How to Separate BSA from Lysozyme

Property Comparison

BSA (Target)

Molecular Weight66,500 Da

TypeSerum Albumin

Isoelectric Point4.7

Net Charge (pH 7)−18

Stokes Radius3.5 nm

Diffusion Coeff.5.9 × 10^-7 cm²/s

Solubility~40 g/L (crystalline)

vs

Lysozyme (Impurity)

Molecular Weight14,300 Da

TypeGlycoside Hydrolase

Isoelectric Point11.35

Net Charge (pH 7)+8

Stokes Radius1.9 nm

Diffusion Coeff.1.04 × 10^-6 cm²/s

Solubility~10 g/L (crystalline)

Why This Separation Works

BSA and lysozyme have a pI difference of 6.65 units—one of the largest among common proteins. At pH 7.5, they carry opposite charges:

Component	Charge at pH 7.5	AEX Binding (Q resin)	Goes To
BSA	−18 (strongly negative)	Binds tightly	Eluate (product)
Lysozyme	+8 (positive)	No binding (repelled)	Flow-through

Selectivity is essentially binary—BSA binds while lysozyme is completely excluded from the anion exchange resin at pH 7.5. This is a textbook separation used in biochemistry teaching labs.

Recommended Process Route

1

Buffer Exchange — Desalting

Exchange sample into 20 mM Tris-HCl, pH 7.5 with <50 mM NaCl. High ionic strength reduces protein binding to ion exchange resin. Use HiPrep desalting column or dialysis cassette.

Feed conditioning

2

Anion Exchange — Q Sepharose

Load onto Q Sepharose Fast Flow at pH 7.5. BSA (pI 4.7) carries net charge −18 and binds strongly. Lysozyme (pI 11.35) carries net charge +8 and flows straight through. Capacity: 100–120 mg BSA per mL resin.

Key separation step

3

NaCl Gradient Elution

Elute BSA with linear NaCl gradient (0–500 mM in 20 mM Tris pH 7.5). BSA elutes at approximately 200 mM NaCl as a single sharp peak. Pool fractions above >90% purity.

Product recovery

4

Optional: CEX for Lysozyme Co-Product

To recover pure lysozyme, load the AEX flow-through onto SP Sepharose at pH 7.5. Lysozyme binds (positive charge). Elute with 0–1 M NaCl gradient. Both proteins recovered in high purity.

Co-product recovery

Expected Results

>95%

BSA Yield

>98%

BSA Purity

3 steps

Total Process Length

This is one of the cleanest protein-protein separations. Single-step AEX routinely achieves >98% purity due to the opposite charges at physiological pH.

Alternative Techniques

Technique	Feasibility	Notes
Size Exclusion Chromatography	Good	4.6× MW difference (66.5 kDa vs 14.3 kDa). Clear separation on Sephacryl S-200. Lower throughput and resolution than IEX.
Ultrafiltration (30 kDa MWCO)	Moderate	BSA (66.5 kDa) retained, lysozyme (14.3 kDa) passes through. Works but ~70–80% purity due to concentration polarization and fouling. Needs diafiltration.
Cation Exchange (pH 7.5)	Good	Reverse approach: lysozyme binds (positive), BSA flows through (negative). Equally effective but less commonly taught.
Ammonium Sulfate Precipitation	Moderate	BSA precipitates at ~75% saturation, lysozyme at ~95%. Differential precipitation feasible but imprecise.

Frequently Asked Questions

Why pH 7.5 and not a different pH?

At pH 7.5, BSA (pI 4.7) carries strong negative charge (−18) while lysozyme (pI 11.35) carries strong positive charge (+8). This maximizes the charge difference. Any pH between 5 and 10 would work, but 7.5 is optimal for stability and binding capacity on AEX resins.

Can I separate these proteins by size alone?

Yes, the 4.6× MW difference (66.5 vs 14.3 kDa) is sufficient for SEC on Sephacryl S-200 or Superdex 75. However, SEC has lower throughput (1–5% column volume sample load) compared to IEX (10–50% capacity utilization). IEX is preferred for preparative scale.

Is this the same as the classic IEX teaching lab?

Yes. BSA/lysozyme separation on Q-Sepharose is one of the most common biochemistry undergraduate labs because the pI difference is so large that it works reliably even with minimal optimization. It demonstrates the fundamental principle of ion exchange chromatography.

Related Separation Guides

IgG from Host Cell Proteins Whey Protein from Lactose Insulin from Host Cell Proteins Lactic Acid from Glucose BSA Properties Ion Exchange Guide