Isoelectric Point (pI) Calculator

About the Isoelectric Point (pI) Calculator

The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. At this pH, the positive charges from protonated groups exactly balance the negative charges from deprotonated groups. For amino acids and proteins, the pI determines behavior in electrophoresis, ion exchange chromatography, solubility, and crystallization.

For a simple amino acid with only α-amino and α-carboxyl groups, pI is the average of the two pKa values: pI = (pKa1 + pKa2) / 2. For amino acids with ionizable side chains (Asp, Glu, Cys, Tyr, His, Lys, Arg), the pI is the average of the two pKa values that bracket the zwitterionic form. Selecting the correct pair of pKa values requires understanding which protonation states carry zero net charge.

For peptides and proteins, the calculation is more complex because multiple ionizable groups contribute to the net charge. The net charge at any pH can be estimated using the Henderson-Hasselbalch equation applied to each ionizable group. The pI is found by iteratively adjusting pH until the net charge is zero. Protein pI values typically range from 4 to 12, and are critical for designing purification strategies using ion exchange chromatography and isoelectric focusing.

When This Page Helps

Essential for biochemistry students, researchers, and protein chemists who need to determine pI for electrophoresis, chromatography, and protein purification. Understand amino acid charge states at any pH.

How to Use the Inputs

1. Select an amino acid preset to load standard pKa values.
2. Or enter custom pKa values for acidic and basic groups.
3. View the calculated pI (isoelectric point).
4. Examine the net charge at various pH values in the charge curve.
5. For peptides, add multiple ionizable groups from the side chain table.
6. Use the reference table to compare pI values across all amino acids.
7. Explore how charge changes near the pI affect electrophoretic mobility.

Example Calculation

Tips & Best Practices

• For amino acids with no ionizable side chain, pI is always between pKa1 (COOH) and pKa2 (NH₃⁺).
• Acidic amino acids (Asp, Glu) have low pI values (2.8-3.2) and are negatively charged at physiological pH.
• Basic amino acids (Lys, Arg, His) have high pI values (7.6-10.8) and are positively charged at pH 7.4.
• Most proteins have pI between 5 and 7, making them slightly negatively charged at physiological pH.
• The pI of a protein can be shifted by post-translational modifications (phosphorylation, glycosylation, acetylation).
• Online tools like ExPASy ProtParam can estimate protein pI from the amino acid sequence.

Amino Acid pKa Values and Side Chain Chemistry

The 20 standard amino acids have α-carboxyl pKa values ranging from 1.82 (Asp) to 2.83 (Thr) and α-amino pKa values from 8.80 (Asp) to 10.60 (Pro). Seven amino acids have ionizable side chains: Asp (3.65), Glu (4.25), His (6.00), Cys (8.18), Tyr (10.07), Lys (10.53), and Arg (12.48). These side chain pKa values are "intrinsic" values — in a folded protein, the actual pKa can differ significantly due to the local electrostatic environment, solvent exposure, and hydrogen bonding networks.

Protein pI Prediction and Applications

For peptides and proteins, pI is calculated by summing the charge contributions of all ionizable groups at each pH until net charge = 0. The accuracy of prediction depends on using appropriate pKa values. For unfolded polypeptides, intrinsic pKa values work well. For folded proteins, computational methods like PROPKA or H++ use structural information to predict shifted pKa values. Two-dimensional gel electrophoresis (2D-GE) separates proteins first by pI (isoelectric focusing) and then by molecular weight (SDS-PAGE), creating a "protein map" of the cell.

Industrial and Pharmaceutical Applications

In monoclonal antibody manufacturing, pI is a critical quality attribute. IgG antibodies typically have pI values of 6-9, and charge heterogeneity (from deamidation, C-terminal lysine clipping, or sialylation) creates charge variants that are separated by ion exchange or capillary isoelectric focusing. The drug's pI affects pharmacokinetics: highly charged antibodies can have altered tissue distribution and clearance rates. In food science, the pI of casein (~4.6) is exploited in cheese making — acidification to pH 4.6 precipitates casein, forming curds.

Frequently Asked Questions

• What is a zwitterion?

  A zwitterion is a molecule with both positive and negative charges but zero net charge. At the pI, amino acids exist primarily as zwitterions: the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻).

• How do you pick which pKa values to average?

  Average the two pKa values that bracket the zero-charge (zwitterionic) species. For acidic amino acids (Asp, Glu), average α-COOH pKa and side chain COOH pKa. For basic amino acids (Lys, Arg, His), average α-NH₃⁺ pKa and side chain pKa.

• Why is protein pI important for purification?

  In ion exchange chromatography, proteins bind to charged resins at pH values away from their pI. At pH < pI, proteins are positively charged (bind cation exchangers). At pH > pI, they are negatively charged (bind anion exchangers). Knowing pI guides column selection.

• What is isoelectric focusing?

  Isoelectric focusing (IEF) is an electrophoresis technique where proteins migrate through a pH gradient until they reach their pI, where their net charge is zero and they stop moving. It provides extremely high resolution separation based on pI differences as small as 0.01 pH units.

• Does protein pI equal the sum of amino acid pI values?

  No. Protein pI depends on all ionizable groups in the folded protein. The local environment (buried vs. surface, nearby charges, hydrogen bonds) can shift individual pKa values by several pH units from their intrinsic values.

• What affects solubility at the pI?

  Proteins are generally least soluble at their pI because they lack net charge, reducing electrostatic repulsion between molecules. This promotes aggregation and precipitation. "Salting out" near the pI is a classic purification technique.