Isoelectric Point Calculator

The Isoelectric Point (pI) of a protein is the pH at which it carries no net electrical charge — a critical value in protein purification, gel electrophoresis, and solubility studies. Paste your protein or peptide sequence (single-letter amino acid codes or FASTA format) into the Isoelectric Point Calculator, then set your pH range and resolution to get your pI. Secondary outputs include molecular weight, sequence length, net charge at pH 7, and counts of positive and negative residues.

Use single letter amino acid codes (A, C, D, E, F, G, H, I, K, L, M, N, P, Q, R, S, T, V, W, Y)

Starting pH for calculation range

Ending pH for calculation range

Step size for pH calculations (smaller = more accurate)

Display charge vs pH curve

Results

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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Frequently Asked Questions

What is the isoelectric point of a protein?

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.

How do you calculate the isoelectric point of peptides and amino acids?

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.

Why is the isoelectric point important in biochemistry?

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.

What amino acids affect the isoelectric point calculation?

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.

How accurate are isoelectric point calculations?

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.

Can this calculator handle modified amino acids?

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.

What is the difference between theoretical and experimental pI?

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.