Protein Isoelectric Point Calculator (pI by Composition)

Protein Isoelectric Point Calculator (pI by Composition) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

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.

{"sequence":"GIVEQCCTSICSLYQLENYCN","decimalPlaces":2}

Outcome

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.

{"sequence":"RPPGFSPFR","decimalPlaces":2}

Run this tool

Examples that match this tool

Continue with connected tools and hubs

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Insulin A-chain: acidic theoretical pI

2. Bradykinin: highly basic theoretical pI

FAQ

Web Image Processing Python Samples

Android Image Processing Java Samples

Android Image Processing Kotlin Samples

Web Image Processing Rust Samples

Absorbance↔Transmittance Converter (A = −log₁₀T)

Enzyme Activity Unit Converter (U ↔ katal)

Molar Extinction Coefficient & Beer-Lambert (A = ε·c·l)

OD600 Cell Density Calculator (E. coli)

Protein Quantification, Enzyme Kinetics, and Sequence Calculators