a chemical compound which upon reaction with cysteine thiol groups produces a yellow product absorbing at 412 nm with a known extinction coefficient (E412 = 11,400 M-1 cm-1). An unknown peptide has molar extinction coefficient at 280 nm of 10,000 M-1 cm-1. A solution of this peptide in a 1 cm cuvette has an absorbance of 0.5 at 280 nm. Upon addition of Ellman's reagent to the peptide, the solution turns dark yellow and the absorbance at 412 nm is 1.14. Calculate the number of Cys residues in the peptide assuming that the total solution volume has not changed after addition of Ellman's reagent and that the stoichiometry of the reaction is 1:1 (i.e. 1 mole of the yellow dye is produced from 1 mole of cysteine).