Protein Extinction Coefficient Calculator
14. Protein Extinction Coefficient
Estimate the molar extinction coefficient ($\varepsilon$) at 280 nm.
Scientific Documentation
The Molar Extinction Coefficient ($\varepsilon$) is a measure of how strongly a chemical species absorbs light at a given wavelength. For proteins, the absorbance at 280 nm is dominated by the aromatic side chains of Tryptophan and Tyrosine, as well as disulfide bonds between Cysteine residues.
The Calculation: Using the Gill and von Hippel method, the coefficient is estimated as: $\varepsilon_{280} = (nW \times 5500) + (nY \times 1490) + (nCys \times 125)$. This calculation provides the theoretical absorbance value for a 1 M solution in a 1 cm path length cuvette.
Experimental Importance: Spectrophotometric quantification is the standard, non-destructive method for determining protein concentration. By accurately calculating $\varepsilon$, researchers can bypass the need for standard curves (like Bradford or BCA assays) in routine purification, allowing for rapid and precise determination of protein concentration from UV absorbance data.
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