Enter your protein or peptide sequence and set pH range and resolution to calculate the isoelectric point (pI). You'll also get molecular weight, sequence length, net charge at pH 7, positive residues, and an optional titration curve.
Isoelectric Point (pI)
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Molecular Weight
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Sequence Length
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Net Charge at pH 7
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Positive Residues
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Negative Residues
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The isoelectric point (pI) is the pH at which a protein or peptide carries no net electrical charge. At this pH, the positive and negative charges on the molecule are balanced, making it electrically neutral.
The pI is calculated by determining the pH where the sum of all positive and negative charges equals zero. This involves considering the pKa values of ionizable groups (amino, carboxyl, and side chains) and using the Henderson-Hasselbalch equation.
The pI is crucial for protein separation techniques like isoelectric focusing, 2D gel electrophoresis, and chromatography. It also affects protein solubility, stability, and interactions with other molecules.
Charged amino acids significantly impact pI: basic residues (lysine, arginine, histidine) increase pI, while acidic residues (aspartate, glutamate) decrease it. The N and C-terminus also contribute charges.
Theoretical pI calculations are generally accurate within ±0.1-0.5 pH units for most proteins. Accuracy depends on the algorithm used and whether post-translational modifications are considered.
This calculator uses standard amino acid pKa values. For modified amino acids or post-translational modifications, experimental determination or specialized software may be needed for accurate results.
Theoretical pI is calculated from amino acid sequence and standard pKa values. Experimental pI may differ due to protein folding, modifications, or environmental conditions that affect ionizable groups.