5-Dodecanoylaminofluorescein as a probe for the determination of critical micelle concentration of detergents using fluorescence anisotropy

A method for determining the critical micelle concentration (CMC) of various detergents based on fluorescence polarization (anisotropy) of the lipophilic probe 5-dodecanoylaminofluorescein is presented. Nonionic, cationic, anionic, and steroid-based detergents can all be evaluated by this method and the determined CMC values of selected detergents agree well with those reported in the literature. In addition, we report the CMC of domiphen bromide, whose CMC value has not previously been described. In the case of ionic detergents, the method described is particularly sensitive at discerning changes in the CMC with increasing ionic strength of the medium and can discriminate detergent CMCs in 5 mM versus 25 mM buffering components. The described fluorescence polarization technique allows very low (submicromolar) concentrations of probe to be employed, thus minimizing the perturbation of micelle formation by 5-dodecanoylaminofluorescein insertion.