Protein Concentration Calculator (A280)

How to calculate protein concentration from A280

1. Type the A280 reading, the dilution factor and the path length.
2. Choose where the extinction coefficient comes from: paste the protein sequence, type the molar coefficient and molecular weight, or type the Abs 0.1%.
3. The concentration appears in mg/mL, with µM when the molecular weight is known.

Frequently asked questions

How is the extinction coefficient worked out from the sequence?

With the method of Pace et al. (1995): 5,500 M⁻¹ cm⁻¹ for each tryptophan, 1,490 for each tyrosine and 125 for each cystine (pair of cysteines in a disulfide bond). Choose Reduced if the cysteines are free.

What is Abs 0.1%?

The absorbance at 280 nm of a 1 mg/mL solution in a 1 cm cell, which is the extinction coefficient divided by the molecular weight. Concentration in mg/mL = A280 ÷ Abs 0.1%.

Why can it not use my protein?

Proteins with no tryptophan or tyrosine absorb almost nothing at 280 nm. Use a dye assay such as Bradford or BCA, or measure at 205 nm.