Peptide Extinction Coefficient Calculator
Enter a single-letter amino acid sequence to estimate its molar extinction coefficient at 280nm (ε280) from tryptophan, tyrosine, and cystine content, using the Edelhoch/Pace method.
Single-letter amino acid codes only (A–Z, standard 20).
Results
- Trp
- —
- Tyr
- —
- Cys
- —
Reduced (no disulfides)
- ε(280)
- —
- A(0.1%)
- —
Disulfide-assumed
- ε(280)
- —
- A(0.1%)
- —
How this calculator works
ε280 = (Trp count × 5500) + (Tyr count × 1490) + (cystine count × 125), in M⁻¹cm⁻¹. Since a linear sequence alone doesn't say which cysteines are disulfide-bonded, this calculator shows both a “reduced” value (no disulfides) and a “disulfide-assumed” value (every cysteine paired). A(0.1%) is ε280 divided by the average molecular weight — the absorbance of a 1 mg/mL solution at a 1 cm path length.
How to calculate a peptide's extinction coefficient
- Count the number of tryptophan (Trp) residues in the sequence.
- Count the number of tyrosine (Tyr) residues.
- Count the number of cystine (paired cysteine) residues, if any are disulfide-bonded.
- Multiply each count by its coefficient — 5,500 for Trp, 1,490 for Tyr, 125 for cystine — and sum the results.
Worked example: a peptide with 1 tryptophan, 2 tyrosines, and no disulfide bonds (reduced) has ε280 = (1 × 5,500) + (2 × 1,490) + 0 = 8,480 M⁻¹cm⁻¹.
Frequently asked questions
What is a peptide's extinction coefficient?
The molar extinction coefficient (ε280) measures how strongly a peptide absorbs UV light at 280nm, in M⁻¹cm⁻¹. It's used with the Beer-Lambert law to estimate a peptide's concentration from a measured absorbance reading, without needing a separate standard curve.
How do I calculate ε280 for a peptide?
Count the tryptophan (Trp), tyrosine (Tyr), and cystine (paired cysteine) residues in the sequence. Multiply Trp count by 5,500, Tyr count by 1,490, and cystine count by 125, then add the three results together — this is the Edelhoch/Pace method.
What's the difference between the "reduced" and "disulfide-assumed" values?
A linear sequence alone doesn't say which cysteines, if any, are paired into disulfide bonds. The "reduced" value assumes no disulfides; the "disulfide-assumed" value assumes every cysteine is paired. The true value depends on the peptide's actual folded structure.
What is A(0.1%) and why does it matter?
A(0.1%) is ε280 divided by the average molecular weight — the absorbance a 1 mg/mL solution would show at a 1 cm path length. Dividing a measured absorbance reading by A(0.1%) gives you the sample's concentration directly in mg/mL.
Does this account for other UV-absorbing groups?
No — the Edelhoch/Pace method only accounts for tryptophan, tyrosine, and cystine, the dominant near-UV absorbers in most peptides. Unusual chromophores, bound cofactors, or modified residues aren't included in this estimate.
Is this a substitute for measuring absorbance experimentally?
No. This is a theoretical estimate from the sequence you enter. A real sample's measured ε280 can differ, particularly for peptides with unusual folding, oligomeric states, or unaccounted-for chromophores.
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Educational tool only, not medical advice — verify with a licensed clinician. See the full medical disclaimer.