ABSTRACT
The fluorescent compound, 5(6)-carboxyfluorescein (CF), has been a powerful tool in liposome methodology since the paper by J. N. Weinstein et al. in 1977. Calcein often contains residual fluorescein and other impurities which may interfere with assays such as liposome-cell interactions. A method for estimation of residual fluorescence of fluorescein in calcein is given by H. Diehl. Impurities in calcein do not appear to cause much interference with the use of quenched calcein in liposomes for permeability measurements. Calcein shares all the advantages of 5(6)-CF in liposome methodology and can be substituted for the latter in any of the experiments in which CF has been used. Calcein exhibits fluorescent quenching at neutral pH with transition metals and this is used as the basis of an assay for liposome-liposome fusion. A detailed description of the properties of calcein can be found in a publication by Diehl.
