Protein concentration converter
Convert an A280 reading into a concentration. Pick the protein first: its extinction coefficient sets the scale.
That's about the same as a working solution. A 20 µL sample at that concentration holds about 20 µg of protein.
What it means
How much protein is that?
A280 assumes a 1 cm path length and a protein with ordinary aromatic content. Nucleic acid contamination absorbs at 280 too and inflates the reading.
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Three things worth knowing
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Concentration = A280 ÷ extinction coefficient
With the coefficient expressed for a 1 mg/mL solution in a 1 cm cuvette. For a protein of unknown sequence, 1.0 is the usual guess.
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Beer and Lambert
Absorbance is proportional to concentration and path length, which is the Beer–Lambert law. It holds until the sample is so concentrated the molecules start shading each other.
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BSA is the odd one out
Albumin has unusually few tryptophans, so its A280 of 0.667 per mg/mL is far below most proteins. Using it as a generic standard underestimates everything else.
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Background
About A280
Proteins absorb ultraviolet at 280 nm, mostly through their tryptophan and tyrosine. Measuring that absorbance is the fastest concentration assay there is — no reagents, no incubation, and the sample survives.
The catch is that the extinction coefficient depends on the protein: an A280 of 1.0 is 0.74 mg/mL of IgG but 1.5 mg/mL of bovine serum albumin.