The Extinction Coefficient Calculator determines the molar absorptivity (ε) of a protein at 280 nm from its amino acid sequence, enabling accurate UV-based protein quantification without a protein standard. Used by biochemists, structural biologists, and protein engineers, it applies the widely validated Pace et al. (1995) formula and integrates a Beer-Lambert concentration converter for direct use with spectrophotometer readings.
Accepts single-letter IUPAC codes. FASTA headers and whitespace are removed automatically.
Disulfide bonds add 125 M⁻¹cm⁻¹ per bond to ε.
Used to calculate A280 per 1 mg/mL (E1%)
🔬 Extinction Coefficient Results
Absorbing Residue Breakdown
| Residue | Count | ε per residue (M⁻¹cm⁻¹) | Contribution |
|---|
Beer-Lambert Concentration Calculator
How to Use the Extinction Coefficient Calculator
This free tool calculates the molar extinction coefficient (ε) of your protein at 280 nm using the Pace et al. (1995) method — the same algorithm used by ExPASy ProtParam. Whether you are setting up protein quantification workflows, validating expression yields, or calibrating a spectrophotometer-based assay, accurate ε values are foundational. Follow the steps below for correct use.
Step-by-Step Instructions
- Enter or upload your protein sequence. Paste the full amino acid sequence in single-letter IUPAC code directly into the text area, or click "Upload .txt / .fasta file" to load a sequence file from disk. FASTA header lines beginning with ">" and all whitespace characters are stripped automatically before processing.
- Select the cysteine condition. Choose Reduced if your protein contains free thiol groups (intracellular proteins, denatured preparations, or samples treated with DTT or β-mercaptoethanol). Choose Oxidized if your protein is in its native folded state with intact disulfide bonds (secreted proteins, antibodies, extracellular domains).
- Enter molecular weight (optional). Providing the protein molecular weight in Daltons activates calculation of the specific absorbance E1% — the absorbance of a 1 mg/mL solution at 1 cm pathlength. This is particularly useful when working with protein concentrations expressed in mass/volume units.
- Click Calculate Extinction Coefficient. The tool instantly outputs the molar ε value, a residue-by-residue breakdown table, and the complete formula used.
- Use the Beer-Lambert Concentration Calculator. After calculating ε, enter your measured A280 absorbance value and your cuvette or pedestal pathlength (typically 1.0 cm for standard cuvettes, or 0.1 cm for NanoDrop short-path measurements). The tool converts A280 to molar concentration (µM and mol/L) and, if MW was provided, to mg/mL.
The Scientific Formula
The molar extinction coefficient at 280 nm is calculated using the Pace et al. (1995) equation:
ε = (nW × 5500) + (nY × 1490) + (nSS × 125)
Where:
- nW = number of tryptophan (Trp/W) residues in the sequence. Trp dominates UV absorption at 280 nm due to its indole ring system, contributing 5500 M⁻¹cm⁻¹ per residue.
- nY = number of tyrosine (Tyr/Y) residues. Tyr contributes 1490 M⁻¹cm⁻¹ per residue via its phenol chromophore.
- nSS = number of disulfide bonds. In oxidized mode, nSS = floor(nC / 2), where nC is the total cysteine count. Each S–S bond adds 125 M⁻¹cm⁻¹. In reduced mode, nSS = 0.
Once ε is known, protein concentration is derived from the Beer-Lambert law: A = ε × c × l, rearranged as c = A / (ε × l).
When to Use This Calculator
This calculator is appropriate whenever you need to quantify a protein by UV absorbance and you know — or can predict — its sequence. Common lab scenarios include: confirming protein concentration after affinity chromatography or size-exclusion chromatography; calculating yield at each step of a multi-stage purification; preparing protein stocks for enzymatic assays, SPR binding experiments, or crystallography; and verifying expression system output by comparing expected versus observed A280 values. It is also useful for quality control of commercial protein reagents when the sequence is known from the manufacturer datasheet.
Common Mistakes to Avoid
- Wrong cysteine mode. Choosing Reduced for a protein with confirmed disulfide bonds (or vice versa) introduces a systematic error in ε and in all downstream concentration calculations. Always confirm the redox state from published structural data, UniProt annotations, or experimental reduction/alkylation assays before selecting a mode.
- Ignoring light scattering from aggregates. If your protein sample contains aggregates or particulates, the apparent A280 will be elevated due to Rayleigh or Mie scattering, leading to overestimated concentrations. Centrifuge or filter samples (0.22 µm) and measure A320 as a scattering baseline; subtract A320 from A280 before entering the absorbance value.
- Incorrect pathlength entry. NanoDrop instruments use a short-path pedestal (typically 0.1 or 0.2 mm), not a standard 1.0 cm cuvette. Always verify the pathlength setting on your instrument and enter the correct value in the Beer-Lambert calculator, as a tenfold pathlength error produces a tenfold concentration error.
