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In-Depth Information
than 100 times stronger than cationic peptides. Thus, the cage
62
recognized peptide containing Trp residues with high selectivity and
affinity and discriminated longer peptides by means of electrostatic,
charge transfer, and steric repulsion between the cage and peptides.
Moreover, the selectivity of the charged peptides can be controlled by
pH conditions owing to the cationic nature of the cage
62
. Similarly,
the bowl-shaped host
71
can recognize aromatic peptides [110].
For example,
71
bound the hexapeptide
69
more strongly than cage
×
5
−
1
62
can
cover a large area and is expected to employ a pinpoint recognition
of protein surface that often mediates protein
(for
71•69
K
= 1.3
10
M
). The bowl-shaped cage
71
a
−
protein interactions.
Fujita
showed that the short peptides can be induced into
R-helix conformations in aqueous solution through binding to an
artificial supramolecular hydrophobic pocket created by three-
dimensional cage similar to
et al.
, which has Pt as the coordination
center instead of Pd [111]. The systematic variation of hydrophobic
residues in short oligopeptides evidenced aromatic
71
−
aromatic
interactions as a key driving force for enclathration; however, the
spacing and preorganization of the residues were also important
factors. Peptides with two aromatic residues showed the highest
affinity for host, thus the artificial cavities could be used to alter and
control biological processes occurring at the protein surface, such as
protein
−
protein recognition and interaction. Furthermore, Fujita
et
al.
used the box-shaped macrocyclic framework to engineer unusual
symmetrical
quadruple
stacks [112,113]. The electrostatic and hydrophobic interactions are
essential in the formation of discrete stacks within the box-shaped
cavities of organic pillared coordination cages. The stacking number
n
A
-
D
-
D
-
A
and dissymmetrical
A
-
D
-
A
-
A
can be uniquely determined by simply adjusting the length of
the organic pillar sets. Thus, the pillared cages enable the precise
control of not only the stacking number but also the stacking order,
providing opportunities to study the hitherto unexplored properties
of discrete stacks of aromatic compounds.
10.10
Concluding Remarks
This chapter is primarily concerned with systematic design and study
of transition metal
−
mediated self-assembly supramolecules and
their associated photophysical and photochemical properties. The
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