Protein Isoelectric Point Calculator

Calculate the isoelectric point (pI) of proteins using accurate Henderson-Hasselbalch equation and binary search algorithm

Protein Charge Analysis

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Isoelectric Point (pI)
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Acidic Residues
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Basic Residues
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Net Charge at pI

Net Charge vs pH

pHNet ChargeCharge State

What is Protein Isoelectric Point (pI)?

The isoelectric point (pI) is the pH at which a protein has no net electrical charge. At this pH, the protein will not migrate in an electric field and is least soluble in solution. pI calculation is essential for protein purification, crystallization, and understanding protein behavior in different pH environments.

How to Use This pI Calculator

Calculate protein isoelectric point in three simple steps:

  1. Enter your protein sequence using single-letter amino acid codes
  2. View the calculated pI and charge analysis automatically
  3. Examine the detailed charge vs pH table for complete analysis

When to Use pI Calculation

This tool is essential when you need to:

  • Design protein purification strategies using isoelectric focusing
  • Predict protein solubility at different pH conditions
  • Optimize crystallization conditions for X-ray crystallography
  • Understand protein charge distribution for drug design
  • Plan electrophoretic separation experiments

Example Input

Histone H3 protein sequence (basic protein):

MARTKQTARKSTGGKAPRKQLATKAARKSAPATGGVKKPHRYRPGTVALREIRRYQKSTELLIRKLPFQRLVREIAQDFKTDLRFQSSAVMALQEACEAYLVGLFEDTNLCAIHAKRVTIMPKDIQLARRIRGERA

This basic protein has a high pI due to many lysine and arginine residues.

Example Output

Expected analysis results:

Isoelectric Point (pI): 11.27 Acidic residues: 8 (D, E) Basic residues: 22 (K, R, H) Net charge at pI: 0.00

Includes detailed charge vs pH analysis table.

FAQ

Q: What pKa values are used for calculations?
A: Standard biochemical pKa values: D=3.86, E=4.25, H=6.00, C=8.33, Y=10.07, K=10.53, R=12.48, N-term=9.69, C-term=2.34.

Q: How accurate is the Henderson-Hasselbalch calculation?
A: Very accurate for most proteins. Uses binary search with 0.01 pH tolerance for precise pI determination.

Q: Can I analyze protein fragments or peptides?
A: Yes, the tool works with any protein sequence length from peptides to full proteins.