Calculator Inputs
Formula Used
The calculator estimates charge with Henderson Hasselbalch fractions.
Acidic group charge: charge = -n / (1 + 10^(pKa - pH))
Basic group charge: charge = n / (1 + 10^(pH - pKa))
Net charge: sum of all acidic and basic group charges
Isoelectric point: the pH where net charge equals the target charge. The default target charge is zero.
How To Use This Calculator
- Enter a sample name for the report.
- Add a peptide sequence with one letter amino acid codes.
- Keep termini enabled for a normal free peptide.
- Change terminal pKa values when special conditions are known.
- Add custom acidic or basic pKa values for modifications.
- Set decimal precision and target charge.
- Press the calculate button.
- Review the result above the form.
- Download the CSV or PDF report.
Example Data Table
| Example | Sequence | Important groups | Expected behavior |
|---|---|---|---|
| Acidic peptide | ACDE | C terminus, D, E, C | Lower pI range |
| Basic peptide | AKRR | N terminus, K, R, R | Higher pI range |
| Histidine rich peptide | AHHHK | H, H, H, K | Strong pH sensitivity near neutral range |
| Custom molecule | Blank | Custom acidic and basic pKa entries | Depends on entered groups |
About This Isoelectric Point Tool
The isoelectric point is the pH where a molecule has zero net charge. This calculator estimates that point for amino acids, peptides, and custom ionizable systems. It is useful for protein purification, electrophoresis planning, buffer choice, and classroom checks.
Why pI Matters
A peptide behaves differently above and below its pI. Below the pI, it carries more positive charge. Above the pI, it carries more negative charge. At the pI, migration in an electric field becomes minimal. Solubility may also fall because repulsion is reduced. That behavior matters during precipitation, focusing, chromatography, and formulation work.
Advanced Input Control
The tool accepts a one letter peptide sequence. It counts acidic and basic side chains automatically. It can include amino and carboxyl termini. You may also add extra acidic or basic pKa values. This supports modified residues, attached labels, blocked termini, and small amphoteric molecules. Decimal precision can be adjusted for quick or detailed output.
Calculation Method
The calculator uses Henderson Hasselbalch charge fractions. Acidic groups become negative as pH rises. Basic groups lose positive charge as pH rises. The program sums every fractional charge at each trial pH. Then it uses a binary search to find the pH where the chosen target charge is reached. The default target is zero.
Interpreting Results
The reported pI is an estimate, not a measured constant. Literature pKa values vary with temperature, ionic strength, solvent, neighboring residues, and protein folding. A buried group may titrate differently from an exposed group. For long proteins, experimental pI values can differ from sequence estimates. Still, this calculation gives a strong starting point.
Using The Output
Review the pI, net charge, positive charge, and negative charge. Check the group table to see which residues drive the result. Export the CSV file for spreadsheets. Download the PDF summary for reports. Compare examples before changing lab buffers. For best practice, combine this result with experimental observation and trusted biochemical references.
Good Inputs Improve Accuracy
Enter clean sequences without spaces or numbers. Repeat residues normally. Use custom pKa entries for known modifications. Avoid mixing predicted and measured pKa sets without notes. Small changes can shift the final pI, especially near histidine rich regions during sensitive separation work.
FAQs
What is an isoelectric point?
It is the pH where a molecule has no net electrical charge. At this point, positive and negative charge contributions balance each other.
Can this calculator handle peptides?
Yes. Enter a one letter peptide sequence. The tool counts common acidic and basic side chains and includes termini when selected.
Which side chains are included automatically?
The calculator includes D, E, C, Y, H, K, and R side chains. These residues have common ionizable groups in peptide calculations.
Can I calculate a modified peptide?
Yes. Add known acidic or basic pKa values in the custom fields. Disable termini if the peptide has blocked ends.
Why is my pI different from a reference value?
Reference values may use different pKa sets. Real pI also changes with temperature, solvent, ionic strength, and molecular structure.
What does target net charge mean?
The default target is zero. You may change it to find the pH where the molecule reaches another selected net charge.
Does this replace laboratory measurement?
No. It is a calculation estimate. Use it for planning, comparison, and learning, then confirm important results experimentally.
How do I export results?
After calculating, use the CSV button for spreadsheet data. Use the PDF button or print option for a report summary.