Synthesis and characterization of dimaleimide fluorogens designed for specific labeling of proteins.

A series of aromatic compounds were prepared bearing two maleimide groups attached directly to the fluorescent cores. The resulting derivatives do not fluoresce until the maleimide groups undergo their typical thiol addition reaction, thus removing their ability to quench fluorescence, as shown by kinetic and spectral characterization studies. In this way, the title compounds serve as fluorogens capable of detection of small thiols or appropriately sized dithiols. Recombinant alpha-helical proteins were then designed to bear two cysteine residues capable of regioselective dithiol addition reaction with the dimaleimide fluorogens, thus acting as spatially encoded substrates that form specifically labeled covalent complexes. The efficiency of this in vitro fluorescent protein-labeling reaction demonstrates the feasibility of the development of a method for the fluorescent labeling of specific recombinant proteins.