Why does the protein need Trp or Tyr?

Absorbance at 280 nm comes almost entirely from tryptophan and tyrosine (and slightly cystine). A protein with neither barely absorbs at 280 nm, so A280 cannot quantify it — the tool needs a non-zero ε.

Reduced or oxidised cysteines?

This uses the reduced (all free cysteine) extinction coefficient, the usual default. Disulfide-bonded cystines add ~125 M⁻¹cm⁻¹ per pair, a small correction shown separately in the math layer.

Protein concentration from A280 calculator

Calculate protein concentration (mg/mL) from an A280 reading and the protein sequence — the extinction coefficient and molecular weight are computed from the sequence for you.

How it works

Formula

Beer–Lambert: molar concentration = A280 ÷ (ε · path). ε (M⁻¹cm⁻¹) is computed from the sequence as nTrp·5500 + nTyr·1490 (reduced cysteines). mg/mL = molar × molecular weight (Da).

Worked example

A protein with ε = 6,990 M⁻¹cm⁻¹ (1 Trp + 1 Tyr) and MW 13,970 Da reading A280 = 1.0: molar = 1.0 ÷ 6,990 = 1.43×10⁻⁴ M, so mg/mL = 1.43×10⁻⁴ × 13,970 ≈ 2.0 mg/mL.

When to use it

To quantify a purified protein from a spectrophotometer/NanoDrop A280, when you know the sequence. It is more accurate than a generic 1 mg/mL = 1 A280 rule because it uses the protein’s own aromatic content.

Sensible defaults

The default is an 88-residue serum-albumin fragment; at A280 = 1.0 it reads about 1.2 mg/mL. Paste your own sequence and A280.

Source

Extinction-coefficient contributions per Pace et al. (1995) Protein Science 4:2411–2423 (Trp 5500, Tyr 1490, cystine 125 M⁻¹cm⁻¹).

FAQ

Why does the protein need Trp or Tyr?: Absorbance at 280 nm comes almost entirely from tryptophan and tyrosine (and slightly cystine). A protein with neither barely absorbs at 280 nm, so A280 cannot quantify it — the tool needs a non-zero ε.
Reduced or oxidised cysteines?: This uses the reduced (all free cysteine) extinction coefficient, the usual default. Disulfide-bonded cystines add ~125 M⁻¹cm⁻¹ per pair, a small correction shown separately in the math layer.