Peptide isoelectric point calculator
Paste a sequence in one-letter code to get its isoelectric point (pI), its net charge at any pH, and the full charge curve from pH 0 to 14.
Load a known peptide
| Peptide | Sequence | Modifications | pI (EMBOSS) | Charge, pH 7 | Load preset |
|---|---|---|---|---|---|
| BPC-157 | GEPPPGKPADDAGLV | — | 3.88 | -2.02 | |
| GHK (GHK-Cu without copper) | GHK | — | 9.70 | +1.22 | |
| Epitalon | AEDG | — | 3.55 | -2.02 | |
| KPV | KPV | — | 9.70 | +0.98 | |
| Selank | TKPRPGP | — | 11.65 | +1.98 | |
| Semax | MEHFPGP | — | 5.36 | -0.78 | |
| DSIP | WAGGDASGE | — | 3.55 | -2.02 | |
| Cartalax | AED | — | 3.55 | -2.02 | |
| Thymosin Alpha-1 | SDAAVDTSSEITTKDLKEKKEVVEEAEN | N-acetyl | 3.81 | -5.99 | |
| Oxytocin | CYIQNCPLG | C-amide, 1 disulfide | 9.35 | +0.97 | |
| Kisspeptin-10 | YNWNSFGLRF | C-amide | 11.31 | +1.97 | |
| Sermorelin | YADAIFTNSYRKVLGQLSARKLLQDIMSR | C-amide | 10.95 | +3.98 | |
| AOD-9604 | YLRIVQCRSVEGSCGF | 1 disulfide | 9.35 | +0.98 | |
| MOTS-c | MRWQEMGYIFYPRKLR | — | 10.64 | +2.98 | |
| VIP | HSDAVFTDNYTRLRKQMAVKKYLNSILN | C-amide | 10.72 | +4.22 | |
| LL-37 | LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES | — | 11.35 | +5.98 | |
| TB-500 fragment (Ac-LKKTETQ) | LKKTETQ | N-acetyl | 7.36 | 0.00 |
How the pI is calculated
Every ionizable group contributes a fractional charge set by its pKa and the pH (the Henderson–Hasselbalch equation). Basic groups (the free N-terminus, Lys, Arg, His) carry positive charge below their pKa; acidic groups (the free C-terminus, Asp, Glu, Cys, Tyr) carry negative charge above theirs. The isoelectric point is the pH where the sum is zero, found here by bisection to better than 0.01 pH units. For BPC-157 with the EMBOSS set that is pH 3.88.
N-terminal acetylation removes the free amine, C-terminal amidation removes the free carboxyl, and cysteines locked in a disulfide bond stop ionizing, so all three options change the result.
pKa values used
| Set | N-term | C-term | Asp | Glu | Cys | Tyr | His | Lys | Arg |
|---|---|---|---|---|---|---|---|---|---|
| EMBOSS | 8.6 | 3.6 | 3.9 | 4.1 | 8.5 | 10.1 | 6.5 | 10.8 | 12.5 |
| Lehninger | 9.69 | 2.34 | 3.86 | 4.25 | 8.33 | 10.0 | 6.0 | 10.5 | 12.4 |
| Solomon | 9.6 | 2.4 | 3.9 | 4.3 | 8.3 | 10.1 | 6.0 | 10.5 | 12.5 |
Questions
What is the isoelectric point of a peptide?
The pH at which the peptide carries no net charge. Below its pI a peptide is net positive; above it, net negative. Solubility is usually lowest near the pI, which is why buffers are often chosen at least one pH unit away from it.
Why do pI calculators give different answers?
Each uses its own pKa values for the termini and the ionizable side chains. This tool lets you switch between the EMBOSS, Lehninger and Solomon sets. Real pKa values also shift with neighbouring residues, temperature and ionic strength, so treat any calculated pI as an estimate.
How is net charge at a given pH calculated?
Each basic group contributes 1/(1+10^(pH−pKa)) and each acidic group −1/(1+10^(pKa−pH)). The net charge is the sum over every group present, including the termini unless they are capped.
Do acetylation and amidation change the pI?
Yes. Acetylation removes the N-terminal positive charge and lowers the pI; amidation removes the C-terminal negative charge and raises it. A peptide with both caps and no charged side chains has no isoelectric point at all.
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Reference tools for laboratory research. Not for human or animal use.