Protein Purification & Chromatography — Full Educational Summary

Page 1 — Big Picture: What is protein purification? 🧪

This opening slide gives you the full workflow.

The figure on the left shows:

Preparation → Capture → Intermediate purification → Polishing

This is the standard purification pipeline.

Purity increases step by step.

Think of it like filtering sand from water:

• first remove rocks
• then remove smaller particles
• finally polish until crystal clear

For proteins, the same logic applies.

You begin with a complex biological mixture:

• cell lysate
• serum
• tissue extract
• bacterial culture

containing:

• your target protein
• thousands of other proteins
• DNA/RNA
• lipids
• salts
• debris

The SDS-PAGE image at the bottom shows this beautifully:

• many bands = crude mixture
• fewer bands = purification progressing
• one band = high purity protein

This is one of the most important visual concepts in purification.

Page 2 — Purification strategy 🧠

This page is extremely important.

Before purifying anything, you must ask:

What is the protein used for?

This determines required purity.

Examples:

Therapeutic use

Needs extremely high purity because contaminants can be dangerous

Often >99%

Structural biology

For:

• X-ray crystallography
• cryo-EM
• NMR

Needs very high purity and homogeneity

Usually 95–99%

Activity assay

Sometimes moderate purity is enough if contaminants do not interfere

Important strategy rules

1. Keep steps minimal

Each step loses protein.

Even 90% yield per step becomes:

• 1 step → 90%
• 2 steps → 81%
• 5 steps → 59%
• 8 steps → 43%

The graph on the slide shows this exponential loss.

This is a key concept.

Many purification failures happen because too many steps are used.

2. Use orthogonal methods

This means each step should separate by different properties.

For example:

• charge → ion exchange
• size → size exclusion
• binding specificity → affinity

This greatly improves purity.

3. Avoid dilution

Dilution can:

• destabilize proteins
• reduce concentration
• increase losses

Very important in lab work.

Page 3 — The CIPP strategy ⭐

This is one of the most important concepts.

C = Capture

Goal:

• isolate
• concentrate
• stabilize

This is your first major purification step.

Example:

His-tag affinity column

This quickly pulls your protein out of a complex lysate.

I = Intermediate purification

Remove most contaminants.

This is often:

• ion exchange
• HIC

P = Polishing

Final cleanup.

Removes:

• aggregates
• closely related contaminants
• oligomeric species

Often done with:

• size exclusion chromatography (SEC)

This gives highly pure protein.

Page 4 — What must you know before purification? 🔬

This slide is very practical.

Required purity

Depends on experiment.

Very important.

Examples from slide:

• therapeutic → >99%
• crystallography → 95–99%
• antibody antigen → <95% can be enough

Analytical methods

You must monitor purification continuously.

SDS-PAGE

Fastest and most common

Shows:

• molecular weight
• approximate purity

MS

Mass spectrometry confirms exact identity

Activity assay

Sometimes purity alone is not enough.

Protein must still be functional.

This is especially important for enzymes.

Stability properties

This is critical.

You must know whether protein tolerates:

• temperature
• pH
• salt
• detergent
• redox conditions

This directly affects buffer design.

Page 5 — Sample extraction and clarification 🧫

Before chromatography, the sample must be clean.

The goal:

remove anything that can clog the column

Examples:

• cell debris
• membranes
• precipitates
• DNA clumps

Methods

Centrifugation

Spins debris down.

Supernatant contains soluble proteins.

Filtration

Removes particles.

Often:

0.22 µm filter

Very common before FPLC.

Precipitation

This is important.

Protein solubility can be manipulated.

Common agents:

• ammonium sulfate
• PEG

The SDS-PAGE image shows selective enrichment.

Bands become fewer after precipitation.

This means some proteins precipitated while others remained soluble.

Very important pre-purification step.

Page 6 — Ammonium sulfate precipitation 🧂

Very exam-relevant.

This table tells how many grams of salt to add.

Example from slide:

To reach 40% saturation

Add 243 g/L

Then from 40% to 70%

Add 205 g/L

This is called fractional precipitation.

Extremely useful.

Different proteins precipitate at different salt concentrations.

This is based on salting out.

Salt removes water from protein surfaces, reducing solubility.

Page 7–13 — Introduction to chromatography 🚰

This section explains the general principle.

Core principle

Chromatography separates molecules by repeated partitioning between:

• stationary phase = matrix
• mobile phase = buffer

Proteins interact differently with the matrix.

