Synthesis of chlorinated fluoresceins for labeling proteins

Two novel chlorinated fluoresceins 4,7,2′,7′-tetrachloro-6-(5-carboxypentyl)fluorescein (8a) and 4,7,4′,5′-tetra- chloro-6-(5-carboxypentyl)fluorescein (8b) were synthesized as fluorescent probes for labeling proteins. These two fluoresceins contain 6-aminohexanoic acid as spacer linker to minimize the fluorescence quenching of the fluorescein molecules by the proteins to be labeled.