Fluorescence studies of the binding of anionic derivatives of pyrene and fluorescein to cationic polyelectrolytes in aqueous solution

The electrostatic binding of the anionic probes pyrenetetrasulfonic acid (4-PSA) and 6-carboxyfluorescein (6-CF) to poly(allylamine hydrochloride) (PAH), poly(diallyldimethylammonium chloride) (PDADMAC), and DADMAC-acrylamide (AAM) copolymers in aqueous solution has been investigated by fluorescence spectroscopy. Binding of the probes to the polyelectrolytes causes changes in their fluorescence emission and excitation spectra, which allows conclusions to be drawn about their binding mechanisms. 4-PSA forms excimers in the presence of PAH or PDADMAC, whereas the presence of polyelectrolyte in 6-CF solutions induces fluorescence self-quenching of 6-CF. Through measurement of the 4-PSA excimer to monomer emission ratio (I-E/I-M) as a function of 4-PSA concentration or through evaluation of the minima in the 6-CF fluorescence concentration curves in the presence of polyelectrolyte, the amount of probe electrostatically bound to the polyelectrolytes and hence the binding stoichiometry at saturation binding conditions were determined. Binding experiments using copolymers of DADMAC and AAM with various charge densities showed that the DADMAC charge density over the range 21-100 mol % had little effect on the binding stoichiometry of 4-PSA but a marked influence on that of 6-CF. The data obtained are evaluated using a theoretical model based on probability considerations in order to examine the role of polyelectrolyte flexibility and binding site topology on the probe-polyelectrolyte interactions.