Some move faster.

Some slower.

This creates separation.

Instrument setup

The system images show:

• solvent reservoirs
• pumps
• injector
• column
• detector
• fraction collector

This is exactly what you see in an FPLC/HPLC system.

The detector commonly measures:

A280

Protein absorbance due to:

• tryptophan
• tyrosine

Chromatographic peaks

Very important concept.

Each peak corresponds to a separated protein species.

The peak position tells:

when it eluted

The peak area tells:

how much protein

Usually Gaussian-shaped.

This is important for fraction collection.

Affinity Chromatography (Pages 14–32) 🎯

This is probably the most important purification method.

Principle

Uses specific biological recognition.

Example:

protein binds ligand specifically

Examples:

• enzyme ↔ cofactor
• antibody ↔ antigen
• His-tag ↔ Ni²⁺
• GST ↔ glutathione

How it works

1. ligand immobilized on beads
2. target binds
3. impurities wash away
4. target specifically eluted

This gives extremely high purity in one step.

Binding equation

Very important:

heta = rac{L}{K_D + L}

Where:

• θ = saturation fraction
• KD = dissociation constant
• L = ligand concentration
  heta = rac{L}{K_D + L}

Lower KD = stronger binding

This is exactly the same concept you’ve worked with before in binding curves.

Leakage concept

Pages 21–22 explain something very important.

If KD is too high:

protein leaks off during washing

For efficient purification:

binding KD must be low during loading

but high during elution

This often requires 1000-fold change

Very important concept.

Elution methods

Nonspecific elution

Change:

• pH
• salt
• chaotrope

This weakens binding

Specific elution

Add competing ligand

Example:

glucose competes off glucose-binding protein

This is usually gentler.

His-tag purification ⭐

This is extremely important.

Nickel / Ni-NTA purification.

His residues coordinate nickel.

This is immobilized metal affinity chromatography (IMAC).

Common for recombinant proteins.

Elution

Usually with:

imidazole

because it competes with histidine

This is likely one of the most useful methods you’ll use in protein biochemistry.

Ion Exchange Chromatography (Pages 33–52) ⚡

Very important.

Separates proteins by net charge.

Basic principle

Opposite charges attract.

Protein charge depends on pH relative to pI.

Above pI

protein is negative

binds anion exchanger

Below pI

protein is positive

binds cation exchanger

This is very important.

Elution

Usually by increasing salt.

Example:

NaCl gradient

Salt ions compete with protein for binding sites.

This causes elution.

The stronger the protein binds, the higher salt needed.

Gradient elution

The slides show:

• linear gradient
• complex gradient
• step gradient

This is extremely common in FPLC.

A salt gradient allows fine separation.

pH effect

pH changes protein charge.

Therefore pH strongly affects retention.

This is why buffer choice is critical.

Hydrophobic Interaction Chromatography (Pages 61–67) 💧

This is often confusing but very important.

Proteins contain hydrophobic surface patches.

These bind hydrophobic matrices.

High salt promotes binding

This is opposite of ion exchange.

High salt strengthens hydrophobic interaction.

Why?

Because salt strips water away from hydrophobic surfaces.

This exposes hydrophobic patches.

Then they bind matrix.

Elution

Decrease salt concentration.

Less hydrophobic interaction → protein elutes.

Hofmeister series

Very important concept.

Some salts are better at salting out proteins.

Especially:

(NH_4)_2SO_4

This is why ammonium sulfate is used so much.

Size Exclusion Chromatography (Pages 68–84) 📏

This is one of the easiest methods conceptually.

Separates by size.

Principle

Porous beads contain holes.

Small proteins enter pores.

Large proteins cannot.

Result

Large proteins

elute first

Small proteins

elute later

This is extremely important.

Students often initially think the opposite.

Hydrodynamic size

Important detail:

separation depends on shape + size, not molecular weight alone

This is why unfolded proteins appear larger.

The later slides explicitly show this.

Very important for interpreting SEC.

Denatured proteins

Pages 81–84 show denatured proteins appearing larger.

This is because unfolded proteins behave like expanded coils.

This is a key biophysical concept.

Very relevant to your background.

Final pages — Strategy + exercises ✨

The lecture ends by bringing everything together.

The key idea is:

choose purification method based on protein property

This is the central takeaway.

High-yield exam summary 🎯

Remember this table: