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.1 Unlike its succinimidyl ester derivative CFSE , 5(6)-carboxyfluorescein is membrane impermeant. As a result, it can be used in studies of membrane permeability.2 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.
1.Fischer, R., Mader, O., Jung, G., et al.Extending the applicability of carboxyfluorescein in solid-phase synthesisBioconjug. Chem.14(3)653-660(2003) 2.Ferdani, R., Li, R., Pajewski, R., et al.Transport of chloride and carboxyfluorescein through phospholipid vesicle membranes by heptapeptide amphiphilesOrg. Biomol. Chem.5(15)2423-2432(2007)