How to Use Isoelectric Point Calculator
The Isoelectric Point Calculator determines the isoelectric point (pI) of an amino acid or charged molecule from its ionisation pKa values. Select a preset amino acid or enter pKa values manually.
- Choose mode – Select Select amino acid to use built-in pKa data for 20 standard amino acids, or Manual pKa input to enter custom values.
- Amino acid mode – Choose the amino acid from the dropdown. The Isoelectric Point Calculator uses the standard α-COOH, α-NH₃⁺, and (where applicable) side chain pKa values.
- Manual mode – Enter the pKa of the α-carboxyl group, the α-ammonium group, and optionally a side chain pKa.
- Review the result – The calculator shows all pKa values, the two values used for averaging, and the final pI.
Formula & Theory - Isoelectric Point Calculator
The Isoelectric Point Calculator uses this core formula or rule: the standard approach for determining pI:
pI = (pKa_a + pKa_b) / 2where pKa_a and pKa_b are the two pKa values bracketing the zero-charge pH point.
How pI is Determined
- For neutral amino acids (no ionisable side chain): pI = (pKa_COOH + pKa_NH₃) / 2
- For acidic amino acids (side chain –COOH): pI = (pKa_COOH + pKa_side) / 2, selecting the two lowest pKa values
- For basic amino acids (side chain –NH₂ or guanidinium): pI = (pKa_NH₃ + pKa_side) / 2, selecting the two highest pKa values
| Amino acid type | pI range |
|---|---|
| Neutral | 5.5 – 7.0 |
| Acidic (Asp, Glu) | 2.8 – 3.2 |
| Basic (Lys, Arg, His) | 7.6 – 10.8 |
Assumptions and Limits
- pKa values in this calculator are standard reference values; actual pKa values in folded proteins can differ significantly due to local electrostatic environment.
- The calculation assumes isolated amino acid behaviour, not a peptide or protein context.
Use Cases for Isoelectric Point Calculator
The Isoelectric Point Calculator is a fundamental tool in biochemistry and protein science. Common uses include:
- Protein purification – Designing ion-exchange chromatography conditions by knowing the pI of the target protein.
- Gel electrophoresis – Predicting isoelectric focusing (IEF) behaviour for 2D gel electrophoresis.
- Amino acid chemistry – Understanding the charge state of amino acids at physiological pH.
- Drug development – Estimating the pI of therapeutic peptides to predict solubility and formulation behaviour.
- Biochemistry coursework – Solving amino acid ionisation and titration curve problems.
The Isoelectric Point Calculator provides clear step-by-step results, making the concept of isoelectric focusing and charge-based separations easy to understand.