Estimate protein charge quickly across any pH. Choose pKa sets, termini states, and common modifications. See residue contributions, export results, and compare sequences easily.
This calculator estimates group ionization using the Henderson–Hasselbalch relationship. Each ionizable group contributes a fractional charge between 0 and ±1.
| Example | Sequence snippet | pH | Estimated net charge | Notes |
|---|---|---|---|---|
| A | ACDEKRRHYYC | 7.40 | ≈ +1.2 | Mixed acidic/basic residues. |
| B | DDDEEEGGGG | 7.40 | ≈ −5.0 | Strongly acidic sequence. |
| C | KKKKRRRHHH | 7.40 | ≈ +8.5 | Strongly basic sequence. |
Net charge affects solubility, binding, and electrophoresis mobility. A charge near zero can increase aggregation risk. Many proteins show lower solubility near their isoelectric region.
The calculator uses your sequence and solution pH. It counts D, E, C, Y, H, K, and R residues. It also models N-terminus and C-terminus contributions. You can block termini for capped constructs.
Different pKa tables shift results slightly. Standard settings use an N-terminus pKa near 9.60. The C-terminus uses a pKa near 2.34. Asp uses about 3.86 and Glu about 4.25. Lys uses about 10.50 and Arg about 12.50.
The plot evaluates charge from pH 0 to 14. It uses a 0.5 pH step for fast viewing. Low pH increases protonation and positive charge. High pH increases deprotonation and negative charge.
N-terminal acetylation neutralizes the N-terminus contribution. C-terminal amidation neutralizes the C-terminus contribution. Phosphorylation sites add an approximate negative contribution. Extra fixed charges support tags and adduct assumptions.
CSV export includes group charges and the curve table. PDF export provides a compact summary for sharing. Use exports to compare sequences under identical settings. Recalculate at multiple pH values for buffer screening.
The estimate is theoretical. It assumes independent groups and average pKa values. Real proteins shift pKa values due to structure and salt. Use this result for planning, then confirm with experiments.
The calculator removes non‑standard letters and symbols. This prevents counting unknown residues. It keeps only the 20 standard amino acids. Always review the cleaned sequence shown in the results.
You can enter any pH from 0 to 14. For most lab buffers, test pH 5.5 to 9.0. If you see a charge near zero, consider nearby pH values for improved solubility.
Block a terminus if it is chemically capped or not ionizable. Examples include peptide cyclization or engineered blocking groups. If you are unsure, leave termini free and compare both settings.
Each site is modeled as an added negative contribution. The model is approximate and pH dependent. It is useful for comparing modified and unmodified forms. It is not a replacement for detailed electrostatics.
The CSV includes inputs, group contributions, and the full charge curve table. The PDF includes a summary with group totals. Exports help you document assumptions, compare variants, and share results with collaborators.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.