In Ellman's assay protocol,it says -determining single cystein and lacking Tryptophan. Does it mean, having tryptophan in the sequence will not give a correct result ? And what's the correct extinction co-efficient ?
There is no reported effect of tryptophan on thiol determination using DTNB. TNB extinction coefficient is usually between 13500 and 14200 M-1.cm-1. Whatever, the best option is always to generate your own standard to calculate your own experimental coefficient.