- Nucleic acid contamination. DNA and RNA absorb strongly at 260 nm and contribute non-trivially at 280 nm. An A260/A280 ratio significantly above 0.57 suggests nucleic acid contamination. Treat the sample with DNase I or use a secondary correction (e.g. Warburg-Christian method) before relying on A280-based quantification.
- Entering partial sequences. Make sure the full-length mature protein sequence is used. Signal peptides are cleaved co-translationally and would artificially inflate or deflate counts if included in the sequence used for calculation; use the processed, mature form.
Interpreting Your Results
The primary output is the molar extinction coefficient ε in M⁻¹cm⁻¹. A higher ε means greater UV absorptivity — proteins rich in tryptophan will have ε values in the tens of thousands, while Trp-free proteins may have ε below 1000 M⁻¹cm⁻¹ (relying solely on Tyr and any disulfide bonds). The residue breakdown table shows exactly how each contributing residue class adds to the total, letting you identify which chromophores dominate. The E1% value (when MW is provided) is a convenient unit for comparing preparations expressed in mg/mL — it tells you the absorbance of a 1% (10 mg/mL) solution in a 1 cm cuvette, a format commonly used in pharmaceutical protein characterisation. The Beer-Lambert converter outputs concentration in µM for direct entry into kinetics software, and in mg/mL for comparison with Bradford or BCA colorimetric assay results.
Frequently Asked Questions
Which amino acid residues contribute to protein absorbance at 280 nm?
Three residue types contribute meaningfully to protein UV absorbance at 280 nm: tryptophan (Trp/W), tyrosine (Tyr/Y), and cysteine (Cys/C) when engaged in disulfide bonds. Tryptophan has the highest individual extinction coefficient at 5500 M⁻¹cm⁻¹, followed by tyrosine at 1490 M⁻¹cm⁻¹. Each disulfide bond contributes an additional 125 M⁻¹cm⁻¹. Free (reduced) cysteines do not absorb meaningfully at 280 nm and are excluded from the calculation. Phenylalanine absorbs weakly at 257–267 nm but is not included in the standard Pace formula due to its negligible contribution at 280 nm.
What is the Pace et al. method for calculating extinction coefficients?
The Pace et al. (1995) method calculates the molar extinction coefficient using the formula: ε = (nW × 5500) + (nY × 1490) + (nSS × 125), where nW is the number of tryptophan residues, nY is the number of tyrosine residues, and nSS is the number of disulfide bonds. This method was validated against experimentally determined extinction coefficients for a large set of proteins and forms the basis of the ExPASy ProtParam tool. It is cited in thousands of biochemistry publications as the standard approach for sequence-based extinction coefficient prediction, and its accuracy has been confirmed to within 5–10% for most well-behaved soluble proteins.
When should I use reduced versus oxidized cysteine mode?
Select Reduced mode when working with proteins that have been denatured, are intracellular (typically lacking disulfide bonds), or have been treated with reducing agents such as DTT or β-mercaptoethanol. Select Oxidized mode for native secreted proteins, antibodies, and any protein where disulfide bond formation is confirmed by mass spectrometry or structural data. When uncertain, calculate both values and compare — the difference is usually small unless the protein is rich in cysteine and contains few tryptophan or tyrosine residues. Using the wrong mode introduces a systematic error that propagates into every downstream molar concentration calculation.
How accurate is sequence-based extinction coefficient prediction?
Sequence-based prediction using the Pace et al. method is accurate within approximately 5–10% for most soluble, properly folded proteins under standard spectrophotometric conditions. The main sources of error include unaccounted post-translational modifications (glycosylation can affect scattering), protein aggregation (which increases apparent absorbance), co-purified nucleic acids (absorbing at 260–280 nm), and non-native cysteine redox states. For the most rigorous quantification, the extinction coefficient should be confirmed experimentally using protein concentration verified by amino acid analysis. However, for routine lab work, sequence-derived values are entirely sufficient and are widely accepted in peer-reviewed publications.
How do I use the Beer-Lambert law to convert A280 to protein concentration?
The Beer-Lambert law states A = ε × c × l, where A is absorbance (unitless), ε is the molar extinction coefficient (M⁻¹cm⁻¹), c is concentration (mol/L), and l is the optical pathlength (cm). Rearranging gives c = A / (ε × l). In this calculator, first determine ε from your protein sequence, then enter your measured A280 value and the cuvette pathlength (typically 1.0 cm for standard cuvettes, 0.1 cm for NanoDrop pedestal measurements) into the Beer-Lambert Concentration Calculator section. The tool outputs concentration in µM and, if you provided molecular weight, also in mg/mL for direct comparison to Bradford or BCA assay results.