Abstract: 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.

Abstract: The invention provides methods of analyzing a sample. In general, the methods involve multi-dimensionally fractionating a sample to produce a set of sub-fractions, identifying a sub-fraction of interest by evaluating binding of a first portion of the sub-fractions to a binding agent; and analyzing the mass of analytes in a second portion of the sub-fraction of interest. Also provided is a system for performing the subject methods. The invention finds use in a variety of different medical, research and proteomics applications.

Abstract: The present invention provides methods for analyzing a peptide or peptides of interest in a protein sample using a combination of a relatively generic isotope tag with a decoupled selection process, allowing simplified customization of the application with a single reagent. These methods comprise providing a first and a second protein sample; labeling the first protein sample with a first Universal Peptide Isotope Tag (U-PIT) reagent and the second protein sample with a second U-PIT reagent; separating the peptide of interest from the combined first and second protein samples; and determining the relative amount of the first U-PIT reagent and the second U-PIT reagent bound to the peptide or peptides of interest. The U-PIT label of the present inventive methods has the following general formula A-B-C wherein A is a nucleophilic reactive group, B is a detectable moiety that can be isotopically labeled, and C is a charge replacement group.