1. What is Peptide Extinction Coefficient?

The peptide extinction coefficient (ε) measures how strongly a peptide absorbs light at a specific wavelength (typically 280 nm). It's the sum of the extinction coefficients of its constituent aromatic amino acids (tryptophan, tyrosine, and cysteine).

2. How Does the Calculator Work?

The calculator uses the following formula:

Where:

• \( \epsilon \) — Extinction coefficient (L/mol·cm)
• \( \epsilon_{aa} \) — Extinction coefficient of individual amino acids

Explanation: Only tryptophan (W), tyrosine (Y), and cysteine (C) contribute significantly to absorption at 280 nm. The values used are:

• Tryptophan (W): 5500 L/mol·cm
• Tyrosine (Y): 1490 L/mol·cm
• Cysteine (C): 125 L/mol·cm (reduced form)

3. Importance of Extinction Coefficient

Details: The extinction coefficient is crucial for determining protein concentration using UV absorbance measurements (Beer-Lambert law: A = εcl).

4. Using the Calculator

Tips: Enter the peptide sequence in single-letter code (case insensitive). The calculator sums the contributions from W, Y, and C residues.

5. Frequently Asked Questions (FAQ)

Q1: Why only W, Y, and C contribute?
A: These are the only amino acids with significant absorbance at 280 nm due to their aromatic side chains.

Q2: How accurate is this calculation?
A: It provides a good estimate, but actual values may vary slightly due to protein folding effects on chromophore exposure.

Q3: What about disulfide bonds?
A: Disulfide bonds between cysteines contribute more (ε ≈ 300 L/mol·cm per bond) than reduced cysteines.

Q4: Can I use this for other wavelengths?
A: This calculator is specific for 280 nm. Other wavelengths would require different coefficients.

Q5: How is this used in practice?
A: Combined with absorbance measurements, it allows protein concentration determination without standard curves.