Interaction of a pyrene-labeled cholesterol-bearing polyanion with surfactant micelles studied by fluorescence quenching

The dissociation of multipolymer aggregates of a cholesterol-bearing polyanion by surfactant micelles in water and the formation of polymer–micelle complexes were investigated using fluorescence techniques. Interchain aggregates of cholesterol residues in a random terpolymer of sodium 2-(acrylamido)-2-methylpropanesulfonate (94 mol%), cholesteryl 6-methacryloyloxyhexanoate (5 mol%), and N-(1-pyrenylmethyl)methacrylamide (1 mol%) are completely disrupted by the addition of nonionic micelles of n-dodecyl hexa(oxyethylene) glycol monoether (C12E6) resulting in polymer–micelle complexes. Fluorescence of pyrene labels in the terpolymer indicated that C12E6 micelles bind predominantly to cholesterol sites while the micelle interacts only weakly with pyrene sites. When a small amount of n-hexadecyltrimethylammonium chloride (CTAC), a cationic surfactant, was added to the C12E6 micelle, positively charged C12E6/CTAC mixed micelles bind to sulfonate sites in the terpolymer. This additional electrostatic interaction strengthens the binding of the micelle to pyrene sites as well as cholesterol sites. The pyrene–micelle interaction thus increased was investigated by steady-state and time-dependent fluorescence quenching techniques using n-hexadecylpyridinium chloride, a quencher, added to the C12E6/CTAC mixed micelle. Fluorescence quenching data for varying micelle charge densities (i.e. the mole fraction of CTAC (Y) in the mixed micelle) were analyzed using a kinetic model. Results made it clear that the residence time of the mixed micelle on the pyrene site increases with increasing Y. A long residence time of ca. 19 μs was observed at Y=0.09 as a result of an interplay of interaction of the micelle charge with the polymer charge and hydrophobic interaction of the micelle with mainly cholesterol sites.