Protein Extinction Coefficient Calculator

Formula

The formula to calculate the Extinction Coefficient (ε) is:

\[ \epsilon = (n_W \cdot \epsilon_W) + (n_Y \cdot \epsilon_Y) + (n_C \cdot \epsilon_C) \]

Where:

\( \epsilon \) is the extinction coefficient
\( n_W \) is the number of tryptophan residues
\( n_Y \) is the number of tyrosine residues
\( n_C \) is the number of cysteine residues
\( \epsilon_W \) is the molar extinction coefficient for tryptophan (5500 M^-1cm^-1)
\( \epsilon_Y \) is the molar extinction coefficient for tyrosine (1490 M^-1cm^-1)
\( \epsilon_C \) is the molar extinction coefficient for cysteine (125 M^-1cm^-1)

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.

Example Calculation

Let's assume the following values:

Number of Tryptophan Residues (n_W) = 3
Number of Tyrosine Residues (n_Y) = 5
Number of Cysteine Residues (n_C) = 2

Using the formula to calculate the Extinction Coefficient:

\[ \epsilon = (3 \cdot 5500) + (5 \cdot 1490) + (2 \cdot 125) = 16500 + 7450 + 250 = 24200 \text{ M}^{-1}\text{cm}^{-1} \]

The Extinction Coefficient is 24200 M^-1cm^-1.