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Chromatography
Methods

Master the three essential chromatographic techniques: Ion Exchange, Size Exclusion, and Affinity Chromatography to separate and purify biomolecules.

Biochemistry

What Is Chromatography?

Chromatography is a laboratory technique for separating mixtures into individual components. A sample dissolved in a mobile phase passes through a stationary phase, and different molecules interact differently with the stationary phase, causing them to migrate at different rates.

Mobile Phase vs. Stationary Phase

Mobile Phase
The liquid (buffer) that carries the sample through the column. Its composition (pH, salt concentration) can be adjusted to control separation.
Stationary Phase
The solid matrix (beads/resin) packed inside the column. Different resins exploit different molecular properties to achieve separation.

Why Chromatography Matters

Protein Purification
Isolate a single protein from thousands in a cell lysate for structural or functional studies.
Drug Development
Pharmaceutical companies purify therapeutic antibodies and recombinant proteins at industrial scale.
Clinical Diagnostics
Detect biomarkers, separate hemoglobin variants, and analyze metabolites in patient samples.

Three Core Methods

Ion Exchange - - - + - Separates by CHARGE Size Exclusion Separates by SIZE Affinity Separates by BINDING SPECIFICITY

Ion Exchange Chromatography

Ion exchange separates molecules based on their net charge. The stationary phase carries fixed charges that attract oppositely charged proteins. Bound proteins are eluted by increasing salt concentration or changing pH.

Cation Exchange
Resin has negative charges (e.g., \(-\text{SO}_3^-\)). Binds positively charged proteins. Use when target protein has \(\text{pI} > \text{pH}\) of buffer.
Anion Exchange
Resin has positive charges (e.g., \(-\text{N}^+(\text{CH}_3)_3\)). Binds negatively charged proteins. Use when target protein has \(\text{pI} < \text{pH}\) of buffer.

Elution in Ion Exchange

Absorbance Elution Volume [NaCl] Protein A (weakly bound) Protein B (moderate) Protein C (strongly bound)

Size Exclusion Chromatography

Also called gel filtration, this technique separates proteins by molecular size (hydrodynamic radius). The column is packed with porous beads: small proteins enter the pores and are delayed, while large proteins are excluded and elute first.

Elution by Molecular Weight

Absorbance Elution Volume \(V_0\) \(V_t\) Large Medium Small

Affinity Chromatography

Affinity chromatography exploits the specific biological interaction between a target molecule and a ligand immobilized on the resin. It is the most selective purification method, often achieving >1000-fold enrichment in a single step.

Ligand
A molecule covalently attached to the resin that specifically recognizes the target: antibodies, substrates, metal ions (\(\text{Ni}^{2+}\)), or cofactors.
His-Tag System
A polyhistidine tag (\(6\times\text{His}\)) fused to a recombinant protein binds \(\text{Ni}^{2+}\)-NTA resin. Eluted with imidazole, which competes for binding.

Bind, Wash, Elute

1. Bind Load sample 2. Wash Remove impurities 3. Elute Collect pure target

Key Distinction: Separation Basis

Ion Exchange Size Exclusion Affinity
Separates by Net charge Molecular size Specific binding
Binding? Yes (electrostatic) No Yes (biological)
Elution Salt gradient or pH Continuous flow Competing ligand
Selectivity Moderate Low Very high
Resolution High Moderate Highest

Designing a Purification Strategy

1
Know your target. Determine the protein's \(\text{pI}\), molecular weight, and known binding partners.
2
Start with capture. Use affinity or ion exchange to capture the target from crude lysate with high selectivity.
3
Intermediate polish. Apply a second orthogonal method (different separation basis) to remove remaining contaminants.
4
Final polish with size exclusion. Use gel filtration as the last step to remove aggregates and exchange into the desired buffer.
5
Verify purity. Run SDS-PAGE to check for a single band at the expected molecular weight.

Example 1: Choosing Ion Exchange Type

Problem: You need to purify a protein with \(\text{pI} = 5.2\) from a bacterial lysate. Your buffer is at \(\text{pH} = 7.5\). Which type of ion exchange should you use?

Example 2: Elution Order Prediction

Problem: You load a mixture of three proteins onto a size exclusion column: Albumin (\(M_r = 67\text{ kDa}\)), Lysozyme (\(M_r = 14\text{ kDa}\)), and Thyroglobulin (\(M_r = 670\text{ kDa}\)). Predict the elution order.

1
Thyroglobulin (\(670\text{ kDa}\)): too large for pores → excluded → elutes first
2
Albumin (\(67\text{ kDa}\)): partially enters pores → delayed → elutes second
3
Lysozyme (\(14\text{ kDa}\)): enters most pores → most delayed → elutes last

Example 3: Multi-Step Purification

Problem: You have expressed a His-tagged recombinant protein in E. coli. Design a three-step purification protocol.

1
Capture: \(\text{Ni}^{2+}\)-NTA Affinity → The His-tag binds the \(\text{Ni}^{2+}\) resin with high selectivity. Wash away >90% of contaminants. Elute with 250 mM imidazole.
2
Intermediate: Ion Exchange → Dialyze the eluate into a low-salt buffer. Load onto an appropriate exchanger based on \(\text{pI}\). Elute with a NaCl gradient.
3
Polish: Size Exclusion → Run on a gel filtration column to remove aggregates, confirm monodispersity, and exchange into storage buffer.

Common Mistakes

✕ Wrong Exchanger
Using cation exchange when the protein is negatively charged (or vice versa). Always compare \(\text{pI}\) to buffer \(\text{pH}\) first.
✕ Reversed Elution Order
Thinking small molecules elute first in size exclusion. Remember: large proteins are excluded from pores and exit the column first.
✕ Overloading the Column
Applying too much sample reduces resolution for all three methods. Follow the manufacturer's recommended binding capacity.

Summary Comparison

Ion Exchange Size Exclusion Affinity
Property used Net charge Size / shape Specific interaction
Key parameter \(\text{pI}\) vs. \(\text{pH}\) Molecular weight (\(M_r\)) Ligand specificity
Elution trigger Salt or pH change None (isocratic) Competing ligand
Typical yield ~5-20x enrichment ~2-5x enrichment ~100-1000x enrichment
Best used for Capture or polish Final polish / desalting Capture (tagged proteins)

Question 1 of 5

A protein with \(\text{pI} = 8.3\) is loaded onto an ion exchange column at \(\text{pH} = 6.0\). Which resin should be used?

Question 2 of 5

In size exclusion chromatography, which protein elutes first?

Question 3 of 5

A researcher purifies a His-tagged protein using \(\text{Ni}^{2+}\)-NTA affinity chromatography. What is used to elute the bound protein?

Question 4 of 5

Which chromatography method does not involve binding of the target molecule to the stationary phase?

Question 5 of 5

In a typical multi-step protein purification, which method is best suited as the final polishing step?