---
title: "What does circular dichroism tell you about a peptide? | Sable Peptides"
description: "How circular dichroism (CD) spectroscopy estimates α-helix, β-sheet and disordered content of peptides in solution, the characteristic far-UV signatures, analysis software, and practical sample limits. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/circular-dichroism-for-peptide-secondary-structure
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# What does circular dichroism tell you about a peptide?

Circular dichroism (CD) measures how a peptide absorbs left- and right-circularly polarised light. In the far UV (190–250 nm) the backbone conformation produces characteristic shapes: α-helices show minima at 208 and 222 nm, β-sheets a minimum near 218 nm, and disordered peptides a minimum near 198 nm. CD estimates secondary structure, not sequence.

## What are the characteristic CD signatures?

| Structure | Far-UV features |
| --- | --- |
| α-helix | Positive ~192 nm; negative 208 and 222 nm |
| β-sheet | Positive ~195 nm; negative ~218 nm |
| Random coil / disordered | Negative ~198 nm; weak near 220 nm |
| Polyproline II | Negative ~206 nm; weak positive ~228 nm |

## How is secondary structure estimated from a spectrum?

Deconvolution software fits the spectrum against reference sets; BeStSel, for example, distinguishes parallel and antiparallel β-structure and can suggest a fold class. Difference CD compares two conditions to isolate a change, such as binding or a modification.

Source: [BeStSel: secondary structure analysis and fold prediction by CD, Methods Mol Biol 2021 (PubMed 33125651)](https://pubmed.ncbi.nlm.nih.gov/33125651/); [Structure analysis of proteins and peptides by difference CD, Protein J 2021 (PubMed 34709521)](https://pubmed.ncbi.nlm.nih.gov/34709521/)

## How is CD used in quality comparison?

Comparing CD (and NMR) of different sources of the same peptide shows whether they adopt the same structure in solution; a 2022 study did this for glucagon products. A matching spectrum supports, but does not prove, equivalence.

Source: [Secondary structure characterisation of glucagon products by CD and NMR, Molecules 2022 (PubMed 36431905)](https://pubmed.ncbi.nlm.nih.gov/36431905/)

## What are the practical limits?

-   Buffers and salts that absorb below 200 nm (chloride, high-concentration phosphate) limit the usable range.
-   Concentration must be known accurately to report mean residue ellipticity.
-   Short peptides are often largely disordered in water; helix-promoting solvents change this.
-   CD says nothing about chemical purity.

## How does Sable document lots?

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.

## Frequently asked questions

### What CD signal indicates an α-helix?

Negative bands at about 208 and 222 nm with a positive band near 192 nm.

### Can CD confirm a peptide's sequence?

No. It reports average secondary structure, not sequence or purity.

### Why avoid chloride buffers in CD?

Chloride absorbs strongly below about 200 nm, obscuring the far-UV spectrum.

## Related

-   [Size-exclusion chromatography](/guides/size-exclusion-chromatography-for-peptide-aggregates)
-   [NMR and qNMR](/guides/nmr-for-peptide-identity-and-qnmr-purity)
-   [Peptide bonds and structure](/learn/peptide-bonds-and-structure)
-   [UV concentration](/guides/uv-absorbance-for-peptide-concentration-a280-and-a205)

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)

-   α-helix: −208/−222 nm; β-sheet: −218 nm; coil: −198 nm
-   CD = structure, not purity
-   References: PubMed 33125651, 34709521, 36431905
-   Updated 2026-10-16