Patent ID: 6902936

Claim:
A method for the analysis of mixtures containing proteins, said method comprising the steps of: (a) reducing disulfide bonds in the proteins of a sample, thereby providing thiol groups in cysteine-containing proteins; (b) blocking free thiols with a blocking reagent in the sample; (c) digesting the proteins in the sample to provide peptides; (d) deprotecting the peptides to remove the blocking reagent, thereby providing thiol groups in cysteine-containing peptides for reaction; (e) reacting cysteine-containing peptides in the sample with a tagging reagent, wherein said tagging reagent comprises a thiol-specific reactive group which is attached to a polymer tag via a linker, wherein the linker can be differentially labeled with stable isotopes and wherein the polymer tag forms a covalent bond with the cysteine-containing peptides; (f) washing the polymer tag to remove non-covalently bound species; (g) eluting the cysteine-containing peptides from the polymer tag; and (h) subjecting the eluted peptides to quantitative mass spectrometry (MS) analysis.