The complexation of the hexacyanoferrate(1r) anion by three modified cyclodextrins [hexakis(6-deoxy-6-amino-)-a-CD (l), heptakis(6-deoxy-6-amino)-p-CD (Z), and octakis(6-deoxy-6-amino)-y-CD (3)] has been studied by cyclic voltammetry and ' H NMR in moderately acidic aqueous media. The binding constants of hexacyanoferrate(I1) with the three hosts were estimated by computer simulation of the voltammetric data, and were found to fall in the ranges 3 x 105-106 dm3 mol-' and 2-5 x lo3 dm3 mol-' for the reduced and oxidized forms of the anion, respectively. The reduced form (-4 charge) of the guest is always more strongly bound to any of the positive charged aminocyclodextrins than the oxidized form (-3 charge). This finding, along with the magnitude of the Gibbs energies of complexation, suggests that the binding interactions are essentially electrostatic in nature. Based on the 'H NMR spectroscopic data, complexation seems to take place by partial inclusion of the anionic guest inside the cavity of the C D receptors.
Self-assembled multilayers of the three modified cyclodextrins
(hexakis(2,3-O-hexyl-6-deoxy-6-amino)-α-cyclodextrin (1),
heptakis(2,3-O-hexyl-6-deoxy-6-amino)-β-cyclodextrin
(2), and octakis(2,3-O-hexyl-6-deoxy-6-amino)-γ-cyclodextrin (3)) on bare gold, as well as
on gold surfaces modified with mercaptopropionic
(4) and mercaptooctanoic (5) acid, were
investigated by cyclic voltammetric, contact angle, FT-IR,
and
quartz crystal microbalance measurements in neutral aqueous media.
The level of organization of the
aggregates formed by the three CD derivatives
(1−3) on the negatively charged surface of
the
mercaptopropionic and mercaptooctanoic acid modified gold electrodes
proved to be substantially better
than that on bare gold electrodes. This finding suggests that
although the amphiphilic character of
compounds 1−3 can induce aggregation on a gold
surface, the electrostatic interaction between the
carboxylic
acid groups and the positively charged cyclodextrins is the primary
force leading to the formation of
well-organized aggregates.
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