Protein assays

Here is an method for determination of the extinction coefficient for a protein of unknown concentration. It's listed as one of the laboratory methods of a biolab in the Rice University.

The method first use a formula to determine the extinction coefficient at 205nm:

E(205nm) = 27 + 120 * (A280/A205)

And then, determine protein concentration by beer's law at 205nm.

protein concentration = A205/E(205nm)

When we know the protein concentration and A280, we can calculate the extinction coefficient also by beer's law.

E(280nm) = protein concentration/A280

References

1. Scopes, RK. Analytical Biochemistry 59: 277. 1974.
2. Stoscheck, CM. Quantitation of Protein. Methods in Enzymology 182: 50-69. 1990.