How to Calculate Molar Extinction Coefficient of a Protein

Molar Extinction Coefficient Equation:

\[ \varepsilon = \frac{A}{c \times l} \]

Absorbance at 280nm (A):

unitless

Protein Concentration:

mg/mL

Molecular Weight:

g/mol

Path Length:

Molar Extinction Coefficient (ε):

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1. What is Molar Extinction Coefficient?

The molar extinction coefficient (ε) is a measure of how strongly a chemical species absorbs light at a given wavelength. For proteins, it is typically measured at 280nm where aromatic amino acids (tryptophan, tyrosine) absorb light.

2. How Does the Calculator Work?

The calculator uses the Beer-Lambert law:

\[ \varepsilon = \frac{A}{c \times l} \]

Where:

\( \varepsilon \) — Molar extinction coefficient (L/mol·cm)
\( A \) — Absorbance at 280nm (unitless)
\( c \) — Molar concentration (mol/L)
\( l \) — Path length (cm)

Explanation: The equation relates the absorption of light to the properties of the material through which the light is traveling. The molar concentration is calculated from protein concentration (mg/mL) and molecular weight (g/mol).

3. Importance of Extinction Coefficient

Details: The molar extinction coefficient is crucial for protein quantification, purity assessment, and concentration determination in biochemical research. It allows researchers to quickly estimate protein concentration from absorbance measurements.

4. Using the Calculator

Tips: Enter absorbance at 280nm, protein concentration in mg/mL, molecular weight in g/mol, and path length in cm. Standard cuvettes typically have 1cm path length. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: Why measure at 280nm?
A: Proteins absorb light at 280nm primarily due to aromatic amino acids (tryptophan and tyrosine), making this wavelength ideal for protein quantification.

Q2: What are typical extinction coefficient values?
A: Most proteins have extinction coefficients between 20,000-60,000 L/mol·cm, but this varies significantly depending on aromatic amino acid content.

Q3: How accurate is this method?
A: This method provides good estimates but accuracy depends on protein purity and the presence of other absorbing substances. For precise measurements, use protein-specific calculated values.

Q4: Can I use this for nucleic acids?
A: No, nucleic acids are typically measured at 260nm and have different extinction coefficients. This calculator is specifically designed for proteins.

Q5: What if my protein lacks aromatic amino acids?
A: Proteins with very few aromatic amino acids will have low extinction coefficients and may require alternative quantification methods like Bradford or BCA assays.