Protein Solubility: Hofmeister, Salting In/Out
Protein solubility depends on ionic strength (Cohn: log S = β - Ks×I), Hofmeister series (SO₄²⁻ salting-out vs SCN⁻ salting-in), pH near pI, and temperature. Essential for purification, crystallization, and formulation.
Why This Chemistry Calculation Matters
Why: Protein solubility governs purification, crystallization, and formulation. Hofmeister ions and ionic strength strongly affect aggregation and precipitation.
How: Enter protein properties, ionic strength, salt type. Cohn equation: log(S/S₀) = β - Ks×I. Hofmeister coefficients modify salting-in/out behavior.
- ●SO₄²⁻ salting-out; SCN⁻ salting-in.
- ●pH near pI minimizes solubility.
- ●Used in ammonium sulfate and PEG precipitation.
Sample Examples
Enter Values
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Chemistry Facts
log(S/S₀) = β - Ks×I. Cohn equation for ionic strength.
— Biochemistry
Hofmeister: SO₄²⁻ > Cl⁻ > Br⁻ > NO₃⁻ > ClO₄⁻ > SCN⁻ (salting-out to salting-in).
— IUPAC
pH near pI minimizes solubility; charge affects aggregation.
— Protein chem
Ammonium sulfate and PEG commonly used for precipitation.
— Purification
📋 Key Takeaways
- • log(S/S₀) = β - Ks×I (Cohn equation); ionic strength affects solubility.
- • Hofmeister series ranks ions by salting-out (SO₄²⁻) vs salting-in (SCN⁻) strength.
- • pH near pI minimizes solubility; charge affects protein-protein interactions.
- • Ionic strength and temperature strongly influence protein solubility.
- • Used in purification, crystallization, and formulation.
1. What is Protein Solubility?
Protein solubility is the maximum amount of protein that can dissolve in a solvent under specific conditions. It depends on pH, temperature, ionic strength, and precipitating agents. Critical for purification, crystallization, and formulation.
log(S/S₀) = β - Ks × I
Cohn equation: S = solubility, I = ionic strength, Ks = salting-out constant
2. How Does It Work?
Hofmeister series ranks ions by salting-out/in strength. Salting-out reduces solubility at high salt; salting-in increases it at low salt or with chaotropic ions. PEG causes precipitation via excluded volume.
3. When to Use
Designing purification protocols, determining salt concentrations for precipitation, optimizing crystallization, analyzing ionic strength effects, planning PEG precipitation, formulation development.
4. Key Formulas
Cohn: log(S/S₀) = β - Ks × I
Ionic strength: I = ½Σcᵢzᵢ²
pH effect: S increases away from pI
5. Example
BSA at 100 mg/mL, (NH₄)₂SO₄ 50% saturation: solubility drops to ~10 mg/mL. At pI (4.7), solubility is minimal; at pH 7, charge repulsion increases solubility.
6. Practical Applications
Protein purification, crystallization, biopharmaceutical formulation, membrane protein solubilization, troubleshooting aggregation.
7. Limitations and Considerations
Cohn equation is empirical. Actual solubility depends on protein structure, buffer, and conditions. Use for estimation; verify experimentally.
8. Hofmeister Series Reference
| Ion | Effect |
|---|---|
| SO₄²⁻ | salting-out |
| Cl⁻ | salting-out |
| Br⁻ | salting-out |
| NO₃⁻ | salting-out |
| ClO₄⁻ | salting-in |
| SCN⁻ | salting-in |
9. 📚 Official Data Sources
⚠️ Disclaimer: Results are approximations. Consult IUPAC, UniProt, ExPASy for critical applications.