Protein Solubility Calculator - Free pH & Temperature Tool

A protein solubility calculator estimates the highest concentration of a protein that stays dissolved in a solvent under a given pH, temperature, and ionic strength. It converts basic chemistry into a single number, in milligrams of protein per milliliter of solvent.

Protein solubility matters in biochemistry, drug formulation, and food science. A protein that will not stay dissolved forms aggregates or precipitates, which can ruin an experiment or make a drug impossible to inject.

How to calculate protein solubility

Solubility depends on five things: the protein's isoelectric point (the pH at which its net charge is zero), its melting point (the temperature at which it starts to unfold), the solvent's dielectric constant (a measure of how well it separates charged particles), the pH of the solution, and the ionic strength (a measure of how much dissolved salt is present).

The calculator starts from each protein's known solubility in water at a set of reference conditions: 25°C, pH 7.0, and an ionic strength of 0.15 M (roughly the salt level in blood). It then multiplies that reference value by four ratios, one for each condition that has changed.

Protein solubility formula

$S = S_{ref} \times \frac{f_{pH}(pH)}{f_{pH}(7.0)} \times \frac{f_{ionic}(I)}{f_{ionic}(0.15)} \times \frac{f_{temp}(T)}{f_{temp}(25)} \times f_{solvent}(\varepsilon)$

• $S_{ref}$ is the protein's known solubility in water at the reference conditions, in mg/mL.
• $f_{pH}(pH) = 1 + 9\left(1 - e^{-|pH - pI|}\right)$, where $pI$ is the protein's isoelectric point. Solubility is lowest at the isoelectric point and rises as pH moves away from it, leveling off within a couple of pH units.
• $f_{ionic}(I) = 10^{\left(\frac{0.8\sqrt{I}}{1 + 1.5\sqrt{I}} - I\right)}$, where $I$ is ionic strength in moles per liter. At low salt levels this term slightly raises solubility ("salting in"). At high salt levels it lowers solubility ("salting out").
• $f_{temp}(T) = e^{0.01(T-25)} \times \dfrac{1}{1 + e^{(T - T_m)/3}}$, where $T_m$ is the protein's melting point in °C. Solubility rises gently with temperature, then falls sharply once the temperature nears $T_m$ and the protein starts to unfold.
• $f_{solvent}(\varepsilon) = 10^{-2000\left(\frac{1}{\varepsilon^2} - \frac{1}{78.4^2}\right)}$, where $\varepsilon$ is the solvent's dielectric constant. Water has a dielectric constant of 78.4, so this term equals 1 in water. Solvents that separate charges less effectively than water, such as ethanol, lower solubility.

Reference values used by the calculator

Solubility categories

Example: albumin in phosphate buffer

Find the solubility of albumin in phosphate buffer at 25°C, pH 7.4, and an ionic strength of 0.15 M.

Albumin's reference values are $pI = 4.7$, $S_{ref} = 500$ mg/mL, and $T_m = 63°C$. Phosphate buffer has a dielectric constant of 76.0.

1. pH term: $f_{pH}(7.4) = 1 + 9(1 - e^{-2.7}) = 9.40$. $f_{pH}(7.0) = 1 + 9(1 - e^{-2.3}) = 9.10$. Ratio: $9.40 / 9.10 = 1.03$.
2. Ionic term: the ionic strength (0.15 M) equals the reference value, so the ratio is 1.
3. Temperature term: the temperature (25°C) equals the reference value, so the ratio is 1.
4. Solvent term: $f_{solvent}(76.0) = 10^{-2000(1/76.0^2 - 1/78.4^2)} = 0.95$.

$S = 500 \times 1.03 \times 1 \times 1 \times 0.95 \approx 492\text{ mg/mL}$

492 mg/mL falls in the "highly soluble" category.

Example: lysozyme in glycerol at low temperature

Find the solubility of lysozyme in a glycerol solution at 4°C, pH 5.0, and an ionic strength of 0.1 M — conditions similar to cold storage of an enzyme.

Lysozyme's reference values are $pI = 11.35$, $S_{ref} = 250$ mg/mL, and $T_m = 72°C$. Glycerol has a dielectric constant of 42.5.

1. pH term: moving from pH 7.0 to pH 5.0 (both far below the pI of 11.35) changes the pH term only slightly. Ratio: about 1.01.
2. Ionic term: dropping ionic strength from 0.15 M to 0.1 M raises solubility slightly through the salting-in effect. Ratio: about 1.06.
3. Temperature term: cooling from 25°C to 4°C, well below the melting point, lowers the term slightly. Ratio: about 0.81.
4. Solvent term: $f_{solvent}(42.5) = 10^{-2000(1/42.5^2 - 1/78.4^2)} \approx 0.17$.

