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

importance of possible electrostatic effects in the complexation of cyclode xtrins has been investigated. The complexation features of the anionic gues t molecule, 2-(p-toluidinyl)naphthalene-6-sulfonate (TNS), with the positiv ely 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 cyc lodextrins, its behavior is still not completely understood. In this paper we shea that the complexation of TNS with beta -CD involves the simultaneou s 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 mo ieties of TNS with two cyclodextrin molecules. In the complexation of TNS w ith NH3+CD, the same species and equilibria are present, although significa nt structural differences of their complexes and variations in the values o f the association constants are observed. Such differences can be explained by the electrostatic interaction between the sulfonate anion of TNS and th e positively charged amino group of NH3+CD. The resolution of these complic ated systems, with four emitting species and four association equilibria, c ould be achieved only by careful and systematic analysis of both steady-sta te and time-resolved fluorescence data, together with direct structural inf ormation from NMR experiments. Moreover, principal components global analys is (PCGA) is presented as an objective method for the detection of complexe s of different stoichiometries and for the precise determination of the inv olved association constants.