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intramolecular 1 : 1 peptoid:metal complex (Figure 11.21). Positioning the ligands at
i
and
i
3 in the sequence matches the pitch of the helix and is designed to orient these groups
in proximity on the same face of the scaffold, separated by one helical turn.
Oligomers
H
1
5
and
H
2
6
were synthesized in good yields [52], and their ability to
bind metal ions was evaluated by UV-vis spectroscopy (Figure 11.22, a and c). Job
plots constructed from UV titration of the peptoids with either Cu
2þ
or Co
2þ
were
consistent with a 2 : 1 (
H
1
5
)
2
M(M
þ
Cu
2þ
or Co
2þ
) duplex and a 1 : 1 (
H
2
6
)M intra-
molecular complex. The peptoid:metal ratio was corroborated further by mass spec-
trometry analysis. CD measurements revealed substantial changes upon metal
complex formation (Figure 11.22, b and d). Solutions of the metal complexes exhib-
ited increases in the magnitude of the CD signals near 200 and 220 nm, relative to
the metal-free peptoids. In the case of (
H
1
5
)
2
M this suggests an increase in confor-
mational order, as the magnitude of the signal reflects the degree of helicity. The
increase in the CD signal was more dramatic for (
H
2
6
)M, consistent with greater
conformational constraint and enhanced secondary structure content due to
¼
Figure 11.22 UV-vis spectra and Job plot for titration of (a) H
1
5 with Cu
2þ
and (c) H
2
6 with
Cu
2þ
. CD spectra for (B) H
1
5, (H
1
5)
2
Cu and (H
1
5)
2
Co. CD spectra for (d) H
2
6, (H
2
6)Cu, and
(H
2
6)Co UV-vis spectra. (a) 54 mM peptoid in MeOH:H
2
O (4 : 1) solution, (c) 40 mM peptoid
in MeOH:H
2
O (4 : 1) solution. Blue ¼ free ligand, red¼metal complex. CD spectra: 100
m
M
MeOH:H
2
O (4 : 1) solutions [52].
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