1. What is the A280 Extinction Coefficient?

The A280 extinction coefficient estimates how strongly a protein absorbs light at 280 nm wavelength. This absorption is primarily due to the presence of tryptophan, tyrosine, and cysteine (disulfide bonds) residues in the protein.

2. How Does the Calculator Work?

The calculator uses the following equation:

Where:

• \( ε \) — Extinction coefficient (M⁻¹ cm⁻¹)
• \( Trp \) — Number of tryptophan residues
• \( Tyr \) — Number of tyrosine residues
• \( Cys \) — Number of disulfide bonds

Explanation: The equation accounts for the contribution of each chromophore (tryptophan, tyrosine, and cystine) to the total absorbance at 280 nm.

3. Importance of Extinction Coefficient

Details: The extinction coefficient is crucial for determining protein concentration using UV absorbance measurements. It's also important for various biochemical and biophysical experiments.

4. Using the Calculator

Tips: Enter the number of tryptophan residues, tyrosine residues, and disulfide bonds in your protein. All values must be non-negative integers.

5. Frequently Asked Questions (FAQ)

Q1: Why are tryptophan residues weighted more heavily?
A: Tryptophan has a much higher molar absorptivity at 280 nm (5500 M⁻¹ cm⁻¹) compared to tyrosine (1490 M⁻¹ cm⁻¹) and cystine (125 M⁻¹ cm⁻¹).

Q2: What if my protein has no tryptophan?
A: The calculation still works - just enter 0 for tryptophan. The extinction coefficient will be based only on tyrosine and disulfide bonds.

Q3: How accurate is this calculation?
A: This provides a theoretical estimate. The actual extinction coefficient can vary slightly depending on protein environment and structure.

Q4: What about other amino acids?
A: Other amino acids don't contribute significantly to absorbance at 280 nm, so they're not included in the calculation.

Q5: Can I use this for protein concentration determination?
A: Yes, once you have the extinction coefficient, you can use Beer's Law (A = εcl) to calculate protein concentration from absorbance measurements.