Peptide extinction coefficient calculator
Paste a sequence to get its molar extinction coefficient at 280 nm, its absorbance at 1 mg/mL, and the concentration that matches an A280 reading.
Load a known peptide
| Peptide | Sequence | Modifications | ε280 (M⁻¹cm⁻¹) | Abs 0.1% | Load preset |
|---|---|---|---|---|---|
| BPC-157 | GEPPPGKPADDAGLV | — | — | — | |
| GHK (GHK-Cu without copper) | GHK | — | — | — | |
| Epitalon | AEDG | — | — | — | |
| KPV | KPV | — | — | — | |
| Selank | TKPRPGP | — | — | — | |
| Semax | MEHFPGP | — | — | — | |
| DSIP | WAGGDASGE | — | 5,500 | 6.480 | |
| Cartalax | AED | — | — | — | |
| Thymosin Alpha-1 | SDAAVDTSSEITTKDLKEKKEVVEEAEN | N-acetyl | — | — | |
| Oxytocin | CYIQNCPLG | C-amide, 1 disulfide | 1,615 | 1.603 | |
| Kisspeptin-10 | YNWNSFGLRF | C-amide | 6,990 | 5.367 | |
| Sermorelin | YADAIFTNSYRKVLGQLSARKLLQDIMSR | C-amide | 2,980 | 0.887 | |
| AOD-9604 | YLRIVQCRSVEGSCGF | 1 disulfide | 1,615 | 0.890 | |
| MOTS-c | MRWQEMGYIFYPRKLR | — | 8,480 | 3.900 | |
| VIP | HSDAVFTDNYTRLRKQMAVKKYLNSILN | C-amide | 2,980 | 0.896 | |
| LL-37 | LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES | — | — | — | |
| TB-500 fragment (Ac-LKKTETQ) | LKKTETQ | N-acetyl | — | — |
How the calculation works
At 280 nm a peptide's absorbance comes almost entirely from tryptophan, tyrosine and disulfide-bonded cysteine pairs (cystines). The calculator uses the Pace values: ε280 = 5,500 × Trp + 1,490 × Tyr + 125 × cystines, in M⁻¹cm⁻¹. It reports the coefficient with every Cys reduced and, when there are two or more, with every Cys pair bonded. Enter disulfide bonds above zero to use the paired value for concentration.
Concentration then follows from the Beer–Lambert law: molar concentration = A280 ÷ (ε × path length). Multiply by the average mass for mg/mL. Many short research peptides, including BPC-157, contain no Trp or Tyr and cannot be quantified this way; the table above marks them with a dash.
Questions
How is a peptide's extinction coefficient at 280 nm calculated?
With the Pace method: ε280 = (Trp × 5,500) + (Tyr × 1,490) + (cystines × 125), in M⁻¹cm⁻¹. Free cysteines do not count; only disulfide-bonded pairs absorb measurably at 280 nm.
What if my peptide has no tryptophan or tyrosine?
Then it barely absorbs at 280 nm and an A280 reading cannot give a reliable concentration. Use absorbance at 205 or 214 nm (the peptide bond), a colorimetric assay or amino-acid analysis instead.
How do I get concentration from an A280 reading?
Beer–Lambert: concentration (M) = A280 ÷ (ε × path length in cm). Multiply by the molecular weight for mg/mL. Blank against the same buffer and keep readings inside the instrument's linear range.
What is Abs 0.1%?
The absorbance of a 1 mg/mL (0.1%) solution in a 1 cm cell, equal to ε divided by the molecular weight. Dividing an A280 reading by it gives mg/mL directly.
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Reference tools for laboratory research. Not for human or animal use.