Isotope-coded affinity tags with tunable reactivities for protein footprinting

(Chemical Equation Presented) Another foot forward: Isotope-coded affinity tag (ICAT) reagents are versatile probes for footprinting dynamic protein structure. To render ICAT footprinting effective, a series of ICAT reagents with alkylation rates that span five orders of magnitude was designed. With this toolkit, an unprecedented range of Cys residues can be footprinted, including those deeply buried in the protein core and those involved in protein-protein interactions of modest affinity.