Protein Molecular Weight Calculator Extinction Coefficient

Extinction Coefficient Formula:

\[ \varepsilon = \frac{MW \times A}{c \times l \times 1000} \]

Molecular Weight (MW):

g/mol

Absorbance (A):

unitless

Concentration (c):

mg/mL

Path Length (l):

Extinction Coefficient (ε):

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1. What is the Extinction Coefficient?

The extinction coefficient (ε) is a measure of how strongly a chemical species absorbs light at a particular wavelength. For proteins, it's typically measured at 280 nm where tryptophan and tyrosine residues absorb light. It's expressed in L/mol·cm and is used to determine protein concentration from absorbance measurements.

2. How Does the Calculator Work?

The calculator uses the extinction coefficient formula:

\[ \varepsilon = \frac{MW \times A}{c \times l \times 1000} \]

Where:

\( \varepsilon \) — Extinction coefficient (L/mol·cm)
\( MW \) — Molecular weight (g/mol)
\( A \) — Absorbance (unitless)
\( c \) — Concentration (mg/mL)
\( l \) — Path length (cm)

Explanation: This equation calculates how much light a protein solution absorbs based on its molecular properties and concentration.

3. Importance of Extinction Coefficient

Details: The extinction coefficient is crucial for quantifying protein concentration in solution using UV spectrophotometry. It's essential for various biochemical applications including protein purification, enzyme kinetics studies, and biophysical characterization.

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: What is a typical extinction coefficient value for proteins?
A: Most proteins have extinction coefficients between 20,000-60,000 L/mol·cm at 280 nm, depending on their tryptophan and tyrosine content.

Q2: Why is the path length important?
A: The path length determines how much light the sample absorbs. Standard cuvettes have a 1 cm path length, but microvolume measurements may use shorter paths.

Q3: Can I use this for nucleic acids?
A: No, nucleic acids have different extinction properties and are typically measured at 260 nm with different calculation methods.

Q4: What affects the accuracy of extinction coefficient calculations?
A: Accuracy depends on precise measurement of absorbance, concentration, and knowledge of the protein's exact molecular weight and amino acid composition.

Q5: How does pH affect extinction coefficient?
A: The extinction coefficient can vary with pH as it affects the ionization state of tyrosine residues, which can change their absorbance properties.