Question 1

How does size-exclusion chromatography separate molecules in a mixture?

Accepted Answer

Size-exclusion chromatography separates molecules based on their size using a column packed with porous material. Large molecules cannot enter the pores and exit first, while small molecules enter the pores and exit last. Only molecules between the exclusion and permeation limits are separated from one another as they spend varying amounts of time diffusing in and out of the pores.

Question 2

What are the three molecular weight categories in size-exclusion chromatography?

Accepted Answer

Molecules fall into three categories: those above the exclusion limit that cannot enter pores and exit together first; those below the permeation limit that freely enter pores and exit together last; and those between these limits that separate based on varying retention times. Molecular weights across one to two orders of magnitude typically fall within the separation range.

Question 3

How does affinity chromatography use ligands to purify target proteins?

Accepted Answer

Affinity chromatography attaches target-specific ligands to the stationary phase. When a mixture flows through the column, target molecules selectively bind to the ligands while other components pass through. The target protein is then collected through elution methods that either compete with the ligand or change solution conditions like pH or ionic strength.

Question 4

What is a polyhistidine tag and how is it used in immobilized metal affinity chromatography?

Accepted Answer

A polyhistidine tag is a short peptide sequence of repeating histidine residues engineered onto target proteins. In immobilized metal affinity chromatography, these tags bind to adhered metal ligands like nickel or cobalt via the imidazole side chain on histidine. The tagged protein is then released using free imidazole via reverse-role specific elution.

Question 5

What steps are involved in the immobilized metal affinity chromatography procedure?

Accepted Answer

The IMAC procedure involves adding the stationary phase directly to the sample mixture to allow his-tagged protein binding. The slurry is poured into a column where unbound compounds drain to waste. After washing to remove unbound components, imidazole buffer is added to release the target protein by competing with the metal ligand.

Question 6

Why might researchers combine size-exclusion and affinity chromatography in protein purification?

Accepted Answer

Researchers combine these techniques to achieve higher purity levels. After affinity chromatography isolates the target protein using ligand binding, size-exclusion chromatography can further purify the sample by separating based on molecular size. This sequential approach is particularly useful when protein tags like polyhistidine may affect protein structure or function.

Question 7

How is gel electrophoresis used to evaluate fractions collected from size-exclusion chromatography?

Accepted Answer

After fractions are collected from the size-exclusion column, gel electrophoresis and other analytical techniques are used to test the quality and identity of the target molecule in each fraction. This verification step ensures that the desired biomolecule has been successfully isolated and separated from other components in the mixture.

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