Quantification of tryptic peptides in quadrupole ion trap using high-mass signals derived from isotope-coded N-acetyl dipeptide tags

Isotope-labeled N-acetyl dipeptides (Ac-Xxx-Ala) are coupled to the primary amines of tryptic peptides and then analyzed by tandem mass spectrometry. Amide bond cleavage between Xxx and Ala provides both low- and high-mass isotope-coded signals for quantification of peptides. Especially, facile cleavage at the modified lysine side chain yields very strong high-mass quantitation signals in a noise-free region. Tagging tryptic peptides with isobaric N-acetyl dipeptides is a viable strategy for accurate quantification of proteins, which can be used with most quadrupole ion trap mass spectrometers carrying the 1/3 mass cut-off problem.