# Protein Extinction Coefficient Calculator (ε280) ε280 = 5500×Trp + 1490×Tyr + 125×cystine (Pace 1995, ±5%) from a sequence or residue counts, with reduced/oxidized brackets, ProtParam molecular weight, A0.1% = ε/MW, and c = A280/ε concentration conversion. Gill & von Hippel / Pace / ProtParam derived. Educational use only. > Canonical page: https://elysiatools.com/en/tools/protein-extinction-coefficient - **Category:** Education - **Keywords:** protein extinction coefficient calculator, epsilon 280 protein, a280 concentration calculator, trp tyr cystine absorbance, pace 1995 extinction coefficient, gill von hippel extinction, protparam extinction coefficient, molar absorption coefficient protein, a0.1% absorbance 1 mg/ml, molecular biology, lab calculator ## Overview Calculate a protein’s molar extinction coefficient at 280 nm (ε280) from its amino acid sequence or Trp, Tyr, and Cys counts. The calculator reports reduced and oxidized cysteine estimates, and can also calculate A0.1% when molecular weight is available. ## Inputs - **Input Mode** (select): Count W/Y/C from a sequence, or enter residue counts directly. - **Protein Sequence (one-letter codes)** (textarea): One-letter codes, 20 standard residues (FASTA headers, whitespace, and '*' are stripped); sequence mode. - **Tryptophan (Trp) Count** (number): Number of tryptophans (counts mode); each contributes 5500 M⁻¹cm⁻¹. - **Tyrosine (Tyr) Count** (number): Number of tyrosines (counts mode); each contributes 1490 M⁻¹cm⁻¹. - **Cysteine (Cys) Count** (number): Number of cysteines (counts mode); each disulfide pair contributes 125 M⁻¹cm⁻¹ when oxidized. - **Cysteine State** (select): Reduced cysteines contribute nothing; oxidized pairs (cystines) add 125 each. - **Molecular Weight (kDa, counts mode)** (number): Protein molecular weight for the A0.1% conversion in counts mode; 0 skips it. - **Decimal Places** (number) ## When to use - Estimate ε280 before using A280 to measure a protein or peptide concentration. - Check how Trp, Tyr, and disulfide-bonded cystines contribute to absorbance. - Convert between molar extinction coefficient and A0.1% for proteins with a known molecular weight. ## How it works - Choose Protein sequence to count W, Y, and C automatically, or choose Residue counts to enter the values directly. - The calculator applies ε280 = 5500×Trp + 1490×Tyr + 125×cystine in M⁻¹cm⁻¹. - Reduced cysteines contribute zero, while oxidized cysteines contribute as disulfide pairs using the integer number of Cys pairs. - For sequence input, molecular weight is estimated with ProtParam average residue masses; A0.1% is calculated as ε divided by molecular weight when available. ## Use cases - Planning UV-based protein concentration measurements from a sequence. - Comparing absorbance estimates for reduced and disulfide-bonded protein forms. - Estimating A0.1% for proteins or peptides with a known molecular weight. ## Frequently asked questions ### What does ε280 measure? ε280 is the molar extinction coefficient describing protein absorbance at 280 nm in M⁻¹cm⁻¹. ### Which residues contribute to ε280? Tryptophan contributes 5500, tyrosine 1490, and each oxidized cystine contributes 125 M⁻¹cm⁻¹. ### How are cysteines treated? Reduced cysteines contribute nothing. Oxidized cysteines are counted as disulfide pairs, so an unpaired cysteine does not contribute. ### Can I enter a protein sequence? Yes. Enter a sequence using the 20 standard one-letter amino acid codes; FASTA headers, whitespace, and asterisks are stripped. ### What if my protein has no Trp or Tyr? Its ε280 may be nearly zero, making A280 unsuitable for concentration measurement; consider another assay such as A205 or BCA. ## Related tools - [PCR Annealing Temperature Calculator (Ta = Tm − 5°C)](https://elysiatools.com/en/tools/annealing-temperature-calculator): PCR Ta by the simple rule (lower primer Tm − 5°C) or the Rychlik 1990 formula (0.3·Tm_primer + 0.7·Tm_product − 14.9), with a ±3°C gradient window and primer-Tm mismatch warning. Derived from Rychlik 1990, Innis & Gelfand 1990, Lorenz 2012. 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