Extinction Coefficient Calculator
Estimate the molar extinction coefficient of a peptide or protein at 280 nm from its sequence. Start with the reduced cysteine estimate. A second result shows what the model would give if as many cysteines as possible formed disulfide pairs within the same chain.
Results
Enter a valid sequence to estimate its molar extinction coefficient at 280 nm.
How the extinction coefficient is calculated
The Pace model adds 5,500 per tryptophan, 1,490 per tyrosine, and 125 per cystine pair in M⁻¹·cm⁻¹. The reduced estimate uses zero cystine pairs. The hypothetical estimate counts only complete pairs and leaves any odd cysteine unpaired.
Read the calculation method →Worked example
WYCCC
Reduced cysteines give 6,990 M⁻¹·cm⁻¹. Pairing two of the three cysteines adds 125 M⁻¹·cm⁻¹, giving 7,115 M⁻¹·cm⁻¹.
Limitations
- The model estimates absorption for a protein in water at 280 nm. Sequence alone does not establish the redox state or disulfide connectivity.
- Estimates are less reliable without tryptophan. Other chromophores, chemical modifications, and solvent conditions can affect measured absorbance.