and F. Toda, J. Am. Chem. Soc., 1993, 115, 5035–5040; (c) H. Ikeda,
Acridine Red, Rhodamine B, and Methyl Orange. In an aque-
ous buffer of pH 7.2, the molecular selectivity profile of host 1
is in the order MO > RhB > AR, while hosts 2 and 3 showed a
selectivity profile of RhB > MO > AR. In aqueous buffer of
pH 2.0, however, hosts 1–3 afforded the same molecular
selectivity sequence of RhB > AR > MO.
From the above comparison, it can be noted that hosts 1–3
gave different molecular selectivity sequences in different pH
systems. That is, the molecular selectivity sequence for Acridine
Red and Methyl Orange was inverted when the pH was changed
from 7.2 to 2.0. This result depends clearly on the transform-
ation of the guest molecule with change of buffer. Whether
in buffer of pH 7.2 or 2.0, of the three hosts the indolyl
substituent of compound 1 is closest to the cavity and would
interfere with the inclusion complexation most effectively. Thus,
it gives a unique molecular binding ability and hence a unique
molecular selectivity sequence.
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Conclusions
In summary, three novel indolyl-contained β-cyclodextrin
derivatives were synthesized and their molecular recognition
behavior was studied using spectroscopic methods. Because
the indolyl moiety may be protonated in acidic conditions, the
spatial position of this substituent is different in solutions
with different pH. This idea may also be extended to other
conformation-changeable modified cyclodextrin systems,
which would potentially provide molecular switches depending
upon the acidity of microenvironment. On the other hand,
the molecular recognition studies revealed that several factors
greatly affect the inclusion complexation of guest dyes with
cyclodextrin derivatives, including not only the shape/charge/
substituent of the guest and the tether of the host but also the
critical changes in host–guest interactions such as hydrophobic,
van der Waals and electrostatic interactions as well as the size/
shape-matching and induced-fit mechanisms. Because the pH
of the system would affect both the host conformation and the
guest structure, the host–guest inclusion complexation behavior
depends on the acidity of the aqueous buffer. Thus, this method
is a convenient and powerful tool for controlling the molecular
binding ability and relative molecular selectivity of cyclo-
dextrins—altering the conformation/structure of the modified
cyclodextrins or guest molecules upon inclusion complex-
ation—by simply changing the pH value of the solution. Fur-
ther studies should consider the solvent itself when discussing
the molecular recognition process of cyclodextrin hosts.
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Acknowledgements
This work was supported by grants of the Natural Science
Foundation (No. 29992590–8 and 29972029) of China, and the
Foundation of Ministry of Education, which are gratefully
acknowledged.
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