Protein Solubility Calculator
Estimate how a protein's solubility changes with salt concentration using the simplified Cohn salting-out/salting-in relationship.
🧬 What is Protein Solubility?
Protein solubility is the maximum amount of a protein that stays dissolved in a solution rather than aggregating and precipitating out. It depends heavily on the solution's ionic strength, the combined effect of every dissolved ion's concentration and charge, because dissolved salt ions compete with the protein for surrounding water molecules and shield charged groups on the protein's surface from each other.
Biochemists rely on this relationship constantly in the lab: ammonium sulfate precipitation, one of the oldest and still most common protein purification steps, deliberately raises ionic strength to precipitate target proteins out of a crude mixture; formulation scientists tune buffer salt concentration to keep a therapeutic protein stable and soluble in storage; and structural biologists screen ionic strength conditions to find crystallization conditions where a protein is just barely soluble enough.
A common point of confusion is assuming that more salt always means less solubility. Over a wide range of ionic strength that is true, the salting-out regime this calculator's default constants describe, but at very low ionic strength many proteins actually become slightly more soluble as a small amount of salt is added, the salting-in regime. This calculator's simplified linear model can represent either behavior depending on the sign of the salting-out constant Ks: positive Ks models salting-out, negative Ks models salting-in.
This calculator applies the simplified Cohn salting-out relationship, log10(S) = beta minus Ks times I, to turn a solubility constant, a salting-out constant, and an ionic strength value into a predicted solubility, a log10 solubility, and a percentage relative to the protein's zero-ionic-strength baseline solubility, along with a chart of the full solubility curve across a range of ionic strength.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Physiological ionic strength
β = 2.5, Ks = 0.5 L/mol, I = 0.15 mol/L (close to blood plasma ionic strength).
Example 2 — High ionic strength (salting-out)
β = 2.5, Ks = 0.5 L/mol, I = 1.5 mol/L (near saturating ammonium sulfate level).
Example 3 — Salting-in with a negative Ks
β = 1.0, Ks = −0.2 L/mol, I = 0.5 mol/L.
❓ Frequently Asked Questions
🔗 Related Calculators
What is the formula for protein solubility vs ionic strength?
The simplified Cohn salting-out equation is log10(S) = beta minus Ks times I, where S is solubility, beta is the log10 solubility at zero ionic strength, Ks is the salting-out constant, and I is the ionic strength of the solution.
What does the salting-out constant Ks represent?
Ks measures how sensitive a specific protein's solubility is to added salt. A larger positive Ks means solubility falls off more sharply as ionic strength increases, which is why different proteins precipitate out of a mixture at different ammonium sulfate concentrations during salting-out purification.
What is salting out?
Salting out is the process where increasing a solution's salt (ionic strength) reduces a protein's solubility, eventually causing it to precipitate out of solution. It happens because high salt concentrations compete with the protein for water molecules, reducing the hydration shell that keeps the protein dissolved.
What is salting in?
Salting in is the opposite effect: at low ionic strength, adding a small amount of salt can actually increase a protein's solubility, because the added ions help shield charged groups on the protein surface from each other, reducing protein-protein attraction. This calculator models salting-in by allowing a negative Ks value.
Why is solubility calculated on a log scale?
Protein solubility can span several orders of magnitude across a realistic ionic strength range, so a log10 scale keeps the relationship linear and easy to work with. This mirrors how the Cohn equation and similar salting-out relationships are always reported in the biochemistry literature.
How is ionic strength calculated?
Ionic strength I equals one half times the sum, over every ion in solution, of that ion's molar concentration times the square of its charge. Use the Ionic Strength Calculator to compute I from a solution's actual ion concentrations before entering it here.
Why does protein solubility matter for purification?
Ammonium sulfate precipitation, a classic protein purification step, exploits exactly this relationship: raising ionic strength selectively precipitates proteins with a higher Ks (or lower beta) first, letting a researcher fractionate a complex mixture by collecting protein at different salt cuts.
What units does the calculator use for solubility?
The output S carries whatever concentration unit beta was originally measured in, commonly grams per liter. Since beta and S share the same unit by definition of the equation, you can treat the calculator's units as matching your own reference data.
Does pH affect the beta and Ks constants?
Yes. Both beta and Ks are empirical constants specific to a given protein at a given pH and temperature, most often measured closest to (or with respect to) the protein's isoelectric point, since solubility minimums and salting behavior both shift with pH. Use the Isoelectric Point Calculator to find where that minimum-solubility pH sits for a given amino acid or known pKa values.
What happens to solubility when ionic strength is zero?
At I = 0, the exponent term Ks times I becomes zero, so log10(S) equals beta exactly, and S equals 10 to the power of beta. This is the baseline solubility the calculator reports as 100% in the percent-of-baseline result.
Is this the exact form of the Cohn equation used in research?
This calculator uses the simplified, widely taught linear form log10(S) = beta minus Ks times I. Some research applications use extended forms with additional terms for pH dependence or a separate salting-in term at low ionic strength, but this simplified form captures the core salting-out trend that dominates over most of the practically useful ionic strength range.