Calculator Input
Enter a peptide sequence for automatic counting, or provide manual residue counts. Manual values override automatic counts when supplied.
Example Data Table
| Case | Trp | Tyr | Disulfides | MW (g/mol) | A280 | Path (cm) | Dilution | ε280 | Estimated Stock (µM) | Mass (mg/mL) |
|---|---|---|---|---|---|---|---|---|---|---|
| Oxidized peptide sample | 1 | 2 | 1 | 1500 | 0.86 | 1.0 | 10 | 8605 | 999.42 | 1.4991 |
| Reduced Tyr-rich sample | 0 | 3 | 0 | 1800 | 0.45 | 1.0 | 1 | 4470 | 100.67 | 0.1812 |
| Forward absorbance estimate | 2 | 1 | 0 | 2200 | — | 1.0 | 5 | 12490 | Known 50 µM | — |
Formula Used
Peptide molar extinction at 280 nm:
ε280 = (5500 × Trp) + (1490 × Tyr) + (125 × disulfide bonds)
Beer–Lambert concentration from measured absorbance:
c = A / (ε × l)
Original stock concentration after dilution correction:
cstock = cmeasured × dilution factor
Predicted absorbance from known concentration:
A = ε × c × l
Reduced cysteine is treated as negligible at 280 nm, while disulfide-linked cystine contributes slightly. Specific absorptivity is shown as ε/MW.
How to Use This Calculator
- Paste a peptide sequence or FASTA entry into the sequence field.
- Optionally override Trp, Tyr, or Cys counts with manual values.
- Select whether cysteines are reduced, auto-paired, or entered manually.
- Enter molecular weight if you also want specific absorptivity or mg/mL results.
- Add measured A280, path length, and dilution to estimate concentration.
- Add a known concentration in µM to predict absorbance instead.
- Click the calculate button to show results above the form.
- Use the CSV or PDF buttons to save the result summary.
Frequently Asked Questions
1. What does this calculator estimate?
It estimates peptide molar extinction at 280 nm from Trp, Tyr, and disulfide contributions. It can also convert absorbance into concentration and predict absorbance from a known concentration.
2. Why are Trp and Tyr so important here?
These aromatic residues dominate peptide absorbance near 280 nm. Tryptophan contributes the strongest signal, tyrosine adds a moderate signal, and disulfide-linked cystine adds a smaller contribution.
3. Does reduced cysteine increase ε280?
Not in this model. Reduced cysteine is treated as having negligible absorbance at 280 nm. Only cystine formed by disulfide bonds contributes to the extinction estimate.
4. What is the difference between oxidized and reduced modes?
Reduced mode sets disulfide contribution to zero. Oxidized mode automatically uses floor(Cys/2). Manual mode lets you specify the exact number of disulfide bonds yourself.
5. Can I use this with a FASTA sequence?
Yes. FASTA headers, spaces, and line breaks are stripped automatically. The calculator keeps only amino acid letters before counting residues.
6. Why might the estimate be less reliable?
Sequence-based ε280 values are most dependable when tryptophan is present. Peptides lacking Trp can show higher relative error, especially if experimental conditions or modifications alter absorbance.
7. What does specific absorptivity mean here?
It is ε divided by molecular weight. That converts the molar coefficient into a mass-based value, useful when your lab reports peptide concentration in mg/mL instead of molarity.
8. Why export CSV or PDF?
CSV is helpful for spreadsheets, batch reviews, and data logging. PDF is better for sharing a clean calculation summary with colleagues, reports, or experimental records.