Enter the number of tryptophan, tyrosine, and cysteine residues in a protein to calculate its extinction coefficient. The extinction coefficient is a measure of how strongly a protein absorbs light at a particular wavelength and is important for determining protein concentration.

Protein Extinction Coefficient Formula

The extinction coefficient (ε) of a protein can be estimated using the following formula:

ε = (n<sub>W</sub> * ε<sub>W</sub>) + (n<sub>Y</sub> * ε<sub>Y</sub>) + (n<sub>C</sub> * ε<sub>C</sub>)

Variables:

  • nW is the number of tryptophan residues
  • nY is the number of tyrosine residues
  • nC is the number of cysteine residues
  • εW is the molar extinction coefficient for tryptophan (5500 M-1cm-1)
  • εY is the molar extinction coefficient for tyrosine (1490 M-1cm-1)
  • εC is the molar extinction coefficient for cysteine (125 M-1cm-1)

To calculate the extinction coefficient of a protein, multiply the number of each type of residue by its respective molar extinction coefficient and sum the results.

What is a Protein Extinction Coefficient?

The protein extinction coefficient is a measure of how much light a protein absorbs at a certain wavelength. It is a critical parameter for determining the concentration of proteins in solution using spectrophotometric methods. The extinction coefficient depends on the protein’s amino acid composition, particularly the presence of aromatic residues like tryptophan, tyrosine, and cysteine, which have significant absorbance at 280 nm.

How to Calculate Protein Extinction Coefficient?

The following steps outline how to calculate the Protein Extinction Coefficient.

  1. Determine the number of tryptophan residues (nW).
  2. Determine the number of tyrosine residues (nY).
  3. Determine the number of cysteine residues (nC).
  4. Use the formula ε = (nW * εW) + (nY * εY) + (nC * εC) to calculate the extinction coefficient.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Number of tryptophan residues (nW) = 5

Number of tyrosine residues (nY) = 3

Number of cysteine residues (nC) = 2