Protein Isolation
Protein isolation separates proteins from complex biological mixtures using electrophoresis and chromatography techniques.
Before a protein can be studied, it has to be separated out of a complex biological mixture. Two major technique families accomplish this: electrophoresis, which separates proteins using an electric field, and chromatography, which separates compounds based on differential interaction between two phases.
Key Takeaways
Electrophoresis separates proteins by charge, size, and shape under an electric field: v = Ez/f.
Native PAGE preserves function; SDS-PAGE denatures and separates purely by size; isoelectric focusing separates by pI, where a protein has zero net charge and stops migrating.
All chromatography relies on partitioning between a stationary phase and a mobile phase.
TLC/paper chromatography separate by polarity and use the Rf value (distance moved by spot ÷ distance moved by solvent) to compare compounds.
Column chromatography includes ion-exchange (charge), size-exclusion (size), and affinity (specific binding) methods.
GC separates vaporized compounds by retention time; HPLC is a computer-controlled, high-precision version of column chromatography.
Electrophoresis
Electrophoresis uses a gel matrix to observe the migration of proteins under an applied electric field. Proteins carry charge, so when a voltage is applied across the gel, each protein migrates toward the electrode of opposite charge. The gel itself acts like a molecular sieve.
Migration speed depends on three factors, captured in the relationship:
v = Ez / f
v — velocity (speed of migration)
E — electric field strength
z — net charge of the protein
f — frictional coefficient (depends on the protein's size and shape)
The most common gel is polyacrylamide gel, a dense, crosslinked matrix that separates proteins by size and charge. There are three key electrophoresis variants tested on the MCAT.
Native PAGE
Native PAGE (native polyacrylamide gel electrophoresis) keeps the protein in its native, folded state — charge and shape are both preserved. Because different proteins have different mass-to-charge ratios, results can be harder to interpret, but this method is valuable for studying protein function or interactions in their natural form.
SDS-PAGE
SDS-PAGE denatures the protein and coats it in a negatively charged detergent, SDS. This masks the protein's native charge and eliminates shape differences, so separation is based purely on size — smaller proteins move faster, larger ones move more slowly. Because the protein is denatured, its original functional form cannot be recovered afterward.
Isoelectric Focusing
Isoelectric focusing separates proteins by their isoelectric point (pI) — the pH at which a protein carries no net charge. The protein sits in a gel with a pH gradient and an electric field, migrating until it reaches the point in the gel where the pH equals its pI. At that point, it stops moving, since it has no net charge left to drive migration. To make a protein move at all during electrophoresis, the surrounding pH must differ from its pI.
Method | Separates By | Native Function Preserved? | Key Use |
|---|---|---|---|
Native PAGE | Size and native charge/shape | Yes | Studying protein function or interactions |
SDS-PAGE | Size only | No (denatured) | Comparing protein sizes |
Isoelectric focusing | Isoelectric point (pI) | Varies | Identifying a protein's pI |
Chromatography: The General Principle
Chromatography separates and identifies compounds based on their physical and chemical properties, and it's especially useful for complex mixtures. Every chromatography technique relies on two phases:
The stationary phase — typically a polar solid that stays in place.
The mobile phase — a liquid or gas that moves through the stationary phase, carrying the sample with it.
As the mobile phase flows through the stationary phase, each compound interacts with both phases to a different degree. A compound with greater affinity for the stationary phase sticks to it more and takes longer to travel; a compound with more affinity for the mobile phase moves faster and elutes earlier. This differential movement — called partitioning — is what separates the mixture.
Thin Layer and Paper Chromatography
Thin layer chromatography (TLC) and paper chromatography are simple, visual methods for separating small amounts of compound, using the same stationary/mobile phase principle above.
In TLC, the stationary phase is a thin layer of silica gel or alumina attached to an inert plate (glass or plastic).
In paper chromatography, the stationary phase is simply paper (cellulose).
In both, the stationary phase is polar and the mobile phase is a nonpolar solvent that moves by capillary action.
The process: a small dot of sample is applied near the bottom of the plate or paper (spotting), then the bottom edge is placed into solvent. As the solvent rises, it carries the sample's components with it — more nonpolar compounds travel farther (they interact more with the mobile phase), while more polar compounds stick to the stationary phase and move less.
Once the solvent front has moved far enough, the distance each spot traveled is measured and used to calculate the Rf value (retardation factor):
Rf = distance moved by the spot ÷ distance moved by the solvent front
Rf values let researchers compare and identify unknown compounds based on how far they travel in a given solvent.
Column Chromatography
Column chromatography uses a vertical column filled with silica or aluminum beads as the stationary phase, providing a large surface area for compounds to interact with. The mobile phase — typically a nonpolar solvent — flows through by gravity or pressure. Compounds that interact more with the stationary phase move slower; compounds that interact less elute faster.
