Compounds, compositions, methods for sequencing proteins and peptides, and methods for identifying proteins and peptides in a mixture, are disclosed. Compounds of formula A-B-C wherein A is a nucleophilic reactive group, B is a detectable moiety capable of being isotopically labeled, and C is a charge replacement group, are used to label the peptides at the N-terminus or the C-terminus. The tagged peptides can then be analyzed by mass spectroscopy.