Protein Isoelectric Point Calculator (pI by Composition) | Online Free Tool | Elysia Tools
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Protein Isoelectric Point Calculator (pI by Composition)
Theoretical pI via the simplified ExPASy/Bjellqvist pKa set with bisection to zero net charge, plus net charge at pH 7.4, ProtParam average molecular weight, and charged composition. Bjellqvist 1993 / Gasteiger 2003 derived. Educational use only.
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Key facts
Category
Education
Input types
textarea, number
Output type
json
Sample coverage
4
API ready
Yes
Overview
Calculate a protein’s theoretical isoelectric point (pI) from its amino acid sequence, along with net charge at pH 7.4 and average molecular weight. The calculator uses a simplified ExPASy/Bjellqvist pKa set and is intended for educational use.
When to use
Estimate whether a protein sequence is acidic or basic from its theoretical pI.
Check the approximate net charge of a protein at pH 7.4.
Calculate the average molecular weight of a sequence before planning educational or laboratory workflows.
How it works
1Enter a protein sequence using the one-letter codes for the 20 standard amino acids.
2FASTA headers, whitespace, and asterisks are removed before calculation.
3The calculator estimates net charge for ionizable groups using the simplified ExPASy/Bjellqvist pKa values.
4It uses bisection to find the pH where the calculated net charge reaches zero, then reports pI, pH 7.4 charge, and average molecular weight.
Use cases
Classify protein sequences as relatively acidic or basic using their theoretical pI.
Compare peptide or protein charge behavior at pH 7.4.
Estimate sequence-based molecular weight for molecular biology coursework and early experiment planning.
Examples
1. Insulin A-chain: acidic theoretical pI
Molecular biology student
Background
A student is reviewing the charge properties of the 21-residue human insulin A-chain sequence.
Problem
They want to estimate its theoretical pI, physiological-pH charge, and average molecular weight.
How to use
Enter `GIVEQCCTSICSLYQLENYCN` and leave Decimal Places set to 2.
The calculator returns pI ≈ 3.79, net charge −2.54 at pH 7.4, and average molecular weight 2383.71 Da, indicating an acidic sequence.
2. Bradykinin: highly basic theoretical pI
Biochemistry learner
Background
A learner is examining the charge of the nine-residue peptide bradykinin, `RPPGFSPFR`.
Problem
They need a quick theoretical estimate of the peptide’s pI and charge at pH 7.4.
How to use
Enter `RPPGFSPFR` with Decimal Places set to 2.
FAQ
What sequence format does the calculator accept?
Enter a sequence using one-letter codes for the 20 standard amino acids. FASTA headers, whitespace, and asterisks are stripped.
What does the pI result mean?
The pI is the estimated pH at which the protein’s calculated net charge is zero.
What additional results are provided?
The calculator also reports net charge at pH 7.4, average molecular weight in daltons, and an acidic or basic interpretation.
Can I change the number of decimal places?
Yes. Set Decimal Places from 0 to 8; the default is 2.
Are the results suitable for experimental decisions?
They are theoretical estimates based on an unfolded-chain pKa model and exclude post-translational modifications, so they are intended for education and preliminary planning.