Tandem mass tags

Overview
Tandem Mass Tags (TMT) are chemical labels used for mass spectrometry (MS) based quantitation and identification of biological macromolecules, especially, proteins, peptides and nucleic acids. They enable gel- and antibody-free quantitation but may also be used in combination with these and other methods.

Versions
There are currently three varieties of TMT available:  TMTzero - a non-isotopically substituted core structure for method development TMTduplex - an isobaric pair of mass tags with a single isotopic substitution TMTsixplex - an isobaric set of six mass tags with five isotopic substitutions 

The tags contain four regions, namely a mass reporter region (M), a cleavable linker region (F), a mass normalization region (N) and a protein reactive group (R). The chemical structures of all the tags are identical but each contains isotopes substituted at various positions, such that the mass reporter and mass normalization regions have different molecular masses in each tag. The combined M-F-N-R regions of the tags have the same total molecular weights and structure so that during chromatographic or electrophoretic separation and in single MS mode, molecules labelled with different tags are indistinguishable. Upon fragmentation in MS/MS mode, sequence information is obtained from fragmentation of the peptide back bone and quantitation data are simultaneously obtained from fragmentation of the tags, giving rise to mass reporter ions.

Bioinformatics Integration
The structures of the tags are made publicly available through the unimod database at unimod.org and hence, mass spectrometry software such as Mascot are able to account for the tag masses. Additionally, as of version 2.2, Mascot has a built in capability to quantitate using TMT without additional software.