--- title: "Measuring peptide concentration by UV: A280 vs A205 | Sable Peptides" description: "How laboratories estimate peptide concentration in solution from UV absorbance: the A280 method for Trp/Tyr-containing peptides, the A205 method for peptides without them, extinction coefficients and the common errors. No dosing; research use only." canonical: https://www.sablepeptides.co.uk/guides/uv-absorbance-for-peptide-concentration-a280-and-a205 dateModified: 2026-10-16 publisher: Sable Peptides Limited (UK company 17488610) note: Research use only. Not for human or veterinary use. --- # Measuring peptide concentration by UV: A280 vs A205 Peptide concentration in solution can be estimated from UV absorbance using Beer–Lambert. At 280 nm the signal comes from tryptophan and tyrosine, so the method only suits peptides that contain them. At 205 nm the peptide bond absorbs, so it works for almost any peptide but is far more sensitive to buffer interference. ## How does the A280 method work? Once a peptide is in solution, the quickest way to check its concentration is UV absorbance. Which wavelength works depends on the sequence: 280 nm needs tryptophan or tyrosine; 205 nm uses the peptide bond itself and works for almost any peptide, with more caveats. At 280 nm, absorbance comes almost entirely from tryptophan (molar extinction coefficient about 5,500 M⁻¹cm⁻¹), tyrosine (about 1,490) and, slightly, cystine (about 125). Summing these for the sequence gives a calculated ε280, and Beer–Lambert (A = ε·c·l) gives concentration. The method was set out by Pace and colleagues in 1995 and is still the basis of most online calculators. | | A280 | A205 | | --- | --- | --- | | Chromophore | Trp, Tyr, cystine | Peptide bond (+ side chains) | | Works for | Peptides with Trp/Tyr | Almost all peptides | | Sensitivity | Moderate | High | | Buffer interference | Low | High (acetate, TFA, DMSO) | | Reference | Pace et al. 1995 | Anthis & Clore 2013 | Source: [How to measure and predict the molar absorption coefficient of a protein, Protein Sci 1995 (PubMed 8563639)](https://pubmed.ncbi.nlm.nih.gov/8563639/) ## When should A205 be used instead? Many short research peptides contain no Trp or Tyr and give essentially no A280 signal. At 205 nm the peptide backbone absorbs strongly, and a sequence-specific method predicts ε205 from the composition, including side-chain contributions. It is much more sensitive but also much more affected by buffers, salts and organic solvents that absorb at low wavelengths. Source: [Sequence-specific determination of protein and peptide concentrations by absorbance at 205 nm, Protein Sci 2013 (PubMed 23526461)](https://pubmed.ncbi.nlm.nih.gov/23526461/) ## What are the common errors? - Using gross powder weight to 'check' a UV result: the difference is often net peptide content, not an assay error. - Buffers with absorbance at 205 nm (acetate, TFA, DMSO, many detergents); blank with the exact solvent. - Light scattering from aggregates inflates apparent absorbance; check A320–350. - Cuvette path length and stray light at absorbances above about 1.5. ## When is UV the wrong method? For peptides with neither aromatic residues nor a clean A205 background, quantitative amino acid analysis or HPLC against a reference standard is more reliable. Reviews of RP-HPLC quantitation describe how detector response can be used without a full standard curve. Source: [Approaches towards quantitative analysis of peptides and proteins by RP-HPLC, J Chromatogr A 2000 (PubMed 11043783)](https://pubmed.ncbi.nlm.nih.gov/11043783/) ## How does this relate to Sable's CoAs? 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. The concentration calculator gives the arithmetic from mass and volume; UV is how you check it. ## Frequently asked questions ### Can I measure every peptide at 280 nm? No. Only peptides containing tryptophan or tyrosine (or cystine) absorb meaningfully at 280 nm. ### Why is A205 more error-prone? Many buffers, salts and solvents also absorb at 205 nm, so the blank must match exactly. ### Why might UV concentration be lower than expected from the weighed mass? Lyophilised peptides contain water and counter-ions, so net peptide content is below gross weight. ## Related - [Concentration calculator](/guides/reconstitution-concentration-calculator) - [Amino acid analysis](/guides/amino-acid-analysis-for-peptide-content) - [Purity vs net peptide content](/guides/peptide-purity-vs-net-peptide-content) - [Peptide aggregation](/guides/peptide-aggregation-in-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) - ε280 contributions: Trp ~5,500; Tyr ~1,490; cystine ~125 M⁻¹cm⁻¹ - A205 method: Anthis & Clore 2013 - References: PubMed 8563639, 23526461, 11043783 - Updated 2026-10-16