BackProtein Purification: Size Exclusion and Ion-Exchange Chromatography
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Protein Purification Techniques
Size Exclusion Chromatography (SEC)
Size Exclusion Chromatography is a method used to separate proteins based on their size. It is also known as gel filtration chromatography. This technique is widely used in biochemistry for protein purification and analysis.
Principle: Larger proteins elute (exit) from the column faster than smaller proteins because they cannot enter the pores of the stationary phase beads and thus take a shorter path through the column.
Stationary Phase: Consists of porous beads. Small proteins enter the pores and are delayed, while large proteins are excluded and elute earlier.
Application: Used to estimate the molecular weight of proteins and to separate protein mixtures.
Example: In a mixture of proteins, the order of elution in SEC will be from largest to smallest molecular weight.
Protein | Molecular Weight (Da) |
|---|---|
Cytochrome C | 12,000 |
Immunoglobulin G | 145,000 |
Ribonuclease A | 13,700 |
Serum Albumin | 68,500 |
Order of Elution: Immunoglobulin G > Serum Albumin > Ribonuclease A > Cytochrome C
Chromatogram: The chromatogram shows peaks corresponding to different proteins as they elute from the column over time.
Key Points and Misconceptions
SEC separates proteins by size, not charge.
It does not require proteins to be denatured; native proteins can be separated using a salt solution.
SEC is not suitable for separating proteins with similar molecular sizes.
Ion-exchange chromatography separates proteins based on charge, not size.
Application: Determining Protein Structure
SEC can be used to estimate the quaternary structure of proteins by comparing elution volumes in native and denaturing conditions.
Example: A protein with a native molecular weight of 240 kDa elutes as a single peak. In the presence of a denaturant (e.g., 6 M guanidine hydrochloride), the protein elutes as two peaks corresponding to subunits of 60 kDa and 31 kDa, indicating a heteromeric structure.
Ion-Exchange Chromatography
Ion-exchange chromatography separates proteins based on their net charge at a given pH.
Anion-exchange chromatography: Binds negatively charged proteins.
Cation-exchange chromatography: Binds positively charged proteins.
The order of elution depends on the protein's isoelectric point (pI) and the pH of the buffer.
Protein | pI |
|---|---|
A | 4.5 |
B | 5.2 |
C | 5.8 |
D | 6.4 |
E | 7.0 |
Order of Elution (Anion-Exchange at pH 6.0): E > D > C > B > A (from least negative to most negative)
Formulas and Equations
Elution Volume (Ve): The volume at which a protein elutes from the column is related to its size.
Isoelectric Point (pI): The pH at which a protein has no net charge.
Summary Table: Comparison of Chromatography Methods
Method | Basis of Separation | Application |
|---|---|---|
Size Exclusion | Size | Protein purification, molecular weight estimation |
Ion-Exchange | Charge | Protein purification, separation of isoforms |
Additional info:
SEC is often used as a final polishing step in protein purification due to its gentle separation conditions.
Ion-exchange chromatography can be optimized by adjusting buffer pH and ionic strength.
