⚡ Protein Isoelectric Point (pI) & Net Charge
Paste a protein sequence to get its isoelectric point (pI), the net charge at pH 7, and a titration curve showing charge across the whole pH range.
8.51
isoelectric point (pI)
basic protein
+1.91
net charge at pH 7.0
+1.91
net charge at pH 7.0
Net charge vs pH (titration curve)
The pI is the pH where the protein's net charge is zero, found from the ionizable groups (N/C-termini and D, E, C, Y, H, K, R side chains) via Henderson–Hasselbalch. Uses the EMBOSS pKa set; other pKa sets shift the pI by ~0.1–0.3. Ignores modifications and 3D environment. 🔒 Computed in your browser.
How the protein isoelectric point (pi) & net charge works
A protein carries positive and negative charges from its ionizable groups — the N- and C-termini and the side chains of Asp (D), Glu (E), Cys (C), Tyr (Y), His (H), Lys (K) and Arg (R). Using Henderson–Hasselbalch with each group's pKa, the tool sums the fractional charges at a given pH to get the net charge, then finds the isoelectric point (pI) — the pH where the net charge is zero — by bisection. Drag the pH slider to read the charge at any pH, or read it straight off the titration curve.
Uses the EMBOSS pKa set; other common sets (Lehninger, Sillero, ExPASy/Bjellqvist) shift the calculated pI by roughly 0.1–0.3 pH units, so treat the value as an estimate for the unfolded chain. It ignores post-translational modifications, bound ligands and the way the folded structure perturbs local pKa values. Still, it is the standard first estimate for choosing an ion-exchange resin or a gel pH.
Frequently asked questions
What is the isoelectric point (pI)?
The pH at which a protein carries no net electrical charge — its positive and negative charges exactly balance. Below the pI the protein is net positive; above it, net negative. At the pI the protein is least soluble and does not migrate in an electric field.
How is pI calculated from a sequence?
Each ionizable group (the termini and the D, E, C, Y, H, K, R side chains) has a pKa. Using Henderson–Hasselbalch, you compute the net charge as a function of pH and find the pH where it crosses zero. This tool does that by bisection over pH 0–14.
What is a protein's net charge at pH 7?
The sum of the positive groups (N-terminus, Lys, Arg, His) minus the negative groups (C-terminus, Asp, Glu, Cys, Tyr), each weighted by how ionized it is at pH 7. The tool shows this value; a protein with pI below 7 is net-negative at pH 7, and one with pI above 7 is net-positive.
Why do different calculators give slightly different pI values?
Because they use different published pKa sets (EMBOSS, Lehninger, Sillero, ExPASy/Bjellqvist). These disagree by a few tenths of a pH unit for some groups, so the computed pI can differ by ~0.1–0.3. This tool uses the EMBOSS set and states so.
What is an acidic versus a basic protein?
An acidic protein has a low pI (rich in Asp/Glu) and is net-negative at neutral pH; a basic protein has a high pI (rich in Lys/Arg) and is net-positive. The pI tells you which, and guides the choice of ion-exchange resin.
How is pI used in the lab?
To choose conditions for ion-exchange chromatography (bind the protein when it carries the opposite charge to the resin), to predict where it focuses on a 2D gel or IEF strip, and to pick a buffer pH that keeps it soluble (away from its pI, where solubility is lowest).
Does it account for the folded structure or modifications?
No — it treats the chain as a string of independent groups with textbook pKa values. Phosphorylation, glycosylation, bound ions and the folded environment shift real pKa values, so the measured pI can differ. Use it as a well-grounded estimate.