Protein Extinction Coefficient Calculator
A protein absorbs ultraviolet light at 280 nm mainly through its tryptophan and tyrosine residues, with a small contribution from disulfide-bonded cystines. The Pace-Edelhoch method estimates the molar extinction coefficient by summing standard per-residue absorbances: 5500 per tryptophan, 1490 per tyrosine, and 125 per cystine. This calculator returns that coefficient and the convenient 0.1 percent absorbance value when you also enter the molecular weight. Enter the residue counts from your sequence; the per-residue constants are the published method values.
Pace-Edelhoch formula
epsilon(280) = nTrp*5500 + nTyr*1490 + nCystine*125
units: inverse molar inverse centimeter
A(0.1%) = epsilon / molecular weight
concentration from Beer-Lambert: A = epsilon * c * path
For 1 Trp, 4 Tyr, and 2 cystines: 5500 + 4*1490 + 2*125 = 5500 + 5960 + 250 = 11,710 inverse molar inverse centimeter.
UV absorbance facts
- Tryptophan is the dominant 280 nm chromophore at 5500 per residue.
- Tyrosine contributes 1490 and cystine 125 per residue.
- Free cysteines in reduced proteins contribute essentially nothing at 280 nm.
- The 0.1 percent value lets you read concentration straight from an A280 reading.
- Estimates are most reliable for tryptophan-containing folded proteins.
Protein extinction coefficient: frequently asked questions
How is a protein extinction coefficient estimated?
The Pace-Edelhoch method sums the contributions of the absorbing residues at 280 nm: each tryptophan contributes 5500, each tyrosine 1490, and each disulfide-bonded cystine 125, all in units of inverse molar inverse centimeter. The total is the molar extinction coefficient of the folded protein at 280 nm.
What are the per-residue values based on?
The values of 5500 for tryptophan, 1490 for tyrosine, and 125 for cystine are the standard molar absorptivities published by Pace and colleagues, widely used in tools such as the ExPASy ProtParam program. They represent the average absorbance contribution of each chromophore at 280 nm.
What is the 0.1 percent absorbance value?
Dividing the molar extinction coefficient by the protein molecular weight gives the absorbance of a 1 gram per liter (0.1 percent) solution in a 1 cm path. This is convenient for estimating protein concentration from an A280 reading using the Beer-Lambert law.
Should I count cystines or cysteines?
Count cystines, which are pairs of cysteine residues joined by a disulfide bond, not free cysteines. Each disulfide bond is one cystine. Reduced proteins with free thiols contribute essentially no absorbance at 280 nm, so set cystines to zero for a fully reduced protein.
How accurate is the estimate?
For folded proteins the estimate is typically within a few percent of the measured value. Accuracy is best for proteins containing tryptophan; proteins with no tryptophan have larger relative uncertainty. Always treat the result as a close estimate rather than an exact constant.
Official sources
- NIST: NIST biomanufacturing measurement resources.
- NCBI: NCBI PubMed Central: Pace et al. method literature.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.