Enter your protein sequence and choose a wavelength and cysteine state (reduced or oxidized) to calculate your protein's molar extinction coefficient — along with its molecular weight, total amino acid count, and the number of tryptophan and tyrosine residues contributing to UV absorbance.
Molar Extinction Coefficient
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Molecular Weight
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Total Amino Acids
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Tryptophan Residues
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Tyrosine Residues
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Cysteine Residues
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The extinction coefficient is calculated using the contributions of tryptophan, tyrosine, and cysteine residues. At 280 nm, tryptophan contributes 5500 M⁻¹cm⁻¹, tyrosine contributes 1490 M⁻¹cm⁻¹, and cysteine (disulfide bonds) contributes 125 M⁻¹cm⁻¹.
280 nm is the standard wavelength for protein analysis as it measures aromatic amino acids. 260 nm is primarily used for nucleic acids but can also be used for proteins, though it's less specific and includes contributions from nucleic acid contamination.
Reduced cysteine residues (free sulfhydryl groups) do not contribute significantly to extinction at 280 nm. Oxidized cysteine residues forming disulfide bridges contribute approximately 125 M⁻¹cm⁻¹ per bridge.
Enter the protein sequence using single letter amino acid codes (e.g., MKTVRQERLK). Spaces and line breaks are automatically ignored. Both uppercase and lowercase letters are accepted.
The extinction coefficient is essential for determining protein concentration using UV spectroscopy, calculating protein yields, and preparing solutions with known concentrations for biochemical experiments.
This method provides theoretical extinction coefficients based on amino acid composition. Actual values may vary by ±5-10% due to protein folding, local environment effects, and measurement conditions.
This calculator works with the 20 standard amino acids. For proteins with post-translational modifications or non-standard amino acids, manual adjustments to the calculated value may be necessary.