A Spectroscopic and Molecular Modelling Study of the Nature of the Association Complexes of Nile Red with Cyclodextrins

The polarity-sensitive fluorescent dye Nile Red forms association complexes with various cyclodextrins in aqueous solution. The formation of such association complexes has a significant effect on the Nile Red fluorescence, with the largest effect being observed in -cyclodextrin solution. When -cyclodextrin is used to increase the Nile Red concentration in solution, the absorption spectrum shows a large blue shift indicating the formation of an inclusion complex, but surprisingly the Nile Red fluorescence is strongly suppressed. A proposed explanation for this observation involves the formation of 1:2 host:guest complexes, in which the Nile Red guests are included as relatively non-fluorescent dimers. When the solutions were prepared by adding -cyclodextrinto a near-saturated aqueous solution of Nile Red (so that 1:1 or 2:1 complexation shouldbe favoured), significant fluorescence enhancement was observed. Analysis of thefluorescence enhancement as a function of host concentration indicated the formationof 2:1 host:guest complexes in these solutions. However, electrospray mass spectroscopic studies show no evidence for the formation of any such inclusion complexes. Furthermore, molecular modelling shows that the formation of a complex involving full insertion of Nile Red in the -cyclodextrin cavity is not stable, and will quickly eject the Nile Red guest molecule. These modelling results suggest that an association complex involving capping (via the association of Nile Red parallel to the cavity opening, or by partial insertion into the cavity) of the -cyclodextrin cavity by one or two Nile Red molecules is much more likely.