Isobaric labeling

Isobaric labeling is a mass spectrometry strategy used in quantitative proteomics. Peptides or proteins are labeled with various chemical groups that are (at least nominally) isobaric, or the same in mass, but which fragment during tandem mass spectrometry to yield reporter ions of different mass. In a typical bottom-up proteomics workflow, proteins are enzymatically digested by a protease to produce peptides, which are then labeled with different isobaric tags. The most common isobaric tags are amine-reactive tags. These amine-reactive groups go through N-hydroxysuccinimide (NHS) reactions, which are based around three types of functional groups. The samples are mixed in equal ratios. During a liquid chromatography-mass spectrometry analysis, the peptides are fragmented to produce sequence-specific product ions, which help to determine the peptide sequence, as well as the reporter tags, whose abundances reflect the relative ratio of the peptide in the samples that were combined.