5(6)-Carboxyfluorescein

5(6)-Carboxyfluorescein (CAS No.: 72088-94-9) is an amine-reactive, pH-sensitive green fluorescent probe widely employed in biomedical research for the labeling and tracking of biomolecules. Comprising a mixture of positional isomers derived from fluorescein, this compound readily forms stable covalent linkages with primary amine groups, enabling efficient conjugation to proteins, peptides, and nucleotides for applications in molecular imaging and biochemical assays. It exhibits strong fluorescence with excitation and emission characteristics compatible with standard laser-based detection systems, making it particularly suitable for fluorescence microscopy, flow cytometry, and high-content screening. Due to its membrane-impermeant nature, 5(6)-Carboxyfluorescein is also utilized in studies of membrane integrity and permeability, as well as in tracing extracellular or compartmentalized processes. Its pH sensitivity further supports its use in monitoring intracellular environments and dynamic biological responses. Although it does not act through a defined molecular target or signaling pathway, its utility lies in its robust physicochemical properties and reliable fluorescence output. In vitro and in vivo applications include tumor localization, biomolecular tracking, and probe-based assay development, with experimental concentrations typically optimized according to specific assay requirements, often within micromolar ranges depending on labeling efficiency and detection sensitivity.