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intramolecular metal complexation. Metal binding to
H
1
5
and
H
2
6
also produced
new CD peaks between 240 and 280 nm, the region corresponding to the 8-hydroxy-
quinoline p-p
transition, which reflects the transmission of the stereogenic charac-
ter of the peptoid scaffold to the metal center. These results indicate the
reciprocating effects of metal binding - the chirality of the peptoid backbone estab-
lishes an asymmetric environment about the metal center while metal complexation
enhances the helical character of the backbone [55]. This synergistic interaction
between helices and metal complexes, which has not been observed before in artifi-
cial folded oligomers, holds potential for applications in asymmetric catalysis and
material science.
The last example of metallopeptoids involves N-benzyloxyethyl cyclic oligomers of
various sizes that bind alkali metal ions, a study conducted in the groups of Izzo and De
Riccardis [56]. The synthesis of the linear N-benzloxyethyl glycine oligomers was
accomplished both in solution [57] and through solid-phase methods. Head to tail macro-
cyclizations of the linear compounds were achieved in the presence of different condens-
ing agents, producing three cyclic peptoids - trimer, tetramer and hexamer (Figure 11.23).
The peptoids were characterized in solution by NMR spectroscopy and in the solid state
by X-ray crystallography. The spectroscopic data of the cyclic trimer revealed a C
3
-sym-
metric all-
cis
“crown” conformation. In the case of the cyclic tetramer, a single crystal
X-ray analysis demonstrated a
ctct
“chair” tetralactam core geometry [58]. The cyclic
hexamer, in contrast, showed conformational disorder in solution, an observation that
prompted metal complexation studies with this peptoid.
Indeed, stepwise addition of sodium picrate induced the formation of a new chemical
species with a remarkably simplified NMR spectrum, suggesting the presence of an
S
6
-
symmetry axis passing through the inner cavity of the sodium cation. Moreover,
Figure 11.23 Izzo and De Riccardis's cyclic peptoids. [56] Reproduced by permission of The
Royal Society of Chemistry.
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