Resolution of the association equilibria of 2-(p-toluidinyl)-naphthalene-6-sulfonate (TNS) with β-cyclodextrin and a charged derivative

importance of possible electrostatic effects in the complexation of cyclodextrins has been investigated. The complexation features of the anionic guest molecule, 2-(p-toluidinyl)naphthalene-6-sulfonate (TNS), with the positively charged cyclodextrin, 6-deoxy-6-amino-beta -cyclodextrin (NH3+CD), and with the neutral beta -cyclodextrin (beta -CD) were compared. Although TNS is a widely used fluorescent probe for the studies of complexation with cyclodextrins, its behavior is still not completely understood. In this paper we shea that the complexation of TNS with beta -CD involves the simultaneous formation of two different 1:1 complexes, resulting from inclusion of the toluidin moiety or the naphthalenesulfonate moiety in a cyclodextrin unit, and a 1:2 complex, which is due to the complexation of the two aromatic moieties of TNS with two cyclodextrin molecules. In the complexation of TNS with NH3+CD, the same species and equilibria are present, although significant structural differences of their complexes and variations in the values of the association constants are observed. Such differences can be explained by the electrostatic interaction between the sulfonate anion of TNS and the positively charged amino group of NH3+CD. The resolution of these complicated systems, with four emitting species and four association equilibria, could be achieved only by careful and systematic analysis of both steady-state and time-resolved fluorescence data, together with direct structural information from NMR experiments. Moreover, principal components global analysis (PCGA) is presented as an objective method for the detection of complexes of different stoichiometries and for the precise determination of the involved association constants.