Calculate Net Protein Charge at a Given pH
Paste a protein sequence (single-letter amino acid code), choose pH, and calculate estimated net charge and pI.
What this protein charge calculator does
This calculator estimates the net charge of a protein at any pH between 0 and 14 using a Henderson–Hasselbalch style model. It evaluates common ionizable groups in proteins: Asp (D), Glu (E), Cys (C), Tyr (Y), His (H), Lys (K), Arg (R), and optionally the free N- and C-termini.
If you are screening buffer conditions, comparing variants, or deciding where your protein might bind in ion-exchange chromatography, this provides a fast first-pass estimate.
How the charge is estimated
1) Sequence parsing
The tool counts ionizable residues in your sequence. Non-standard letters and symbols are ignored.
2) Fraction protonated/deprotonated
For basic groups (K, R, H, N-terminus), the calculator estimates how much remains protonated at your selected pH. For acidic groups (D, E, C, Y, C-terminus), it estimates how much is deprotonated.
3) Net charge and estimated pI
Net charge is computed as total positive contribution minus total negative contribution. The page also estimates the isoelectric point (pI) numerically by finding where predicted net charge approaches zero.
pKa values used in this model
| Group | pKa (used) | Type |
|---|---|---|
| Asp (D) | 3.90 | Acidic |
| Glu (E) | 4.10 | Acidic |
| Cys (C) | 8.30 | Acidic (weak) |
| Tyr (Y) | 10.10 | Acidic (weak) |
| His (H) | 6.00 | Basic (weak) |
| Lys (K) | 10.50 | Basic |
| Arg (R) | 12.40 | Basic |
| N-terminus | 9.69 | Basic |
| C-terminus | 2.34 | Acidic |
How to use it effectively
- Use a clean amino-acid sequence in one-letter code.
- Try several pH values to see charge transitions across your buffer range.
- Compare variants to identify substitutions that shift net charge or pI.
- Use results as screening guidance, then confirm experimentally.
Important limitations
Real proteins do not behave like isolated amino acids in solution. Microenvironment, salt, temperature, folding state, cofactors, PTMs, and local electrostatics can shift effective pKa values. So this model is best for quick estimation, not as a substitute for measured titration data.
Practical interpretation tips
- Positive net charge: protein may bind more strongly to cation-exchange repelled? Actually it generally binds anion exchangers more readily.
- Negative net charge: protein often has greater affinity for cation exchangers.
- Near-zero net charge: aggregation risk can increase near pI for some proteins.
- Histidine-rich proteins: expect strong charge sensitivity around pH 5.5–7.0.
Bottom line
This protein charge calculator is a fast, practical tool for planning purification, formulation, and buffer optimization. Use it to generate hypotheses quickly, then refine with experimental data.