pI Calculator

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Isoelectric Point Calculator

Calculate the pI of amino acids and proteins with ease

The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. This tool helps you determine the pI for amino acids and proteins based on their ionizable groups.

How to Use:

  • Select the type of calculation (Amino Acid or Protein)
  • Enter your sequence or select amino acids
  • Adjust pKa values if needed (defaults are provided)
  • Click "Calculate pI" to get results
  • View the charge vs. pH graph and detailed information

Amino Acid pI Calculator

pKa Adjustments
Results

Select an amino acid and click "Calculate pI" to see results.

Protein pI Calculator

Enter single-letter amino acid codes (A-Z, no spaces or numbers)
pKa Adjustments
Results

Enter a protein sequence and click "Calculate pI" to see results.

Amino Acid Composition

Amino acid composition will appear here after calculation.

Advanced pI Calculator

Advanced Parameters
Advanced Results

Enter a sequence and adjust parameters to see advanced results.

Help & Information

About Isoelectric Point (pI)

The isoelectric point (pI) is the pH at which a particular molecule carries no net electrical charge. For amino acids and proteins, the pI depends on the pKa values of their ionizable groups.

How to Use This Calculator

This tool provides three calculation modes:

  1. Amino Acid pI: Calculate the pI of individual amino acids
  2. Protein pI: Calculate the pI of protein sequences
  3. Advanced pI: Calculate pI with environmental factors

Key Concepts

The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. At this pH, the molecule is least soluble in water and may precipitate.

For amino acids with two ionizable groups, pI = (pKa1 + pKa2)/2. For proteins, the pI is calculated by finding the pH where the sum of positive charges equals the sum of negative charges.

pI is crucial for protein purification (e.g., isoelectric focusing), predicting solubility, and understanding protein behavior in different pH environments.

Default pKa Values

Group pKa
N-terminal (α-amine) 9.69
C-terminal (α-carboxyl) 2.34
Aspartic acid (D) 3.90
Glutamic acid (E) 4.07
Histidine (H) 6.04
Cysteine (C) 8.37
Tyrosine (Y) 10.46
Lysine (K) 10.54
Arginine (R) 12.48
Quick Tips
  • Use single-letter codes for protein sequences
  • Adjust pKa values for different conditions
  • Check the charge vs. pH graph for behavior
  • Save important calculations for reference
  • Use the advanced mode for precise calculations
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