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.
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Key facts
Category
Education
Input types
select, textarea, number
Output type
json
Sample coverage
4
API ready
Yes
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.
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
1Choose Protein sequence to count W, Y, and C automatically, or choose Residue counts to enter the values directly.
3Reduced cysteines contribute zero, while oxidized cysteines contribute as disulfide pairs using the integer number of Cys pairs.
4For 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.
Examples
1. Insulin A-chain sequence with reduced cysteines
Protein researcher
Background
A researcher needs the expected 280 nm absorbance coefficient for the human insulin A-chain sequence.
Problem
The sequence contains two tyrosines, four cysteines, and no tryptophan, so the reduced and oxidized estimates should be compared.
How to use
Select Protein sequence and enter GIVEQCCTSICSLYQLENYCN. Set Cysteine State to All reduced.
Outcome
The calculator counts 0 Trp, 2 Tyr, and 4 Cys, giving ε280 = 2980 M⁻¹cm⁻¹ when reduced and an oxidized bound of 3230 M⁻¹cm⁻¹. The sequence-based A0.1% is approximately 1.25.
2. 42 kDa enzyme entered by residue counts
Biochemistry student
Background
A 42 kDa enzyme is known to contain 3 tryptophans, 6 tyrosines, and 4 cysteines.
Problem
The student wants an ε280 estimate for the fully oxidized form and the corresponding A0.1% value.
How to use
Select Residue counts, enter Trp = 3, Tyr = 6, Cys = 4, choose All oxidized, and enter 42 kDa as the molecular weight.
FAQ
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.