New Thiol-Reactive Chromophores

Sometimes commercially available chromophores are not perfectly suited for the experiment one wishes to do. In my case, I want an extremely solvatochromic chromophore so that I can perform a single covalent labeling near the binding center of GGBP that will report local motions corresponding to global protein rearrangements upon glucose/galactose binding. The chromophore needs to be excitable preferably in the blue to green spectrum (optimal for Ti:Al₂O₃ lasers). Nile Red is an excellent dye in this case, however, there is no commercially available form that is thiol-reactive (covalent bonding to cysteine amino acid sidechain).
About a year ago, I began synthesizing a nile red maleimide, but I had a lot of difficulty getting maleimide to link to nile red phenol. It took some trained organic chemists at Kent State University (S. Y. Kim et al., J. Phys. Chem. B 109, 24517 (2005).) about the same amount of time to figure out that a protecting group is necessary. A simple way of protecting the maleimide and later deprotecting it is through a Diels-Alder/Retro-Diels-Alder set of reactions. I have shown a rough schematic of the process below. I am now optimizing the set of reactions and preparing to attach the chromophore to GGBP for single molecule studies.