Isoelectric Point Calculator
Average pKa values surrounding the neutral charge state to approximate the isoelectric point of amphoteric molecules.
Carboxyl group pKa for amino acids.
Amino group pKa for amino acids.
Use for ionizable side chains (Lys, Asp, His, etc.).
Isoelectric point
6
How to Use This Calculator
Collect pKa values
Gather acid, base, and optional side chain pKa values for the molecule of interest.
Select charge state
Choose which protonation states define zero net charge. Default uses backbone pKa values.
Compute pI
The calculator averages the two pKa values that straddle the neutral charge state.
Apply to separation planning
Use the estimated pI to select buffer pH values for electrophoresis or precipitation experiments.
Formula
pI = (pKa i + pKa j) / 2
Select the two pKa values that surround the neutral charge state. For simple amino acids, use the backbone acid and base pKa values.
Example
Glycine with pKa1 = 2.3 and pKa2 = 9.6 gives pI = (2.3 + 9.6) / 2 about 5.95.
Full Description
The isoelectric point marks the pH where a molecule carries no net charge, which strongly influences solubility, adsorption, and electrophoretic mobility.
Averaging surrounding pKa values provides a quick approximation. Detailed calculations may require accounting for multiple ionizable groups and activity effects.
Frequently Asked Questions
How accurate is the average method?
It provides a useful estimate for mono- and di-basic molecules. Complex peptides may require more detailed charge balance calculations.
Which pKa values should I use?
Use solution pKa values measured under similar ionic strength and temperature conditions for your experiment.
Can I include more than one side chain pKa?
This simple tool accepts one side chain pKa. For multiple ionizable groups consider iterative charge balance methods.
What does the charge state selector do?
It chooses which pKa pair to average. For acidic side chains use the negative option, for basic side chains use the positive option.
Does temperature affect pKa?
Yes. pKa values can shift with temperature and ionic strength. Adjust values if conditions differ significantly from data sources.