Protein Extinction Coefficient Calculator
How to calculate a protein extinction coefficient
- Paste the protein sequence in one-letter code. For a protein of several chains, such as an antibody, paste each chain as its own FASTA record, once for every copy: two heavy and two light chains for an IgG.
- Choose whether the cysteines form disulfide bonds, as in most secreted and purified folded proteins, or are reduced, as in a sample with DTT or in the cytoplasm.
- The molar extinction coefficient at 280 nm appears in M⁻¹ cm⁻¹, with the Abs 0.1% and the molecular weight.
Frequently asked questions
How is the extinction coefficient calculated?
By the method of Pace et al. (1995): ε280 = 5,500 × the number of tryptophans + 1,490 × the number of tyrosines + 125 × the number of cystines, in M⁻¹ cm⁻¹. A cystine is a pair of cysteines joined by a disulfide bond. ExPASy ProtParam uses the same values, so the results match.
What is Abs 0.1%?
The absorbance at 280 nm of a 1 mg/mL solution (0.1%) in a 1 cm cell: the molar extinction coefficient divided by the molecular weight. Divide an A280 reading by it to get the concentration in mg/mL. It is also written E0.1% or, for a 1% solution, E1% = 10 × Abs 0.1%.
What is the extinction coefficient of an antibody?
For a typical human or mouse IgG, the Abs 0.1% is about 1.4 (E1% about 14), which for 150 kDa is a molar coefficient of about 210,000 M⁻¹ cm⁻¹. The real value depends on the variable regions, so paste the heavy and light chains for the figure of your own antibody.
How accurate is a coefficient from the sequence?
For most proteins it is within about 5% of the measured value, as long as the protein has tryptophans. Proteins with no tryptophan are less certain, and those with neither tryptophan nor tyrosine barely absorb at 280 nm. Bound cofactors such as heme or flavins absorb too and are not counted.