Isoelectric Point (pI) Calculator
Determine the isoelectric point (pI) of a peptide or protein sequence by calculating net electrical charge across pH 1-14.
Peptide Sequence Entry
Enter 1-letter amino acid codes for your target peptide/protein.
Isoelectric Point (pI)
Net Charge at Physiological pH 7.40
Ion Exchange Chromatography Tip
If buffer pH < pI, protein is positively charged (binds cation exchangers like SP-Sepharose). If buffer pH > pI, protein is negatively charged (binds anion exchangers like Q-Sepharose).
Standard Ionizable Residue pKa Values (EMBOSS Scale) Reference
| Residue / Terminal Group | Single Letter | Side Chain Type | Standard pKa Value |
|---|---|---|---|
| N-Terminal Amine | N-term | Basic (Positive) | 9.60 |
| C-Terminal Carboxyl | C-term | Acidic (Negative) | 2.34 |
| Lysine / Arginine / Histidine | K / R / H | Basic (Positive) | K: 10.50 | R: 12.40 | H: 6.00 |
| Aspartate / Glutamate / Cys / Tyr | D / E / C / Y | Acidic (Negative) | D: 3.90 | E: 4.20 | C: 8.30 | Y: 10.10 |
Calculation Methodology & Equations
Net Charge Sum Equation
Henderson-Hasselbalch ionization ftions are summed across all positive (N-term, K, R, H) and negative (C-term, D, E, C, Y) groups.
Bisection Algorithm
Iterative binary search between pH 1.0 and 14.0 converges on exact zero net-charge pH.
Frequently Asked Questions
What is the Isoelectric Point (pI) of a protein? ▼
The isoelectric point (pI) is the specific pH at which the net electrical charge of a protein or peptide molecule is exactly zero.
Why is protein solubility lowest at its pI? ▼
At its pI, net electrostatic repulsive forces between protein molecules vanish, allowing hydrophobic intetions to cause aggregation and precipitation.
Which amino acid side chains contribute to net charge calculations? ▼
Seven amino acids have ionizable side chains: Lysine (K, pKa ~10.5), Arginine (R, pKa ~12.4), Histidine (H, pKa ~6.0), Aspartate (D, pKa ~3.9), Glutamate (E, pKa ~4.2), Cysteine (C, pKa ~8.3), and Tyrosine (Y, pKa ~10.1), in addition to the N-terminal amine (pKa ~9.6) and C-terminal carboxyl (pKa ~2.34).
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