1. What is Molar Extinction Coefficient?

The molar extinction coefficient (ε) measures how strongly a chemical species absorbs light at a given wavelength. For peptides, it's calculated from the sum of contributions from individual amino acids, particularly tryptophan, tyrosine, and cysteine residues.

2. How Does the Calculator Work?

The calculator uses the extinction coefficient formula:

Where:

• \( ε \) — Molar extinction coefficient (M⁻¹ cm⁻¹)
• \( ε_{amino\ acid} \) — Extinction coefficient per residue
• \( Number \) — Number of residues
• \( Corrections \) — Additional correction factors

Standard Values:

• Tryptophan: 5500 M⁻¹ cm⁻¹
• Tyrosine: 1490 M⁻¹ cm⁻¹
• Cysteine: 125 M⁻¹ cm⁻¹

3. Importance of Extinction Coefficient

Details: The extinction coefficient is essential for quantifying protein concentration using UV spectrophotometry, determining protein purity, and studying protein-ligand interactions.

4. Using the Calculator

Tips: Select the amino acid type, enter the number of residues, and add any necessary correction factors. The calculator will compute the total extinction coefficient at 280 nm.

5. Frequently Asked Questions (FAQ)

Q1: Why are only three amino acids considered?
A: Tryptophan, tyrosine, and cysteine are the primary amino acids that absorb significantly at 280 nm, which is the standard wavelength for protein quantification.

Q2: What are typical extinction coefficient values?
A: For most proteins, extinction coefficients range from 10,000 to 100,000 M⁻¹ cm⁻¹, depending on the number of aromatic residues.

Q3: When are correction factors needed?
A: Correction factors account for disulfide bonds, unusual amino acid modifications, or specific buffer conditions that may affect absorbance.

Q4: How accurate is this calculation?
A: This provides a theoretical estimate. For precise work, experimental determination is recommended, especially for proteins with unusual structures.

Q5: Can this be used for protein concentration calculation?
A: Yes, once you have the extinction coefficient, you can calculate protein concentration using Beer-Lambert law: Concentration = Absorbance / (ε × path length).