Calculator Input
Leave the sequence blank to use manual ionizable counts. Enter a sequence to auto-count residues and terminals.
Example Data Table
These illustrative examples help you test the calculator quickly and compare acidic, balanced, and basic protein behavior.
| Example | Asp | Glu | Cys | Tyr | His | Lys | Arg | N-term | C-term | Expected Trend |
|---|---|---|---|---|---|---|---|---|---|---|
| Acidic enzyme fragment | 6 | 7 | 1 | 2 | 1 | 3 | 1 | 1 | 1 | Lower pI |
| Balanced teaching sample | 3 | 3 | 1 | 1 | 2 | 4 | 2 | 1 | 1 | Near-neutral pI |
| Basic binding protein | 1 | 2 | 0 | 1 | 2 | 8 | 4 | 1 | 1 | Higher pI |
Formula Used
The calculator treats each ionizable group with a Henderson-Hasselbalch style fractional charge model. Basic groups contribute positive charge, while acidic groups contribute negative charge.
This gives a practical pI estimate for classroom work, screening studies, and fast comparative analysis. Real proteins can shift slightly because microenvironment, folding, solvent, and post-translational effects alter effective pKa values.
How to Use This Calculator
- Enter a sample name so exports and reports look organized.
- Paste a protein sequence for automatic counting, or leave it blank and enter manual ionizable group counts.
- Adjust pKa values if your model, solvent, or teaching reference uses different constants.
- Set plot limits and step size for a smoother or faster net-charge curve.
- Click Calculate Protein pI to place the result block below the header and above the form.
- Review pI, net charge at pH 7, acidic/basic site totals, and the plotted titration-like curve.
- Use the export buttons to save CSV data or a PDF summary.
FAQs
1) What does pI mean for a protein?
The isoelectric point is the pH where the predicted net charge becomes approximately zero. At this point, a protein often shows minimum electrophoretic mobility and altered solubility behavior, though real measurements can vary with buffer and structure.
2) Why can my experimental pI differ from this result?
This calculator uses idealized pKa values and independent group behavior. Real proteins experience folding effects, neighboring residues, salt concentration, ligand binding, and modifications such as phosphorylation, all of which can shift the observed pI.
3) Should I use sequence mode or manual counts?
Use sequence mode when you have the amino acid string and want automatic residue counting. Use manual mode when composition is already summarized, when comparing hypothetical proteins, or when modeling processed fragments with custom terminal counts.
4) Which residues affect the calculation most directly?
Aspartate, glutamate, cysteine, tyrosine, histidine, lysine, arginine, plus the N- and C-termini contribute directly. Other residues matter indirectly only through sequence context in real systems, not through explicit charge terms in this model.
5) Why include a net charge curve?
The curve shows how charge changes across pH rather than only reporting one pI value. That helps with buffer selection, teaching acid-base transitions, comparing variants, and spotting whether neutrality occurs sharply or across a flatter region.
6) Can I change the pKa values?
Yes. Advanced fields let you replace the default pKa values with your preferred literature set, laboratory convention, or teaching standard. This is helpful when comparing calculator behavior against textbooks, software packages, or measured systems.
7) What if the sequence contains unusual letters?
Unsupported letters are ignored and listed after calculation. That prevents the solver from failing while still alerting you that the input included ambiguous or nonstandard codes which may need manual interpretation.
8) Is this calculator suitable for published biochemical characterization?
It is best used for estimation, screening, and teaching. For publication-grade work, compare against experimental isoelectric focusing, curated biophysical software, and context-specific pKa models that include three-dimensional structure and environmental effects.