gel, a denser gel (commonly 7.5% to 12% acrylamide, pH 8.8) with smaller pores. This is where the actual size-based separation happens. The percentage of acrylamide you choose depends on the proteins you’re trying to separate. A higher percentage creates a tighter mesh that’s better for resolving small proteins, while a lower percentage works better for large ones.
Estimating Molecular Weight
One of the most useful features of SDS-PAGE is its ability to estimate a protein’s molecular weight. The relationship is remarkably predictable: the distance a protein travels through the gel varies linearly with the logarithm of its molecular mass. In practical terms, if you plot migration distance against log(molecular weight) for a set of known proteins, you get a straight line. You can then use that line to estimate the weight of any unknown protein by measuring how far it traveled.
To make this work, researchers load a “ladder” of pre-made protein standards with known molecular weights alongside their samples. These standards produce a reference pattern of bands that lets you read off approximate sizes for every band in the gel. The technique is reliable across a range of gel densities, with studies showing strong linear correlations (R² values of 0.85 or higher) across different acrylamide concentrations.
Choosing a Buffer System
The standard buffer system for SDS-PAGE uses a combination of Tris and glycine. This works well for most proteins, generally those in the range of about 10 to 250 kilodaltons (kDa). For very small proteins or peptides below 10 kDa, a different buffer system using Tris and tricine is preferred. Tricine gels can resolve proteins as small as 2 kDa, which Tris-glycine gels simply cannot do.
Running the Gel
A typical SDS-PAGE run on a standard mini-gel takes anywhere from 1.5 to 5 hours, depending on the gel length and the voltage applied. Modified high-speed protocols can cut that time dramatically, with some finishing in under 20 minutes. Higher voltage speeds things up but generates more heat, which can distort the bands if not managed properly. Most labs run gels at a moderate, steady voltage and monitor the blue tracking dye to know when to stop.
Visualizing Protein Bands
After electrophoresis, the gel is essentially invisible. Proteins need to be stained to see where they ended up. The two most common methods offer very different levels of sensitivity.
Coomassie Brilliant Blue is the workhorse stain for routine work. It’s simple, inexpensive, and detects protein bands containing 0.3 to 1 microgram of protein. For most applications, like checking whether a protein purification worked, Coomassie staining is more than adequate.
Silver staining is far more sensitive, detecting as little as 2 to 5 nanograms of protein per band. That’s roughly 100 to 200 times more sensitive than Coomassie. It’s used when protein quantities are very low or when you need to detect faint contaminant bands that Coomassie would miss.
Common Uses in the Lab
The most frequent application of SDS-PAGE is checking protein purity. After a researcher purifies a protein through multiple steps, they run a sample on a gel to see whether it appears as a single clean band or whether contaminating proteins are still present. A single band at the expected molecular weight is the gold standard for purity.
SDS-PAGE also serves as a semi-quantitative tool. By comparing the intensity of an unknown protein band to standards of known concentration, researchers can estimate how much protein is present. This is done through densitometry, where software measures the darkness or area of each band and converts it to a quantity. Labs routinely use the protein standards in molecular weight ladders as built-in references for this kind of analysis.
Beyond purity and quantification, SDS-PAGE is the first step in Western blotting, a technique where proteins are transferred from the gel onto a membrane and then probed with antibodies to identify specific proteins. It’s also used in quality control for pharmaceutical proteins, in forensic analysis, and as a teaching tool in virtually every undergraduate biochemistry course.