Extinction Coefficient Calculator

This calculator determines the molar extinction coefficient of proteins, which is essential for accurate protein concentration measurements using UV-Vis spectroscopy. The extinction coefficient represents how strongly a protein absorbs light at a specific wavelength, allowing you to convert absorbance readings into precise concentration values.

The Beer-Lambert Law

Protein concentration determination relies on the Beer-Lambert Law, which relates absorbance to concentration:

Where $A$ is absorbance (dimensionless), $\varepsilon$ is the molar extinction coefficient (M⁻¹cm⁻¹), $c$ is the molar concentration (M), and $l$ is the path length (cm, typically 1 cm for standard cuvettes).

Why 280 nm?

Proteins absorb UV light at 280 nm primarily due to aromatic amino acid residues. Three amino acids contribute to this absorption:

• Tryptophan (W): Contains an indole ring with strong UV absorption (ε = 5,500 M⁻¹cm⁻¹)
• Tyrosine (Y): Contains a phenol ring with moderate UV absorption (ε = 1,490 M⁻¹cm⁻¹)
• Cystine (disulfide bonds): Oxidized cysteine pairs contribute weak UV absorption (ε = 125 M⁻¹cm⁻¹ per disulfide bond)

The Pace Method

This calculator uses the method developed by Pace et al. (1995), which calculates the extinction coefficient by summing the individual contributions of each chromophore:

Where $n_{Trp}$, $n_{Tyr}$, and $n_{Cys}$ are the number of tryptophan, tyrosine, and disulfide bond residues, respectively. For the reduced form, the cystine contribution is omitted.