A modified cyclodextrin containing a dansyldiethylenetriamine
metal-binding group (6-deoxy-6-N-(N‘‘-dansyldiethylenetriamino)-β-cyclodextrin,
CD-dien-DNS) was synthesized. The conformation
of CD-dien-DNS was studied by 2D NMR (ROESY spectra) 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)-β-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.