Demonstration of structure-selective binding of organic cations to anionic polyelectrolytes

The binding of diprotonated trans-4,4′-diaminostilbene (DASH₂²⁺) to poly (styrenesulfonate) (PSS) and poly(vinylsulfonate) (PVS) in aqueous solution have been studied by following the changes in the absorption, fluorescence and polarization emission spectra of the complexed chromophors. It was found that in the DASH₂²⁺-PSS complex most of the DASH₂²⁺ molecules are bound in their trans-isomer form, while in the DASH₂²⁺-PVS complex, which is less stable and lack π-π interactions, the DASH₂²⁺ molecules are bound in the electrostatically preferred cis conformation. The higher stability of the DASH₂²⁺-PSS complex is due to structural fitting between trans-DASH₂²⁺ and p-styrene sulfonic units. This structural fitting allows formation of π-π bonds between aromatic moieties in combination with electrostatic attraction between the protonated amine and sulfonate charged groups. Our findings demonstrate the delicate interplay between electrostatic and non-polar interactions which controls the binding of relatively large molecules to high-molecular weight receptors, and may be used to model a binding site in biological systems. These findings are corroborated by molecular mechanics simulation of the complexes.