A spectroscopic and molecular modelling study of the ...

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Title A spectroscopic and molecular modelling study of the nature of the association complexes of Nile Red with cyclodextrins
Author(s) Brian D. Wagner, N. Stojanovic, G. Leclair, C. K. Jankowski
Journal Journal of Inclusion Phenomena and Macrocyclic Chemistry
Date 2003
Volume 45
Issue 3-4
Start page 275
End page 283
Abstract 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 gamma-cyclodextrin solution. When gamma-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 gamma-cyclodextrin to a near-saturated aqueous solution of Nile Red (so that 1:1 or 2:1 complexation should be favoured), significant fluorescence enhancement was observed. Analysis of the fluorescence enhancement as a function of host concentration indicated the formation of 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 gamma-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 gamma-cyclodextrin cavity by one or two Nile Red molecules is much more likely.

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