Biology Reference
In-Depth Information
especially striking inward bowing of two of the four ethynylpyridine
linkages. The driving force for the cavity collapse is apparently van
der Waals interactions between the porphyrins as the cavity of the
rectangle is too narrow to bind large guest. A strong difunctional
Lewis base, such as 1,4-diazabicyclo[2.2.2]octane (DABCO), bound
to
4
5m
in a 2:1 stoichiometry with a binding constant of
K
= 5
×
10
a
−
1
M
, whereas the monofunctional Lewis base, quinuclidine, also
bound
5m
in a 2:1 stoichiometry with a binding constant of
K
= 2
×
a
5
−
1
, four times larger than the binding constant for DABCO. This
observation suggests that the guest molecules were bounded to the
exterior of
10
M
, consistent with the strong van der Waals interaction
between the two long-axial-bridging ligands that prevent an open-
cavity internal binding event.
5m
Hunter
et al.
reported the self-assembly of Zn
−
trisporphyrin
(
) induced by coordination of DABCO to form a stable 2:3
double-decker molecular coordination cage
47
[91]. The cage
showed a flexible nature, which allows breathing of the cavity
height between dimensions of 0 and 7 Å. Such a large flexibility
enabled the recognition of large molecules of complementary size
and shape through an induced-fit mechanism. It was found that
the cage
46
46
was able to recognize benzene-1,3,5-tricarboxamides
48
. The tricarboxamides bound inside the central cavity of
the cage complex to form a triple helical hydrogen-bonding motif.
The driving force for the recognition of tricarboxamides is probably
the formation of six N
and
49
H•••O=C hydrogen bonds as well as other
aromatic interactions. The formation of such a structural motif was
monitored by
−
1
H NMR titration of the cage
46
and the tricarboxamide
guest
. The downfield shift of 0.1 ppm observed for both NH
protons of cage
49
indicated the formation
of a hydrogen-bonded complex. Additionally, the aromatic protons
of the tricarboxamide were upfield shifted, which is indicative of the
aromatic stacking. The isothermal titration calorimetry (ITC) was
employed to determine the binding constants of the cage
46
and tricarboxamide
49
46
with
the guests
48
and
49
. Analysis of the ITC data for both guests
48
and
49
suggested the formation of a 1:1 host
−
guest complex and the
3
3
−
1
binding constants of 9
×
10
and 14
×
10
M
, respectively.
Search WWH ::
Custom Search