$S = 250 \times 1.01 \times 1.06 \times 0.81 \times 0.17 \approx 36\text{ mg/mL}$

36 mg/mL falls in the "soluble" category. The low dielectric constant of glycerol, not the cold temperature, is what pulls the result down from the reference value.

How to use the protein solubility calculator

1. Choose a protein from the list: albumin, insulin, lysozyme, hemoglobin, myoglobin, collagen, casein, or fibrinogen.
2. Choose a solvent: water, phosphate buffer, Tris buffer, glycerol, DMSO, methanol, ethanol, or acetone.
3. Enter the temperature in °C (0 to 100), the pH (0 to 14), and the ionic strength in molar (0 to 2).
4. Read the calculated solubility in mg/mL and its category, shown alongside a bar that visualizes the result.

What affects protein solubility

pH and the isoelectric point. Every protein has an isoelectric point, the pH at which its surface carries no net electrical charge. At that pH, protein molecules do not repel each other, so they clump together and fall out of solution more easily. Moving the pH away from the isoelectric point, in either direction, gives the protein a net charge. Charged molecules repel each other, which keeps them apart and dissolved.

Ionic strength. Dissolved salt ions surround a protein's charged surface groups. At low to moderate concentrations, these ions shield charges that would otherwise attract two protein molecules together, which increases solubility. This is called salting in. At high salt concentrations, ions compete with the protein for water molecules, stripping away the layer of water that keeps the protein dissolved. This is called salting out, and it is the basis of a common purification method that uses ammonium sulfate to precipitate proteins out of a mixture.

Temperature. Warming a solution generally increases solubility, up to a point. Once the temperature approaches a protein's melting point, the protein begins to unfold. Unfolding exposes hydrophobic (water-repelling) sections of the protein that are normally hidden inside its folded structure. These exposed sections stick to each other, causing aggregation, so solubility drops sharply near and above the melting point.

Solvent. A solvent's dielectric constant measures how effectively it weakens the electrical attraction between charged particles. Water has a high dielectric constant and separates charges well, which is why most proteins dissolve better in water than in organic solvents like ethanol or acetone. Solvents with a low dielectric constant let opposite charges on different protein molecules attract each other more strongly, encouraging aggregation.

Limitations

The calculator uses a simplified model built on a small set of reference proteins and standard solvents. It does not account for protein purity, additives such as detergents or stabilizers, or unusual buffer components. Results are estimates for planning experiments, not a substitute for measuring solubility directly in the lab.

Frequently asked questions

What is protein solubility? Protein solubility is the maximum concentration of a protein that stays fully dissolved in a solvent, without forming visible aggregates or precipitate.

Why is a protein least soluble at its isoelectric point? At the isoelectric point, the protein carries no net electrical charge, so protein molecules no longer repel each other electrically. Without that repulsion, they are more likely to stick together and fall out of solution.

What is the difference between salting in and salting out? Salting in happens at low salt concentrations, where dissolved ions shield the protein's charged groups from each other and increase solubility. Salting out happens at high salt concentrations, where ions strip water away from the protein's surface and decrease solubility.

Does higher temperature always increase solubility? No. Solubility tends to rise with temperature only up to a point. Once temperature approaches a protein's melting point, the protein starts to unfold, which usually causes aggregation and a sharp drop in solubility.

Why do proteins dissolve less in ethanol or acetone than in water? Water has a high dielectric constant, meaning it weakens the electrical attraction between charged parts of protein molecules very effectively. Ethanol and acetone have lower dielectric constants, so charged groups on different protein molecules attract each other more strongly in these solvents, making aggregation more likely.

How accurate is this calculator? It gives an estimate based on a physical model and a small set of reference proteins and solvents. It is useful for narrowing down conditions to test, but experimental measurement remains the only way to confirm solubility for a specific protein sample.

Further reading

• Arakawa, T., & Timasheff, S. N. (1984). Mechanism of protein salting in and salting out by divalent cation salts. Biochemistry, 23(25), 5912–5923.
• Trevino, S. R., Scholtz, J. M., & Pace, C. N. (2008). Measuring and increasing protein solubility. Journal of Pharmaceutical Sciences, 97(10), 4155–4166.
• Cohn, E. J., & Edsall, J. T. (1943). Proteins, Amino Acids and Peptides as Ions and Dipolar Ions. Reinhold Publishing.