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Protein Extinction Coefficient Equation:
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The protein extinction coefficient (ε) at 280 nm is a measure of how strongly a protein absorbs light at this wavelength. It is primarily determined by the content of tryptophan, tyrosine, and cystine residues in the protein, as these amino acids have strong absorbance at 280 nm.
The calculator uses the extinction coefficient equation:
Where:
Explanation: The equation calculates the molar extinction coefficient at 280 nm based on the specific absorbance contributions of each chromophoric amino acid residue.
Details: The extinction coefficient is crucial for determining protein concentration using UV absorbance measurements. It is widely used in protein biochemistry, purification, and quantification workflows.
Tips: Enter the number of tryptophan, tyrosine, and cystine residues in your protein. All values must be non-negative integers or zero. The result is given in M⁻¹cm⁻¹ units.
Q1: Why are these specific amino acids used in the calculation?
A: Tryptophan, tyrosine, and cystine are the primary amino acids that absorb light at 280 nm due to their aromatic structures.
Q2: What are typical extinction coefficient values for proteins?
A: Extinction coefficients typically range from thousands to hundreds of thousands M⁻¹cm⁻¹, depending on the protein's size and amino acid composition.
Q3: How accurate is this calculation method?
A: This method provides a good estimate, but the actual extinction coefficient can be affected by protein folding and local environment effects on the chromophores.
Q4: Can this be used for modified proteins or fusion proteins?
A: For modified proteins with additional chromophores or fusion proteins with tags that absorb at 280 nm, additional considerations may be needed.
Q5: What if my protein contains no tryptophan residues?
A: The calculation still works as the coefficients for tyrosine and cystine will contribute to the total extinction coefficient.