Calculate the distance between solution pH and protein pI, convert NaCl concentration to ionic strength, and review conditions to test. These inputs cannot predict a solubility value.
Advanced options
How to use our Protein Solubility Calculator
- Enter "Solution pH" and "Protein pI" for the same protein and solution conditions.
- Choose "Salt entry method" and fill in the visible "Ionic strength (M)" or "NaCl concentration (mM)" input.
- Enter "Temperature (C)". In "Advanced options", optionally record "Protein concentration (mg/mL)" and "Protective additive present".
- Click "Calculate". Read "Distance from pI (pH units)" and "Ionic strength", then review "Conditions to check".
- Compare candidate buffers using protein-specific experiments or an established protocol. The tool does not rank conditions by predicted solubility.
Definitions
Solution pH describes the acidity of the solution. Protein pI is the pH at which the protein has approximately zero net charge under the relevant conditions.
Distance from pI is the absolute difference between the two entered values. It is not a solubility measurement.
Ionic strength combines the concentrations and squared charges of dissolved ions. Fully dissociated NaCl contributes approximately its molar concentration to ionic strength.
pH 8.5 and pI 6.5 give a distance of 2 pH units. This describes the pH offset; it does not predict a percentage dissolved or a favorable-solubility band.
Common mistakes and quick fixes
Do not enter 150 mM into "Ionic strength (M)" as 150. For a NaCl-only solution, use "NaCl concentration (mM)" = 150, giving 0.15 M.
Do not assume that increasing "Distance from pI (pH units)" always improves solubility. Changing pH can also alter protein structure, activity and stability.
An additive, low temperature or a low protein concentration is not an automatic guarantee of solubility. Check the specific protein, buffer and intended assay.
Limitations & Key Assumptions / Boundary Conditions
This is a pH comparison and salt conversion tool with qualitative experimental context. It does not calculate dissolved fraction, mg/mL solubility, an aggregation probability or a calibrated comparison score.
The supported pH and pI input range is 0 to 14. The NaCl conversion assumes a fully dissociated 1:1 salt; buffer ions and other salts are not included in that conversion.
Protein identity, sequence, structure, buffer composition, temperature, concentration and additives can all matter. A generic set of offsets cannot establish which conditions will work for a particular protein.
Methodology
Calculations retained
Distance from pI = |Solution pH - Protein pI|.
Ionic strength is defined as I = 0.5 × sum(c_i × z_i²), with ion concentration c_i in mol/L and charge number z_i. For fully dissociated NaCl alone, equal sodium and chloride concentrations give I = c_NaCl(mM) / 1000.
For pH 8.5, pI 6.5 and 150 mM NaCl, the distance is 2 pH units and the NaCl contribution to ionic strength is 0.15 M.
Interpretation
Many proteins show reduced solubility near their isoelectric point, but this is not a universal quantitative law. Salt effects depend on the protein and salt. Temperature, concentration and additive inputs are reported as experimental context, without invented bonuses or penalties.