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); Structure analysis of proteins and peptides by difference CD, Protein J 2021 (PubMed 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.
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
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