Calculate the isoelectric point (pI) of amino acids, peptides and proteins from their sequence. Includes net charge analysis and step-by-step calculations.
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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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.
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.
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.
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.
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.
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.
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.
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.