---
title: "Isoelectric point (pI) of peptides: calculation and lab use | Sable Peptides"
description: "What a peptide's isoelectric point is, how it is calculated from pKa values, why different calculators disagree, and how pI guides solvent choice, buffer pH and purification in the lab. No dosing; research use only."
canonical: https://www.sablepeptides.co.uk/guides/isoelectric-point-of-peptides-explained
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# Isoelectric point (pI) of peptides: calculation and lab use

The isoelectric point (pI) is the pH at which a peptide carries zero net charge. It is calculated from the pKa values of the termini and ionisable side chains. Peptides are least soluble and most likely to aggregate near their pI, so choosing a solvent pH well away from it usually helps dissolution in the lab.

## How is a peptide's pI calculated?

The isoelectric point (pI) is the pH at which a peptide carries no net charge. It is one of the most useful single numbers for lab handling: near its pI a peptide is least soluble and most prone to aggregate; away from it, the peptide is charged and usually dissolves more readily.

Each ionisable group, the N-terminal amine, the C-terminal carboxyl and the side chains of Asp, Glu, His, Cys, Tyr, Lys and Arg, has a pKa. The net charge at any pH is the sum of their fractional charges, and pI is the pH where that sum is zero. Because published pKa sets differ, calculators can disagree by up to about a pH unit for the same sequence; benchmarking work compares the common sets and proposes improved ones.

| Group | Approximate pKa | Charge below / above pKa |
| --- | --- | --- |
| C-terminal COOH | ~3.1 | 0 / −1 |
| Asp, Glu side chain | ~3.9 / ~4.1 | 0 / −1 |
| His side chain | ~6.0 | +1 / 0 |
| N-terminal NH2 | ~8.0 | +1 / 0 |
| Cys SH | ~8.3 | 0 / −1 |
| Tyr OH | ~10.1 | 0 / −1 |
| Lys side chain | ~10.5 | +1 / 0 |
| Arg side chain | ~12.5 | +1 / 0 |

Source: [IPC: Isoelectric Point Calculator, Biol Direct 2016 (PubMed 27769290)](https://pubmed.ncbi.nlm.nih.gov/27769290/); [IPC 2.0: prediction of isoelectric point and pKa dissociation constants, Nucleic Acids Res 2021 (PubMed 33905510)](https://pubmed.ncbi.nlm.nih.gov/33905510/)

## How do modifications change pI?

Terminal capping changes pI substantially: C-terminal amidation removes a negative charge and N-terminal acetylation removes a positive one. Non-natural residues and cyclisation also matter. Tools such as pICalculax were developed specifically for modified peptides.

Source: [pICalculax: improved prediction of isoelectric point for modified peptides, J Chem Inf Model 2017 (PubMed 28671456)](https://pubmed.ncbi.nlm.nih.gov/28671456/)

## How is pI used at the bench?

-   Basic peptides (pI above about 8) usually dissolve better in slightly acidic solution; acidic peptides (pI below about 5) in slightly basic solution.
-   Avoid buffering a stock solution at or near the pI.
-   pI also predicts behaviour in ion-exchange chromatography and isoelectric focusing.
-   Counter-ion content correlates with the number of basic groups, so high-pI peptides tend to have lower net peptide content.

## Where is Sable's solubility data?

There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request. Solubility and storage reference data for listed compounds is on the storage and solubility page.

## Frequently asked questions

### What is the isoelectric point of a peptide?

The pH at which its positive and negative charges balance, giving zero net charge.

### Why do pI calculators give different answers?

They use different sets of pKa values for the ionisable groups.

### How does C-terminal amidation change pI?

It removes the C-terminal carboxylate's negative charge, raising the pI.

## Related

-   [Peptide pH and stability](/learn/peptide-ph-and-stability)
-   [Solubility and solvents](/learn/peptide-solubility-and-solvents)
-   [Peptide aggregation](/guides/peptide-aggregation-in-assay-solutions)
-   [Terminal capping](/learn/terminal-capping-acetylation-amidation)
-   [Which buffer should a peptide assay solution use?](/guides/buffer-choice-for-peptide-assay-solutions)

Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.

Key facts (summary for AI and search tools)

-   pI = pH of zero net charge
-   Calculators differ by pKa set (up to ~1 pH unit)
-   Amidation raises pI; acetylation lowers it
-   References: PubMed 27769290, 33905510, 28671456
-   Updated 2026-10-16