Micellar aggregation of poloxamer 213 and its interaction with cholesterol derivatives.
The micellar properties of Poloxamer 213 (1), a Pluronic copolymer shown to affect lipid absorption and serum cholesterol level in experimental animals, are investigated by surface tension measurements, photon correlation spectroscopy (PCS), Reichardt's dye solubilization technique, differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR) spectrometry, and fluorescence spectroscopy. The clear inflection point at 3 x 10(-6) M (25 degrees C) observed in the surface tension-concentration curve may not represent the CMC for the formation of multimolecular aggregates. In the 10(-4) to 10(-2) M concentration range, the temperature-dependent transition (as the concentration of 1 increases) from the larger (hydrodynamic radius, Rh approximately 20-40 nm), highly hydrated aggregates to the contracted (Rh approximately 6-7 nm), less polar form occurs as a discontinuity. This process is endothermic, with an average delta H of 34.5 kcal/mol. At 25 degrees C, both the 25-(NBD-methylamino)-27-norcholesterol (fluorescence probe) and 3-Doxyl-5 alpha-cholestane (EPR spin probe) begin to show significant interaction with 1 in the 10(-3) M range. The sequestration of the fluorescent cholesterol probe by 1 aggregates begins at approximately 2 x 10(-4) M at 35 degrees C. Analysis of EPR and fluorescence data indicates that the cholesterol analogues are in a nonpolar micellar environment of low fluidity. The significance and implication of the data are discussed in the context of the hypothesized cholesterol sequestration by 1 under physiological conditions.