2-(p-toluidino)-6-naphthalene sulfonate as a fluorescent probe for flocculation studies of cationic potato amylopectin and nanosized silica particles 1. 2-p-(toluidino)-6-naphthalene sulfonate binding to cationic amylopectin

2-(p-toluidino)-6-naphthalene sulfonate (TNS) is a probe that fluoresces strongly when bound to certain proteins and polymers, but weakly in aqueous solution. Absorption and fluorescence spectroscopy were used to study the interaction of TNS with native amylopectin potato starch (NApS) and cationized amylopectin potato starch (CApS) in aqueous solution. The anionic TNS binds to CApS at a single type of binding site, with an affinity which has both electrostatic and nonelectrostatic contributions (including hydrogen bonding), whereas binding to NApS occurs at the same type of site but only by nonelectrostatic means. The affinity to CApS decreases strongly with increasing salt concentration, due to screening of the electrostatic attraction, whereas with NApS increasing salt concentration slightly enhances the binding affinity, most likely due to screening of a weak repulsive interaction between TNS and phosphate residues on NApS. The association constant for binding of TNS to CApS in 5 mM NaCl is 110 ± 20 M⁻¹. This comparatively weak binding makes TNS a useful probe in kinetic investigations of the flocculation of anionic silica particles by CApS. Received: 14 August 1998 Accepted in revised form: 4 December 1998