Isoelectric point calculator explained
The isoelectric point (pI) is the pH at which an amphoteric molecule carries no net charge. For simple amino acids, pI equals the average of the two p values that bracket the neutral species. This calculator averages the supplied {pka} and {pkb} (or second p) values to return the isoelectric point {iso_point}.
Use it to plan isoelectric focusing runs, pick buffer pH values for protein purification, or predict solubility minima.
How the conversion works
For amino acids with one acidic and one basic group:
For basic residues (lysine, arginine, histidine), average the two highest p values; for acidic residues (aspartate, glutamate), average the two lowest. The calculator simply averages the two inputs you supply, so choose the pair that corresponds to the neutral zwitterion.
Units and conversions
| Parameter | Meaning | Notes |
|---|---|---|
| Protonation equilibrium | Select the value that protonates the neutral form. | |
| or second | Deprotonation equilibrium | Use the dissociation that removes the positive charge. |
| Isoelectric point | Output pH where net charge is zero. |
Worked examples
- Glycine
(carboxyl), (amino).
Glycine is neutral near pH 6.1, so it is least soluble there.
- Lysine
Relevant p values for neutral zwitterion: (alpha-amine) and (side-chain amine).
Choose buffer pH well away from 9.7 to maintain lysine solubility or charge.
Tips and pitfalls
- Always average the two p values that straddle the zwitterionic form; averaging the wrong pair yields misleading pI values.
- For peptides, calculate pI by considering all ionizable groups; software or titration curves are more accurate than simple averaging.
- Ionic strength and temperature shift p values, so adjust inputs if your process runs far from 25 deg C.
- Use pI to predict electrophoretic mobility: molecules migrate toward the anode below pI and toward the cathode above pI.