Isoelectric Point (pI) Calculator

The isoelectric point (pI) is the pH at which a protein or amino acid carries no net electrical charge — a critical value in protein purification, gel electrophoresis, and drug formulation. Select Single amino acid from the Calculation Mode and choose from the dropdown, or select Peptide or protein sequence and paste your sequence in single-letter code to calculate the Isoelectric Point (pI). Secondary outputs include Molecular Weight, Net Charge at pH 7.0, Total Residues, and an optional net charge vs pH chart.

Calculation Mode *

Use single letter amino acid codes. Non-amino acid characters will be ignored.

Results

Isoelectric Point (pI)

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Molecular Weight

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Net Charge at pH 7.0

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Total Residues

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Frequently Asked Questions

What is the isoelectric point (pI) of a protein?

The isoelectric point is the pH at which a protein carries no net electrical charge. At this pH, the positive and negative charges on the protein are balanced, making it electrically neutral.

How is the pI calculated for simple amino acids?

For neutral amino acids, pI is approximated as the average of the pKa values of the carboxyl group (COOH) and amino group (NH3+). The formula is: pI = (pKa,COOH + pKa,NH3+) / 2.

Why is knowing the pI important in biochemistry?

The pI is crucial for protein purification techniques like isoelectric focusing, 2D gel electrophoresis, and chromatography. It also affects protein solubility, stability, and interactions at different pH values.

How does the calculator handle peptides and proteins?

For peptides and proteins, the calculator considers all ionizable groups including the N-terminus, C-terminus, and ionizable side chains (Asp, Glu, Cys, Tyr, His, Lys, Arg). It calculates the net charge across different pH values to find where it crosses zero.

What amino acid sequence formats are accepted?

The calculator accepts single letter amino acid codes in plain text or FASTA format. It automatically removes non-amino acid characters and works with both uppercase and lowercase letters.

How accurate are the pI predictions?

The calculator uses established pKa values for amino acids and provides good theoretical estimates. However, actual pI values may vary due to protein structure, salt concentration, and other environmental factors.

What is the net charge vs pH chart showing?

The chart displays how the net charge of the protein changes with pH. The point where the line crosses zero (net charge = 0) represents the isoelectric point.

Can I calculate pI for modified amino acids or non-standard residues?

This calculator uses standard amino acids only. Modified amino acids, post-translational modifications, or non-standard residues are not included in the calculations and may affect the actual pI value.