2-(p-toluidino)-6-naphthalene sulfonate as a fluorescent probe for flocculation studies of cationic potato amylopectin and nanosized silica particles1. 2-p(toluidino)-6-naphthalene sulfonate binding to cationic amylopectin

2-(p-toluidino)-6-naphthalene sulfonate (TNS) is a probe that fluoresces st rongly when bound to certain proteins and polymers, but weakly in aqueous s olution. Absorption and fluorescence spectroscopy were used to study the in teraction of TNS with native amylopectin potato starch (NApS) and cationize d amylopectin potato starch (CApS) in aqueous solution. The anionic TNS bin ds to CApS at a single type of binding site, with an affinity which has bot h electrostatic and nonelectrostatic contributions (including hydrogen bond ing), whereas binding to NApS occurs at the same type of site but only by n onelectrostatic means. The affinity to CApS decreases strongly with increas ing salt concentration, due to screening of the electrostatic attraction, w hereas with NApS increasing salt concentration slightly enhances the bindin g affinity, most likely due to screening of a weak repulsive interaction be tween TNS and phosphate residues on NApS. The association constant for bind ing 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 flocculat ion of anionic silica particles by CApS.