Protein Extinction Coefficient Calculator (ε280)

Protein Extinction Coefficient Calculator (ε280) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

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.
2The calculator applies ε280 = 5500×Trp + 1490×Tyr + 125×cystine in M⁻¹cm⁻¹.
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.

Trp: 3; Tyr: 6; Cys: 4; cysteine state: all oxidized; molecular weight: 42 kDa; decimal places: 4.

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Insulin A-chain sequence with reduced cysteines

2. 42 kDa enzyme entered by residue counts

FAQ

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