Fluorescent chemosensor for organic guests and copper(II) ion based on dansyldiethylenetriamine-modified beta-cyclodextrin

A modified cyclodextrin containing a dansyldiethylenetriamine metal-binding group (6-deoxy-6-N(N''-dansyldiethylenetriamino)-beta-cyclodextrin, CD-dien-DNS) was synthesized. The conformation of CD-dien-DNS was studied by 2D NMR (ROESY spectral in D2O, by circular dichroism, and by fluorescence. The results were compared with those previously obtained with the analogous 6-deoxy-6-N-(N'-dansylethylenediamino)-beta-cyclodextrin (CD-en-DNS) and were consistent with the self-inclusion of the dansyl group within the macrocycle cavity. However, the orientation of the dansyl group for CD-dien-DNS was found to be equatorial, whereas for CD-en-DNS it was axial, suggesting a dependence of the orientation of the dansyl group upon the length of the linker. In the presence of lipophilic organic molecules, CD-dien-DNS showed sensing properties similar to those observed for CD-en-DNS, suggesting a similar ''in-out'' movement of the dansyl group, due to competitive inclusion of the guest. Unlike CD-en-DNS, CD-dien-DNS was found to be a fluorescent chemosensor for copper(II) ion, with a linear response up to a 1:1 molar ratio, suggesting that a more flexible conformation of the linker and the presence of additional binding sites allow binding of the metal ion by the amino and sulfonamidate groups. Good selectivity for Cu(II), when compared with Fe(II), Co(II), Ni(II), and Zn(II), was observed. The CD-dien-DNS copper(II) complex was shown to behave as a chemosensor for bifunctional molecules, such as amino acids. In fact, upon addition of alanine, tryptophan, and thyroxine, the negligible fluorescence intensity of Cu(CD-dien-DNS) complex was ''switched on'', with a response dependent on the amino acid side chain.