Type | Mechanism |
|---|---|
Ion-exchange chromatography | Beads coated with charged substances bind compounds of the opposite charge |
Size-exclusion chromatography | Beads have small pores; small molecules enter the pores and are slowed down, while large molecules bypass the pores and move through faster |
Affinity chromatography | Beads are coated with a receptor, enzyme, or antibody that specifically binds the target compound; everything else washes out, and the target is released later with an elution solution |
Gas Chromatography and HPLC
Gas chromatography (GC) separates vaporized compounds based on how strongly each adheres to the column's absorbent. The stationary phase is a coil (usually crushed metal or a polymer) inside the column; the mobile phase is a nonreactive inert gas, like helium or nitrogen. The sample is injected, vaporized, and carried through the column by the gas — compounds that adhere strongly take longer to pass through, while those that interact less elute first. The output is a graph of peaks corresponding to each component's retention time.
High-Performance Liquid Chromatography (HPLC) works similarly to standard column chromatography but is computer-controlled and far more precise, using carefully controlled solvent mixtures and temperature gradients for better separation and detection. HPLC is especially valuable when the sample size is small or when subtle physical forces (like capillary action) could otherwise interfere with results — making it a preferred method when accuracy and reproducibility matter most.
Method | Stationary Phase | Mobile Phase | Separates By |
|---|---|---|---|
TLC | Silica gel or alumina on a plate | Nonpolar solvent (capillary action) | Polarity |
Paper chromatography | Cellulose paper | Nonpolar solvent (capillary action) | Polarity |
Ion-exchange | Charged beads | Solvent (gravity/pressure) | Charge |
Size-exclusion | Porous beads | Solvent (gravity/pressure) | Size |
Affinity | Beads coated with a specific binding partner | Solvent, then elution solution | Specific binding affinity |
Gas chromatography | Coil (metal or polymer) | Inert gas (helium/nitrogen) | Volatility / adherence to stationary phase |
HPLC | Column packing (as in standard column chromatography) | Solvent, computer-controlled gradients | Same principle as column chromatography, with greater precision |
Common MCAT Mistakes
Confusing Native PAGE and SDS-PAGE. Native PAGE separates by both size and charge/shape, and preserves the protein's function; SDS-PAGE denatures the protein and coats it with a uniform negative charge, so separation is by size alone.
Thinking isoelectric focusing separates by size. It separates by pI — the protein stops moving at the pH point where it carries zero net charge, regardless of its size.
Mixing up stationary and mobile phase roles. The stationary phase stays fixed and interacts with the sample; the mobile phase moves through it and carries the sample along. A compound that clings to the stationary phase moves slower, not faster.
Misreading the Rf value. Rf is the ratio of distance traveled by the spot to distance traveled by the solvent front — a highly polar compound (which sticks to a polar stationary phase) has a lower Rf, not a higher one.
MCAT-Style Concept Check
Question: A researcher runs a protein sample using SDS-PAGE and separately using Native PAGE. In the SDS-PAGE gel, all proteins migrate strictly according to molecular size. In the Native PAGE gel, two proteins of identical molecular weight migrate to different positions. What best explains this difference?
A) SDS-PAGE preserves native protein charge, while Native PAGE removes it
B) Native PAGE separates by charge and shape in addition to size, since the protein remains folded and its native charge is retained
C) SDS-PAGE uses a pH gradient to separate proteins by isoelectric point
D) Native PAGE denatures proteins, eliminating differences in their native charge
Answer: B
Explanation: SDS coats every protein in a uniform negative charge and unfolds it, so SDS-PAGE separation depends on size alone — this is why all proteins in that gel migrate strictly by molecular weight. Native PAGE keeps the protein folded, so its natural shape and charge are preserved; two proteins with the same molecular weight can still differ in native charge or shape, causing them to migrate to different positions. Option A reverses which method preserves charge. Option C describes isoelectric focusing, not SDS-PAGE. Option D reverses which method denatures the protein.
FAQ
What is the difference between Native PAGE and SDS-PAGE?
Native PAGE keeps the protein folded, so separation depends on the protein's native size, charge, and shape together. SDS-PAGE denatures the protein and coats it in a uniform negative charge, so separation depends on size alone.
What does the isoelectric point (pI) mean for a protein's movement in a gel?
The pI is the pH at which the protein carries no net charge. In isoelectric focusing, the protein migrates through a pH gradient until it reaches the point where the surrounding pH equals its pI, at which point it stops moving.
What is partitioning in chromatography?
Partitioning is the differential interaction of a compound between the stationary phase and the mobile phase. A compound that favors the stationary phase moves slower and elutes later; a compound that favors the mobile phase moves faster and elutes earlier.
How is the Rf value calculated in TLC or paper chromatography?
Rf equals the distance the sample spot traveled divided by the distance the solvent front traveled. It lets researchers compare and identify compounds based on how far each one moves in a given solvent.