Protein Solubility Calculator
Predict protein solubility patterns using either the Cohn salting-out model or pH-dependent isoelectric precipitation curves.
Solubility Model Parameters
Select calculation model and enter salt or pH parameters.
Estimated Protein Solubility
Hofmeister Lyotropic Series
Kosmotropic anions precipitate proteins (Salting-out): SO₄²⁻ > HPO₄²⁻ > CH₃COO⁻ > Cl⁻ > NO₃⁻ > ClO₄⁻.
Ammonium Sulfate Saturation Precipitation Table (at 20°C) Reference
| Protein Ftion | % Saturation Range | Ammonium Sulfate Conc (M) | Precipitation Chateristic |
|---|---|---|---|
| Fibrinogen / Large Immunoglobulins | 0% – 25% Saturation | 0.0 – 1.0 M | Precipitates very large / hydrophobic proteins first |
| IgG Antibodies / Globulins | 33% – 50% Saturation | 1.3 – 2.0 M | Standard antibody ftionation cut |
| Serum Albumin (BSA) | 55% – 75% Saturation | 2.2 – 3.0 M | Highly soluble hydrophilic protein cut |
Calculation Methodology & Equations
Cohn Salting-Out Model
Relates protein solubility $S$ (mg/mL) to salt molarity, where $eta$ is theoretical zero-salt solubility intercept and $K_s$ is the salting-out constant.
pH Isoelectric Model
Models parabolic recovery of protein solubility as solution pH shifts away from the isoelectric point ($pI$).
Frequently Asked Questions
What is the Cohn Salting-Out Equation? ▼
The Cohn equation describes protein solubility $S$ as a function of salt concentration (ionic strength $I$): $log_{10}(S) = β - K_s times []$, where $β$ is zero-salt solubility intercept and $K_s$ is the salting-out constant.
Why does high salt concentration precipitate proteins? ▼
At high salt concentrations, salt ions compete with proteins for hydration by water molecules. Dehydrated hydrophobic surfaces on proteins intet and form insoluble aggregates.
Why is ammonium sulfate (NH4)2SO4 widely used for salting-out? ▼
Ammonium sulfate has high water solubility, low temperature coefficient, high ionic strength per mole, and stabilizes protein tertiary structures without denaturation.