5(6)-Carboxyfluorescein is a mixture of 5-carboxy and 6-carboxy derivatives of fluorescein. It is commonly used to label biomolecules through a reaction involving the carboxyl group. Unlike its succinimidyl ester derivative CFSE , 5(6)-carboxyfluorescein is membrane impermeant. As a result, it can be used in studies of membrane permeability. 5(6)-Carboxyfluorescein displays excellent fluorescence (excitation/emission at 492/514 nm, respectively), and its excitation maximum closely matches the 488 nm spectral line of argon-ion lasers.
