Peptide Molar Extinction Calculator

Input Type Sequence Manual counts

Peptide sequence (one-letter codes) Only A C D E F G H I K L M N P Q R S T V W Y. Whitespace is ignored.

Tryptophan W

Tyrosine Y

Cysteine C

Peptide molecular weight (optional, g/mol) Needed to convert between mg/mL and molar concentration if sequence is not provided.

Disulfide handling Auto from Cys count Reduced (no disulfide) Manual entry

Number of disulfide bonds

Path length (cm)

Calculation mode Predict A280 from concentration Infer concentration from A280

Concentration value

Concentration unit µM M mg/mL

Measured A280

Calculate

• ε₂₈₀ = 5500 × n(W) + 1490 × n(Y) + 125 × n(disulfide)
• Disulfide bonds from sequence: floor(C / 2). Reduced state sets n(disulfide) = 0.
• A₂₈₀ = ε × c × l, with c in mol/L and l in cm.
• From mg/mL to mol/L requires MW (g/mol): c(M) = c(mg/mL) / MW.
• Molecular weight estimated from sequence as sum of residue masses plus H₂O.

1) What wavelengths does this support?

It focuses on 280 nm using Tyr Trp and disulfide contributions which dominate aromatic absorption in proteins and peptides.

2) How accurate is ε₂₈₀ from sequence?

It is a widely used approximation. Microenvironment effects can shift real spectra. For highest accuracy measure A280 experimentally using matched buffer and path length.

3) Why is molecular weight needed?

It enables conversion between molar concentration and mass units like mg/mL and allows reporting A280 at 1 mg/mL based on ε and MW.

4) How are disulfide bonds handled?

You can force reduced state set a manual count or let the tool infer pairs from Cys content as floor(C/2).

5) Can I paste long sequences?

Yes. The tool is optimized for peptides and small proteins. Very large proteins may be slower to process on shared hosting.