A spectrophotometric method for quantitation of carboxyl group modification of proteins using Woodward's Reagent K.

Reaction of proteins with Woodward's Reagent K in 0.05 ionic strength Tris-HCl, pH 7.8, followed by removal of excess reagent by chromatography on Sephadex G-25 in the same buffer, results in covalently attached chromophores with an absorption maximum at 340 nm and an extinction coefficient of 7000 M-1 cm-1. This absorbance can be used to quantitate the reaction of Woodward's Reagent K with carboxyl groups in proteins, provided sulfhydryl groups do not react. The chromophore also enables specific detection and identification of carboxyl-modified peptides upon separation by chromatography or electrophoresis.