Biology Reference
In-Depth Information
4
−
1
chemical shifts for adipamide by 1.5 ppm (
K
= 3.6
×
10
M
) and
a
×
4
−
1
1
for terephthalamide of 1.23 ppm (
H NMR. The
strong binding of the diamides occurred because both hydrogen-
bonding pockets of the host
K
= 2
10
M
) in
a
simultaneously interacted with
size-matching diamide guests. The monoamides showed negligible
hydrogen-binding affinities with the host, further supporting the
aforementioned argument. Similarly, the larger analogue
33
was
found to preferentially and strongly bind larger guest amides, such
as biphenyldicarboxamide. Analysis of the
34
1
H NMR chemical shift
change (see Fig. 10.6) gave a binding constant of
−
×
4
1
.
It was also found that the introduction of different substituents
(e.g., OMe or NO
K
= 4.5
10
M
a
position of the corner pyridine rings
affected the electron density on the pyridine nitrogen. Consequently,
the host with electron-donating group (OMe) showed weak binding
strength toward the amide guests, whereas the host with the
electron-withdrawing substituent (NO
) at the
para
2
) displayed strong binding
2
affinity to the amide guests.
10.5
Chiral Recognition and Sensing by
Metallacyclic Supramolecules
The chirality in the supramolecule can be introduced either by using
(a) a chiral auxiliary
−
coordinated metal complex, (b) an inherently
chiral octahedral metal complex, (c) an optically active linker ligands,
or (d) the combination of the mentioned methods [17,84]. The use
of chiral phosphine ligands such as 2,2
′
-bis(diphenylphosphino)-1,1-
binaphthyl (BINAP) or 2,3-
-diisopropylidene-2,3-dihydroxy-1,4-
bis(diphenylphosphino)-butane (DIOP) into the molecular assembly
made it possible to monitor the host
O
−
guest interaction via circular
dichroism (CD) spectroscopy along with other spectral studies [85].
CD spectroscopy has been used extensively as a powerful tool to
study the substrate binding in the biological systems [86]. A series
of chiral molecular squares (Scheme 10.4) was reported by Stang
et al.
[85]. Interaction of the chiral squares with silver cation was
monitored by a significant change in the intensity and shape of CD
absorption, while the stoichiometric addition of respective guests
to receptors was followed by the subsequent decrease in absorption
intensities and variation of signal shape. For example, upon addition
of two equivalents of silver triflate, a decrease in the Cotton effect
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