Ortho Green Fluorescence Protein Synthetic Chromophore; Excited-State Intramolecular Proton Transfer via a Seven-Membered-Ring Hydrogen-Bonding System

Green fluorescence protein (GFP), which serves as an energy acceptor and emitter for bioluminescence in the sea pansy Renilla reniformis and the jellyfish Aequorea Victoria, has drawn much attention because of its applications in molecular biology and biochemistry.1 GFP takes advantage of the presence of a chro-mophore that is anchored both covalently and via a hydrogen-bond network, 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (p-HBDI, see Scheme 1), which undergoes excited-state proton transfer (ESPT)2 via the proton relay of water molecules and a remote residue such as E222,3 resulting in a very effective and intense anion fluorescence. Nevertheless, studies reveal a strong cutoff between the properties of wild type GFP (or certain GFP mutants) and the synthetic analogue chromophores of p-HBDI.4 In view of photophysics, the fluorescence yield of the protein-free chromophore in fluid solvent